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Digital Image Processing

Week 1

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Evaluation

1(base) + 3 (course test) + 3 (lab activity)+ 3 (article presentation)

gt 45

Weeks 1 ndash 7 ndash coursesWeek 8 ndash (course) testWeeks 9 ndash 12 ndash labWeeks 13 ndash 14 ndash article lab evaluationWeek 15 or 16 ndash article presentation

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Bibliography

bull RC Gonzales RE Woods Digital Image Processing Prentice Hall2008 3rd ed

bull RC Gonzales RE Woods SL Eddins Digital Image Processing Using MATLAB Prentice Hall 2003

bull httpwwwimageprocessingplacecom

bullM Petrou C Petrou Image Processing the fundamentals John Wiley 2010 2nd ed

bull W Burger MJ Burge Digital Image Processing An Algorithmic Introduction Using Java Springer 2008

Digital Image ProcessingDigital Image Processing

Week 1Week 1

bull Image Processing Toolbox (httpwwwmathworkscomproductsimage)

bull C Solomon T Breckon Fundamentals of Digital Image Processing A Practical Approach with Examples in Matlab Wiley-Blackwell 2011

bullWK Pratt Digital Image Processing Wiley-Interscience 2007

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Meet LenaThe First Lady of the Internet

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Lenna Soderberg (Sjoumloumlblom) and Jeff Seideman taken in May 1997Imaging Science amp Technology Conference

Digital Image ProcessingDigital Image Processing

Week 1Week 1

What is Digital Image Processing

f(xy) = intensity gray level of the image at spatial point (xy)

x y f(xy) ndash finite discrete quantities -gt digital image

Digital Image Processing = processing digital images by means of a digital computer

A digital image is composed of a finite number of elements (location value of intensity)

These elements are called picture elements image elements pels pixels

( )i j ijx y f

3 f D

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Image processing is not limited to the visual band of the electromagnetic (EM) spectrum

Image processing gamma to radio waves ultrasound electron microscopy computer-generated images

image processing image analysis computer vision

Image processing = discipline in which both the input and the output of a process are images

Computer Vision = use computer to emulate human vision (AI) ndashlearning making inferences and take actions based on visual inputs

Image analysis (image understanding) = segmentation partitioning images into regions or objects

(link between image processing and image analysis)

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Distinction between image processing image analysis computer vision

low-level mid-level high-level processes

Low-level processes image preprocessing to reduce noise contrast enhancement image sharpening both input and output are images

Mid-level processes segmentation partitioning images into regions or objects description of the objects for computer processing classificationrecognition of individual objects inputs are generally images outputs are attributes extracted from the input image (eg edges contours identity of individual objects)

High-level processes ldquomaking senserdquo of a set of recognized objects performing the cognitive functions associated with vision

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Digital Image Processing (Gonzalez + Woods) =

processes whose inputs and outputs are images +

processes that extract attributes from images recognition of individual objects

(low- and mid-level processes)

Example

automated analysis of text = acquiring an image containing text preprocessing the image (enhancement sharpening) extracting (segmenting) the individual characaters describing the characters in a form suitable for computer processing recognition of individual characters

Digital Image ProcessingDigital Image Processing

Week 1Week 1

The Origins of DIP

Newspaper industry pictures were sent by submarine cable between London and New York

Before Bartlane cable picture transmission system (early 1920s) ndash 1 week

With Bartlane system less than 3 hours

Specialized printing equipment coded pictures for cable transmission and reconstructed them at the receiving end (1920s -5 distict levels of gray 1929 ndash 15 levels)

This example is not DIP the computer is not involved

DIP is linked and devolps in the same rhythm as digital computers (data storage display and transmission)

Digital Image ProcessingDigital Image Processing

Week 1Week 1

A digital pictureproduced in 1921from a coded tapeby a telegraph printer withspecial type faces(McFarlane)

A digital picturemade in 1922from a tapepunched after the signals hadcrossed the Atlantic twice(McFarlane)

Digital Image ProcessingDigital Image Processing

Week 1Week 1

1964 Jet Propulsion Laboratory (Pasadena California) processed pictures of the moon transmitted by Ranger 7 (corrected image distortions)

The first picture of the moon by a US spacecraft Ranger 7 took this image July 31 1964 about 17 minutes before impacting the lunar surface(Courtesy of NASA)

Digital Image ProcessingDigital Image Processing

Week 1Week 1

1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

1970s ndash invention of CAT (computerized axial tomography)

CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

Digital Image ProcessingDigital Image Processing

Week 1Week 1

loz geographers use DIP to study pollution patterns from aerial and satellite imagery

loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

loz astronomy biology nuclear medicine law enforcement industry

DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Examples of Fields that Use DIP

Images can be classified according to their sources (visual X-ray hellip)

Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Electromagnetic waves can be thought as propagating sinusoidal

waves of different wavelength or as a stream of massless particles

each moving in a wavelike pattern with the speed of light Each

massless particle contains a certain amount (bundle) of energy Each

bundle of energy is called a photon If spectral bands are grouped

according to energy per photon we obtain the spectrum shown in the

image above ranging from gamma-rays (highest energy) to radio

waves (lowest energy)

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Gamma-Ray Imaging

Nuclear medicine astronomical observations

Nuclear medicine

the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

Images are produced from the emissions collected by gamma-ray detectors

Images of this sort are used to locate sites of bone pathology (infections tumors)

PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Examples of gamma-ray imaging

Bone scan PET image

Digital Image ProcessingDigital Image Processing

Week 1Week 1

X-ray imaging

Medical diagnosticindustry astronomy

A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Angiography = contrast-enhancement radiography

Angiograms = images of blood vessels

A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

X-rays are used in CAT (computerized axial tomography)

X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

Industrial CAT scans are useful when the parts can be penetreted by X-rays

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Examples of X-ray imaging

Chest X-rayAortic angiogram

Head CT Cygnus LoopCircuit boards

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Imaging in the Ultraviolet Band

Litography industrial inspection microscopy biological imaging astronomical observations

Ultraviolet light is used in fluorescence microscopy

Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

and then it separates the much weaker radiating fluorescent light from the brighter excitation light

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Imaging in the Visible and Infrared Bands

Light microscopy astronomy remote sensing industry law enforcement

LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

Weather observations and prediction produce major applications of multispectral image from satellites

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Satellite images of Washington DC area in spectral bands of the Table 1

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Examples of light microscopy

Taxol (anticancer agent)magnified 250X

Cholesterol(40X)

Microprocessor(60X)

Nickel oxidethin film(600X)

Surface of audio CD(1750X)

Organicsuperconductor(450X)

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Automated visual inspection of manufactured goods

a bc de f

a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Imaging in the Microwave Band

The dominant aplication of imaging in the microwave band ndash radar

bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Spaceborne radar image of mountains in southeast Tibet

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Imaging in the Radio Band

medicine astronomy

MRI = Magnetic Resonance Imaging

This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

Digital Image ProcessingDigital Image Processing

Week 1Week 1

MRI images of a human knee (left) and spine (right)

Digital Image ProcessingDigital Image Processing

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Images of the Crab Pulsar covering the electromagnetic spectrum

Gamma X-ray Optical Infrared Radio

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Other Imaging Modalities

acoustic imaging electron microscopy synthetic (computer-generated) imaging

Imaging using sound geological explorations industry medicine

Mineral and oil exploration

For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Digital Image ProcessingDigital Image Processing

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Biometry - iris

Digital Image ProcessingDigital Image Processing

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Biometry - fingerprint

Digital Image ProcessingDigital Image Processing

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Face detection and recognition

Digital Image ProcessingDigital Image Processing

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Gender identification

Digital Image ProcessingDigital Image Processing

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Image morphing

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Fundamental Steps in DIP

methods whose input and output are images

methods whose inputs are images but whose outputs are attributes extracted from those images

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Outputs are images

bull image acquisition

bull image filtering and enhancement

bull image restoration

bull color image processing

bull wavelets and multiresolution processing

bull compression

bull morphological processing

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Outputs are attributes

bull morphological processing

bull segmentation

bull representation and description

bull object recognition

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Image acquisition - may involve preprocessing such as scaling

Image enhancement

bull manipulating an image so that the result is more suitable than the original for a specific operation

bull enhancement is problem oriented

bull there is no general sbquotheoryrsquo of image enhancement

bull enhancement use subjective methods for image emprovement

bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Image restoration

bull improving the appearance of an image

bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

Color image processing

bull fundamental concept in color models

bull basic color processing in a digital domain

Wavelets and multiresolution processing

representing images in various degree of resolution

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Compression

reducing the storage required to save an image or the bandwidth required to transmit it

Morphological processing

bull tools for extracting image components that are useful in the representation and description of shape

bull a transition from processes that output images to processes that outputimage attributes

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Segmentation

bull partitioning an image into its constituents parts or objects

bull autonomous segmentation is one of the most difficult tasks of DIP

bull the more accurate the segmentation the more likley recognition is to succeed

Representation and description (almost always follows segmentation)

bull segmentation produces either the boundary of a region or all the poits in the region itself

bull converting the data produced by segmentation to a form suitable for computer processing

Digital Image ProcessingDigital Image Processing

Week 1Week 1

bull boundary representation the focus is on external shape characteristics such as corners or inflections

bull complete region the focus is on internal properties such as texture or skeletal shape

bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

Object recognition

the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

Knowledge database

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Simplified diagramof a cross sectionof the human eye

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Digital Image ProcessingDigital Image Processing

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Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

The cornea is a tough transparent tissue that covers the anterior surface of the eye

Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

Digital Image ProcessingDigital Image Processing

Week 1Week 1

The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

Fovea = the place where the image of the object of interest falls on

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

Blind spot region without receptors

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Image formation in the eye

Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

distance between lens and retina along visual axix = 17 mm

range of focal length = 14 mm to 17 mm

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Digital Image ProcessingDigital Image Processing

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Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

Digital Image ProcessingDigital Image Processing

Week 1Week 1

All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

Digital Image ProcessingDigital Image Processing

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Optical illusions

Digital Image ProcessingDigital Image Processing

Week 1Week 1

ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

quantities that describe the quality of a chromatic light source radiance

the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

Digital Image ProcessingDigital Image Processing

Week 1Week 1

For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

Digital Image ProcessingDigital Image Processing

Week 1Week 1

the physical meaning is determined by the source of the image

( )f D f x y

Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

f(xy) ndash characterized by two components

i(xy) = illumination component the amount of source illumination incident on the scene being viewed

r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

( ) ( ) ( )

0 ( ) 0 ( ) 1

f x y i x y r x y

i x y r x y

Digital Image ProcessingDigital Image Processing

Week 1Week 1

r(xy)=0 - total absorption r(xy)=1 - total reflectance

i(xy) ndash determined by the illumination source

r(xy) ndash determined by the characteristics of the imaged objects

is called gray (or intensity) scale

In practice

min 0 0 max min min min max max max( ) L l f x y L L i r L i r

indoor values without additional illuminationmin max10 1000L L

black whitemin max0 1 0 1 0 1L L L L l l L

min maxL L

Digital Image ProcessingDigital Image Processing

Week 1Week 1

Digital Image Processing

Week 1

Image Sampling and Quantization

- the output of the sensors is a continuous voltage waveform related to the sensed

scene

converting a continuous image f to digital form

- digitizing (x y) is called sampling

- digitizing f(x y) is called quantization

Digital Image Processing

Week 1

Digital Image Processing

Week 1

Continuous image projected onto a sensor array Result of image sampling and quantization

Digital Image Processing

Week 1

Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

(00) (01) (0 1)(10) (11) (1 1)

( )

( 10) ( 11) ( 1 1)

f f f Nf f f N

f x y

f M f M f M N

image element pixel

00 01 0 1

10 11 1 1

10 11 1 1

( ) ( )

N

i jN M N

i j

M M M N

a a aa f x i y j f i ja a a

Aa

a a a

f(00) ndash the upper left corner of the image

Digital Image Processing

Week 1

M N ge 0 L=2k

[0 1]i j i ja a L

Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

Digital Image Processing

Week 1

Digital Image Processing

Week 1

Number of bits required to store a digitized image

for 2 b M N k M N b N k

When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

Digital Image Processing

Week 1

Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

Measures line pairs per unit distance dots (pixels) per unit distance

Image resolution = the largest number of discernible line pairs per unit distance

(eg 100 line pairs per mm)

Dots per unit distance are commonly used in printing and publishing

In US the measure is expressed in dots per inch (dpi)

(newspapers are printed with 75 dpi glossy brochures at 175 dpi)

Intensity resolution ndash the smallest discernible change in intensity level

The number of intensity levels (L) is determined by hardware considerations

L=2k ndash most common k = 8

Intensity resolution in practice is given by k (number of bits used to quantize intensity)

Digital Image Processing

Week 1

Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

150 dpi (lower left) 72 dpi (lower right)

Digital Image Processing

Week 1

Reducing the number of gray levels 256 128 64 32

Digital Image Processing

Week 1

Reducing the number of gray levels 16 8 4 2

Digital Image Processing

Week 1

Image Interpolation - used in zooming shrinking rotating and geometric corrections

Shrinking zooming ndash image resizing ndash image resampling methods

Interpolation is the process of using known data to estimate values at unknown locations

Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

same spacing as the original and then shrink it so that it fits exactly over the original

image The pixel spacing in the 750 times 750 grid will be less than in the original image

Problem assignment of intensity-level in the new 750 times 750 grid

Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

This technique has the tendency to produce undesirable effects like severe distortion of

straight edges

Digital Image Processing

Week 1

Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

where the four coefficients are determined from the 4 equations in 4 unknowns that can

be written using the 4 nearest neighbors of point (x y)

Bilinear interpolation gives much better results than nearest neighbor interpolation with a

modest increase in computational effort

Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

nearest neighbors of the point 3 3

0 0

( ) i ji j

i jv x y c x y

The coefficients cij are obtained solving a 16x16 linear system

intensity levels of the 16 nearest neighbors of 3 3

0 0

( )i ji j

i jc x y x y

Digital Image Processing

Week 1

Generally bicubic interpolation does a better job of preserving fine detail than the

bilinear technique Bicubic interpolation is the standard used in commercial image editing

programs such as Adobe Photoshop and Corel Photopaint

Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

then zooming the reduced image back to its original size To generate Fig 1(d) nearest

neighbor interpolation was used (both for shrinking and zooming)

Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

from 1250 dpi to 150 dpi (instead of 72 dpi)

Digital Image Processing

Week 1

Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

Digital Image Processing

Week 1

Neighbors of a Pixel

A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

This set of pixels called the 4-neighbors of p denoted by N4 (p)

The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

and are denoted ND(p)

The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

N8 (p)

If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

fall outside the image

Digital Image Processing

Week 1

Adjacency Connectivity Regions Boundaries

Denote by V the set of intensity levels used to define adjacency

- in a binary image V 01 (V=0 V=1)

- in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

We consider 3 types of adjacency

(a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

(b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

(c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

m-adjacent if

4( )q N p or

( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

ambiguities that often arise when 8-adjacency is used Consider the example

Digital Image Processing

Week 1

binary image

0 1 1 0 1 1 0 1 1

1 0 1 0 0 1 0 0 1 0

0 0 1 0 0 1 0 0 1

V

The three pixels at the top (first line) in the above example show multiple (ambiguous)

8-adjacency as indicated by the dashed lines This ambiguity is removed by using

m-adjacency

Digital Image Processing

Week 1

A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

is a sequence of distinct pixels with coordinates

and are adjacent 0 0 1 1

1 1

( ) ( ) ( ) ( ) ( )( ) ( ) 12

n n

i i i i

x y x y x y x y s tx y x y i n

The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

Depending on the type of adjacency considered the paths are 4- 8- or m-paths

Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

in S if there exists a path between them consisting only of pixels from S

S is a connected set if there is a path in S between any 2 pixels in S

Let R be a subset of pixels in an image R is a region of the image if R is a connected set

Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

that are not adjacent are said to be disjoint When referring to regions only 4- and

8-adjacency are considered

Digital Image Processing

Week 1

Suppose that an image contains K disjoint regions 1 kR k K none of which

touches the image border

the complement of 1

( )K

cu k u u

k

R R R R

We call all the points in Ru the foreground of the image and the points in ( )cuR the

background of the image

The boundary (border or contour) of a region R is the set of points that are adjacent to

points in the complement of R (R)c The border of an image is the set of pixels in the

region that have at least one background neighbor This definition is referred to as the

inner border to distinguish it from the notion of outer border which is the corresponding

border in the background

Digital Image Processing

Week 1

Distance measures

For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

function or metric if

(a) D(p q) ge 0 D(p q) = 0 iff p=q

(b) D(p q) = D(q p)

(c) D(p z) le D(p q) + D(q z)

The Euclidean distance between p and q is defined as 1

2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

The pixels q for which ( )eD p q r are the points contained in a disk of radius r

centered at (x y)

Digital Image Processing

Week 1

The D4 distance (also called city-block distance) between p and q is defined as

4( ) | | | |D p q x s y t

The pixels q for which 4( )D p q r form a diamond centered at (xy)

4

22 1 2

2 2 1 0 1 22 1 2

2

D

The pixels with D4 = 1 are the 4-neighbors of (x y)

The D8 distance (called the chessboard distance) between p and q is defined as

8( ) max| | | |D p q x s y t

The pixels q for which 8( )D p q r form a square centered at (x y)

Digital Image Processing

Week 1

8

2 2 2 2 22 1 1 1 2

2 2 1 0 1 22 1 1 1 22 2 2 2 2

D

The pixels with D8 = 1 are the 8-neighbors of (x y)

D4 and D8 distances are independent of any paths that might exist between p and q

because these distances involve only the coordinates of the point

Digital Image Processing

Week 1

Array versus Matrix Operations

An array operation involving one or more images is carried out on a pixel-by-pixel basis

11 12 11 12

21 22 21 22

a a b ba a b b

Array product

11 12 11 12 11 11 12 12

21 22 21 22 21 21 22 21

a a b b a b a ba a b b a b a b

Matrix product

11 12 11 12 11 11 12 21 11 12 12 21

21 22 21 22 21 11 22 21 21 12 22 22

a a b b a b a b a b a ba a b b a b a b a b a b

We assume array operations unless stated otherwise

Digital Image Processing

Week 1

Linear versus Nonlinear Operations

One of the most important classifications of image-processing methods is whether it is

linear or nonlinear

( ) ( )H f x y g x y

H is said to be a linear operator if

images1 2 1 2

1 2

( ) ( ) ( ) ( )

H a f x y b f x y a H f x y b H f x y

a b f f

Example of nonlinear operator

the maximum value of the pixels of image max ( )H f f x y f

1 2

0 2 6 5 1 1

2 3 4 7f f a b

Digital Image Processing

Week 1

1 2

0 2 6 5 6 3max max 1 ( 1) max 2

2 3 4 7 2 4a f b f

0 2 6 51 max ( 1) max 3 ( 1)7 4

2 3 4 7

Arithmetic Operations in Image Processing

Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

The two random variables are uncorrelated when their covariance is 0

Digital Image Processing

Week 1

Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

used in image enhancement)

1

1( ) ( )K

ii

g x y g x yK

If the noise satisfies the properties stated above we have

2 2( ) ( )

1( ) ( ) g x y x yE g x y f x yK

( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

and g respectively The standard deviation (square root of the variance) at any point in

the average image is

( ) ( )1

g x y x yK

Digital Image Processing

Week 1

As K increases the variability (as measured by the variance or the standard deviation) of

the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

means that ( )g x y approaches f(x y) as the number of noisy images used in the

averaging process increases

An important application of image averaging is in the field of astronomy where imaging

under very low light levels frequently causes sensor noise to render single images

virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

images respectively

Digital Image Processing

Week 1

Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

100 noisy images

a b c d e f

Digital Image Processing

Week 1

A frequent application of image subtraction is in the enhancement of differences between

images

(a) (b) (c)

Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

significant bit of each pixel (c) the difference between the two images

Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

difference between images (a) and (b)

Digital Image Processing

Week 1

Mask mode radiography ( ) ( ) ( )g x y f x y h x y

h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

bloodstream taking a series of images called live images (denoted f(x y)) of the same

anatomical region as h(x y) and subtracting the mask from the series of incoming live

images after injection of the contrast medium

In g(x y) we can find the differences between h and f as enhanced detail

Images being captured at TV rates we obtain a movie showing how the contrast medium

propagates through the various arteries in the area being observed

Digital Image Processing

Week 1

a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

Digital Image Processing

Week 1

An important application of image multiplication (and division) is shading correction

Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

When the shading function is known

( )( )( )

g x yf x yh x y

h(x y) is unknown but we have access to the imaging system we can obtain an

approximation to the shading function by imaging a target of constant intensity When the

sensor is not available often the shading pattern can be estimated from the image

Digital Image Processing

Week 1

(a) (b) (c)

Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

Digital Image Processing

Week 1

Another use of image multiplication is in masking also called region of interest (ROI)

operations The process consists of multiplying a given image by a mask image that has

1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

image and the shape of the ROI can be arbitrary but usually is a rectangular shape

(a) (b) (c)

Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

Digital Image Processing

Week 1

In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

min( )mf f f

0 ( 255)max( )

ms

m

ff K K K

f

Digital Image Processing

Week 1

Spatial Operations

- are performed directly on the pixels of a given image

There are three categories of spatial operations

single-pixel operations

neighborhood operations

geometric spatial transformations

Single-pixel operations

- change the values of intensity for the individual pixels ( )s T z

where z is the intensity of a pixel in the original image and s is the intensity of the

corresponding pixel in the processed image

Digital Image Processing

Week 1

Neighborhood operations

Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

in an image f Neighborhood processing generates new intensity level at point (x y)

based on the values of the intensities of the points in Sxy For example if Sxy is a

rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

intensity by computing the average value of the pixels in Sxy

( )

1( ) ( )xyr c S

g x y f r cm n

The net effect is to perform local blurring in the original image This type of process is

used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

largest region of an image

Digital Image Processing

Week 1

Geometric spatial transformations and image registration

- modify the spatial relationship between pixels in an image

- these transformations are often called rubber-sheet transformations (analogous to

printing an image on a sheet of rubber and then stretching the sheet according to a

predefined set of rules

A geometric transformation consists of 2 basic operations

1 a spatial transformation of coordinates

2 intensity interpolation that assign intensity values to the spatial transformed

pixels

The coordinate system transformation ( ) [( )]x y T v w

(v w) ndash pixel coordinates in the original image

(x y) ndash pixel coordinates in the transformed image

Digital Image Processing

Week 1

[( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

Affine transform

11 1211 21 31

21 2212 22 33

31 32

0[ 1] [ 1] [ 1] 0

1

t tx t v t w t

x y v w T v w t ty t v t w t

t t

(AT)

This transform can scale rotate translate or shear a set of coordinate points depending

on the elements of the matrix T If we want to resize an image rotate it and move the

result to some location we simply form a 3x3 matrix equal to the matrix product of the

scaling rotation and translation matrices from Table 1

Digital Image Processing

Week 1

Affine transformations

Digital Image Processing

Week 1

The preceding transformations relocate pixels on an image to new locations To complete

the process we have to assign intensity values to those locations This task is done by

using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

In practice we can use equation (AT) in two basic ways

forward mapping scan the pixels of the input image (v w) compute the new spatial

location (x y) of the corresponding pixel in the new image using (AT) directly

Problems

- intensity assignment when 2 or more pixels in the original image are transformed to

the same location in the output image

- some output locations have no correspondent in the original image (no intensity

assignment)

Digital Image Processing

Week 1

inverse mapping scans the output pixel locations and at each location (x y)

computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

It then interpolates among the nearest input pixels to determine the intensity of the output

pixel value

Inverse mappings are more efficient to implement than forward mappings and are used in

numerous commercial implementations of spatial transformations (MATLAB for ex)

Digital Image Processing

Week 1

Digital Image Processing

Week 1

Image registration ndash align two or more images of the same scene

In image registration we have available the input and output images but the specific

transformation that produced the output image from the input is generally unknown

The problem is to estimate the transformation function and then use it to register the two

images

- it may be of interest to align (register) two or more image taken at approximately the

same time but using different imaging systems (MRI scanner and a PET scanner)

- align images of a given location taken by the same instrument at different moments

of time (satellite images)

Solving the problem using tie points (also called control points) which are

corresponding points whose locations are known precisely in the input and reference

image

Digital Image Processing

Week 1

How to select tie points

- interactively selecting them

- use of algorithms that try to detect these points

- some imaging systems have physical artifacts (small metallic objects) embedded in

the imaging sensors These objects produce a set of known points (called reseau

marks) directly on all images captured by the system which can be used as guides

for establishing tie points

The problem of estimating the transformation is one of modeling Suppose we have a set

of 4 tie points both on the input image and the reference image A simple model based on

a bilinear approximation is given by

1 2 3 4

5 6 7 8

x c v c w c v w cy c v c w c v w c

(v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

Digital Image Processing

Week 1

When 4 tie points are insufficient to obtain satisfactory registration an approach used

frequently is to select a larger number of tie points and using this new set of tie points

subdivide the image in rectangular regions marked by groups of 4 tie points On the

subregions marked by 4 tie points we applied the transformation model described above

The number of tie points and the sophistication of the model required to solve the register

problem depend on the severity of the geometrical distortion

Digital Image Processing

Week 1

a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

Digital Image Processing

Week 1

Probabilistic Methods

zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

p(zk) = the probability that the intensity level zk occurs in the given image

( ) kk

np zM N

nk = the number of times that intensity zk occurs in the image (MN is the total number of

pixels in the image) 1

0( ) 1

L

kk

p z

The mean (average) intensity of an image is given by 1

0( )

L

k kk

m z p z

Digital Image Processing

Week 1

The variance of the intensities is 1

2 2

0( ) ( )

L

k kk

z m p z

The variance is a measure of the spread of the values of z about the mean so it is a

measure of image contrast Usually for measuring image contrast the standard deviation

( ) is used

The n-th moment of a random variable z about the mean is defined as 1

0( ) ( ) ( )

Ln

n k kk

z z m p z

( 20 1 2( ) 1 ( ) 0 ( )z z z )

3( ) 0z the intensities are biased to values higher than the mean

( 3( ) 0z the intensities are biased to values lower than the mean

Digital Image Processing

Week 1

3( ) 0z the intensities are distributed approximately equally on both side of the

mean

Fig1 (a) Low contrast (b) medium contrast (c) high contrast

Figure 1(a) ndash standard deviation 143 (variance = 2045)

Figure 1(b) ndash standard deviation 316 (variance = 9986)

Figure 1(c) ndash standard deviation 492 (variance = 24206)

Digital Image Processing

Week 1

Intensity Transformations and Spatial Filtering

( ) ( )g x y T f x y

f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

neighborhood of (x y)

- the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

and much smaller in size than the image

Digital Image Processing

Week 1

- spatial filtering the operator T (the neighborhood and the operation applied on it) is

called spatial filter (spatial mask kernel template or window)

( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

( )s T r

s and r are denoting respectively the intensity of g and f at (x y)

Figure 2 left - T produces an output image of higher contrast than the original by

darkening the intensity levels below k and brightening the levels above k ndash this technique

is called contrast stretching

Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

Digital Image Processing

Week 1

Figure 2 right - T produces a binary output image A mapping of this form is called

thresholding function

Some Basic Intensity Transformation Functions

Image Negatives

The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

- equivalent of a photographic negative

- technique suited for enhancing white or gray detail embedded in dark regions of an

image

Digital Image Processing

Week 1

Original Negative image

Digital Image Processing

Week 1

Log Transformations - constant ( ) log(1 ) 0s T r c r c r

Some basic intensity transformation functions

Digital Image Processing

Week 1

This transformation maps a narrow range of low intensity values in the input into a wider

range An operator of this type is used to expand the values of dark pixels in an image

while compressing the higher-level values The opposite is true for the inverse log

transformation The log functions compress the dynamic range of images with large

variations in pixel values

Figure 4(a) ndash intensity values in the range 0 to 15 x 106

Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

Digital Image Processing

Week 1

Power-Law (Gamma) Transformations

- positive constants( ) ( ( ) )s T r c r c s c r

Plots of gamma transformation for different values of γ (c=1)

Digital Image Processing

Week 1

Power-law curves with 1 map a narrow range of dark input values into a wider range

of output values with the opposite being true for higher values of input values The

curves with 1 have the opposite effect of those generated with values of 1

1c - identity transformation

A variety of devices used for image capture printing and display respond according to a

power law The process used to correct these power-law response phenomena is called

gamma correction

Digital Image Processing

Week 1

a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

Digital Image Processing

Week 1

Piecewise-Linear Transformations Functions

Contrast stretching

- a process that expands the range of intensity levels in an image so it spans the full

intensity range of the recording tool or display device

a b c d Fig5

Digital Image Processing

Week 1

11

1

2 1 1 21 2

2 1 2 1

22

2

[0 ]

( ) ( )( ) [ ]( ) ( )

( 1 ) [ 1]( 1 )

s r r rrs r r s r rT r r r r

r r r rs L r r r L

L r

Digital Image Processing

Week 1

1 1 2 2r s r s identity transformation (no change)

1 2 1 2 0 1r r s s L thresholding function

Figure 5(b) shows an 8-bit image with low contrast

Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

in the image respectively Thus the transformation function stretched the levels linearly

from their original range to the full range [0 L-1]

Figure 5(d) - the thresholding function was used with 1 1 0r s m

2 2 1r s m L where m is the mean gray level in the image

The original image on which these results are based is a scanning electron microscope

image of pollen magnified approximately 700 times

Digital Image Processing

Week 1

Intensity-level slicing

- highlighting a specific range of intensities in an image

There are two approaches for intensity-level slicing

1 display in one value (white for example) all the values in the range of interest and in

another (say black) all other intensities (Figure 311 (a))

2 brighten (or darken) the desired range of intensities but leaves unchanged all other

intensities in the image (Figure 311 (b))

Digital Image Processing

Week 1

Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

highlight the major blood vessels that appear brighter as a result of injecting a contrast

medium Figure 6(middle) shows the result of applying the first technique for a band near

the top of the scale of intensities This type of enhancement produces a binary image

Highlights intensity range [A B] and reduces all other intensities to a lower level

Highlights range [A B] and preserves all other intensities

Digital Image Processing

Week 1

which is useful for studying the shape of the flow of the contrast substance (to detect

blockageshellip)

In Figure 312(right) the second technique was used a band of intensities in the mid-gray

image around the mean intensity was set to black the other intensities remain unchanged

Fig 6 - Aortic angiogram and intensity sliced versions

Digital Image Processing

Week 1

Bit-plane slicing

For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

This technique highlights the contribution made to the whole image appearances by each

of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

(plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

Digital Image Processing

Week 1

Digital Image Processing

Week 1

The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

  • DIP 1 2017
  • DIP 02 (2017)

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Evaluation

    1(base) + 3 (course test) + 3 (lab activity)+ 3 (article presentation)

    gt 45

    Weeks 1 ndash 7 ndash coursesWeek 8 ndash (course) testWeeks 9 ndash 12 ndash labWeeks 13 ndash 14 ndash article lab evaluationWeek 15 or 16 ndash article presentation

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Bibliography

    bull RC Gonzales RE Woods Digital Image Processing Prentice Hall2008 3rd ed

    bull RC Gonzales RE Woods SL Eddins Digital Image Processing Using MATLAB Prentice Hall 2003

    bull httpwwwimageprocessingplacecom

    bullM Petrou C Petrou Image Processing the fundamentals John Wiley 2010 2nd ed

    bull W Burger MJ Burge Digital Image Processing An Algorithmic Introduction Using Java Springer 2008

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    bull Image Processing Toolbox (httpwwwmathworkscomproductsimage)

    bull C Solomon T Breckon Fundamentals of Digital Image Processing A Practical Approach with Examples in Matlab Wiley-Blackwell 2011

    bullWK Pratt Digital Image Processing Wiley-Interscience 2007

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Meet LenaThe First Lady of the Internet

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Lenna Soderberg (Sjoumloumlblom) and Jeff Seideman taken in May 1997Imaging Science amp Technology Conference

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    What is Digital Image Processing

    f(xy) = intensity gray level of the image at spatial point (xy)

    x y f(xy) ndash finite discrete quantities -gt digital image

    Digital Image Processing = processing digital images by means of a digital computer

    A digital image is composed of a finite number of elements (location value of intensity)

    These elements are called picture elements image elements pels pixels

    ( )i j ijx y f

    3 f D

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Image processing is not limited to the visual band of the electromagnetic (EM) spectrum

    Image processing gamma to radio waves ultrasound electron microscopy computer-generated images

    image processing image analysis computer vision

    Image processing = discipline in which both the input and the output of a process are images

    Computer Vision = use computer to emulate human vision (AI) ndashlearning making inferences and take actions based on visual inputs

    Image analysis (image understanding) = segmentation partitioning images into regions or objects

    (link between image processing and image analysis)

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Distinction between image processing image analysis computer vision

    low-level mid-level high-level processes

    Low-level processes image preprocessing to reduce noise contrast enhancement image sharpening both input and output are images

    Mid-level processes segmentation partitioning images into regions or objects description of the objects for computer processing classificationrecognition of individual objects inputs are generally images outputs are attributes extracted from the input image (eg edges contours identity of individual objects)

    High-level processes ldquomaking senserdquo of a set of recognized objects performing the cognitive functions associated with vision

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Digital Image Processing (Gonzalez + Woods) =

    processes whose inputs and outputs are images +

    processes that extract attributes from images recognition of individual objects

    (low- and mid-level processes)

    Example

    automated analysis of text = acquiring an image containing text preprocessing the image (enhancement sharpening) extracting (segmenting) the individual characaters describing the characters in a form suitable for computer processing recognition of individual characters

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    The Origins of DIP

    Newspaper industry pictures were sent by submarine cable between London and New York

    Before Bartlane cable picture transmission system (early 1920s) ndash 1 week

    With Bartlane system less than 3 hours

    Specialized printing equipment coded pictures for cable transmission and reconstructed them at the receiving end (1920s -5 distict levels of gray 1929 ndash 15 levels)

    This example is not DIP the computer is not involved

    DIP is linked and devolps in the same rhythm as digital computers (data storage display and transmission)

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    A digital pictureproduced in 1921from a coded tapeby a telegraph printer withspecial type faces(McFarlane)

    A digital picturemade in 1922from a tapepunched after the signals hadcrossed the Atlantic twice(McFarlane)

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    1964 Jet Propulsion Laboratory (Pasadena California) processed pictures of the moon transmitted by Ranger 7 (corrected image distortions)

    The first picture of the moon by a US spacecraft Ranger 7 took this image July 31 1964 about 17 minutes before impacting the lunar surface(Courtesy of NASA)

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

    1970s ndash invention of CAT (computerized axial tomography)

    CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    loz geographers use DIP to study pollution patterns from aerial and satellite imagery

    loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

    loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

    loz astronomy biology nuclear medicine law enforcement industry

    DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

    loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Examples of Fields that Use DIP

    Images can be classified according to their sources (visual X-ray hellip)

    Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Electromagnetic waves can be thought as propagating sinusoidal

    waves of different wavelength or as a stream of massless particles

    each moving in a wavelike pattern with the speed of light Each

    massless particle contains a certain amount (bundle) of energy Each

    bundle of energy is called a photon If spectral bands are grouped

    according to energy per photon we obtain the spectrum shown in the

    image above ranging from gamma-rays (highest energy) to radio

    waves (lowest energy)

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Gamma-Ray Imaging

    Nuclear medicine astronomical observations

    Nuclear medicine

    the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

    Images are produced from the emissions collected by gamma-ray detectors

    Images of this sort are used to locate sites of bone pathology (infections tumors)

    PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Examples of gamma-ray imaging

    Bone scan PET image

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    X-ray imaging

    Medical diagnosticindustry astronomy

    A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

    The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Angiography = contrast-enhancement radiography

    Angiograms = images of blood vessels

    A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

    X-rays are used in CAT (computerized axial tomography)

    X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

    Industrial CAT scans are useful when the parts can be penetreted by X-rays

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Examples of X-ray imaging

    Chest X-rayAortic angiogram

    Head CT Cygnus LoopCircuit boards

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Imaging in the Ultraviolet Band

    Litography industrial inspection microscopy biological imaging astronomical observations

    Ultraviolet light is used in fluorescence microscopy

    Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

    other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

    and then it separates the much weaker radiating fluorescent light from the brighter excitation light

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Imaging in the Visible and Infrared Bands

    Light microscopy astronomy remote sensing industry law enforcement

    LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

    Weather observations and prediction produce major applications of multispectral image from satellites

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Satellite images of Washington DC area in spectral bands of the Table 1

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Examples of light microscopy

    Taxol (anticancer agent)magnified 250X

    Cholesterol(40X)

    Microprocessor(60X)

    Nickel oxidethin film(600X)

    Surface of audio CD(1750X)

    Organicsuperconductor(450X)

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Automated visual inspection of manufactured goods

    a bc de f

    a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Imaging in the Microwave Band

    The dominant aplication of imaging in the microwave band ndash radar

    bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

    bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

    bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

    An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Spaceborne radar image of mountains in southeast Tibet

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Imaging in the Radio Band

    medicine astronomy

    MRI = Magnetic Resonance Imaging

    This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

    Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

    The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    MRI images of a human knee (left) and spine (right)

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Images of the Crab Pulsar covering the electromagnetic spectrum

    Gamma X-ray Optical Infrared Radio

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Other Imaging Modalities

    acoustic imaging electron microscopy synthetic (computer-generated) imaging

    Imaging using sound geological explorations industry medicine

    Mineral and oil exploration

    For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Biometry - iris

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Biometry - fingerprint

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Face detection and recognition

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Gender identification

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Image morphing

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Fundamental Steps in DIP

    methods whose input and output are images

    methods whose inputs are images but whose outputs are attributes extracted from those images

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Outputs are images

    bull image acquisition

    bull image filtering and enhancement

    bull image restoration

    bull color image processing

    bull wavelets and multiresolution processing

    bull compression

    bull morphological processing

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Outputs are attributes

    bull morphological processing

    bull segmentation

    bull representation and description

    bull object recognition

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Image acquisition - may involve preprocessing such as scaling

    Image enhancement

    bull manipulating an image so that the result is more suitable than the original for a specific operation

    bull enhancement is problem oriented

    bull there is no general sbquotheoryrsquo of image enhancement

    bull enhancement use subjective methods for image emprovement

    bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Image restoration

    bull improving the appearance of an image

    bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

    Color image processing

    bull fundamental concept in color models

    bull basic color processing in a digital domain

    Wavelets and multiresolution processing

    representing images in various degree of resolution

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Compression

    reducing the storage required to save an image or the bandwidth required to transmit it

    Morphological processing

    bull tools for extracting image components that are useful in the representation and description of shape

    bull a transition from processes that output images to processes that outputimage attributes

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Segmentation

    bull partitioning an image into its constituents parts or objects

    bull autonomous segmentation is one of the most difficult tasks of DIP

    bull the more accurate the segmentation the more likley recognition is to succeed

    Representation and description (almost always follows segmentation)

    bull segmentation produces either the boundary of a region or all the poits in the region itself

    bull converting the data produced by segmentation to a form suitable for computer processing

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    bull boundary representation the focus is on external shape characteristics such as corners or inflections

    bull complete region the focus is on internal properties such as texture or skeletal shape

    bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

    Object recognition

    the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

    Knowledge database

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Simplified diagramof a cross sectionof the human eye

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

    The cornea is a tough transparent tissue that covers the anterior surface of the eye

    Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

    The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

    The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

    Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

    Fovea = the place where the image of the object of interest falls on

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

    Blind spot region without receptors

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Image formation in the eye

    Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

    Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

    distance between lens and retina along visual axix = 17 mm

    range of focal length = 14 mm to 17 mm

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Optical illusions

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

    gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

    quantities that describe the quality of a chromatic light source radiance

    the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

    measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

    brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    the physical meaning is determined by the source of the image

    ( )f D f x y

    Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

    f(xy) ndash characterized by two components

    i(xy) = illumination component the amount of source illumination incident on the scene being viewed

    r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

    ( ) ( ) ( )

    0 ( ) 0 ( ) 1

    f x y i x y r x y

    i x y r x y

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    r(xy)=0 - total absorption r(xy)=1 - total reflectance

    i(xy) ndash determined by the illumination source

    r(xy) ndash determined by the characteristics of the imaged objects

    is called gray (or intensity) scale

    In practice

    min 0 0 max min min min max max max( ) L l f x y L L i r L i r

    indoor values without additional illuminationmin max10 1000L L

    black whitemin max0 1 0 1 0 1L L L L l l L

    min maxL L

    Digital Image ProcessingDigital Image Processing

    Week 1Week 1

    Digital Image Processing

    Week 1

    Image Sampling and Quantization

    - the output of the sensors is a continuous voltage waveform related to the sensed

    scene

    converting a continuous image f to digital form

    - digitizing (x y) is called sampling

    - digitizing f(x y) is called quantization

    Digital Image Processing

    Week 1

    Digital Image Processing

    Week 1

    Continuous image projected onto a sensor array Result of image sampling and quantization

    Digital Image Processing

    Week 1

    Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

    (00) (01) (0 1)(10) (11) (1 1)

    ( )

    ( 10) ( 11) ( 1 1)

    f f f Nf f f N

    f x y

    f M f M f M N

    image element pixel

    00 01 0 1

    10 11 1 1

    10 11 1 1

    ( ) ( )

    N

    i jN M N

    i j

    M M M N

    a a aa f x i y j f i ja a a

    Aa

    a a a

    f(00) ndash the upper left corner of the image

    Digital Image Processing

    Week 1

    M N ge 0 L=2k

    [0 1]i j i ja a L

    Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

    Digital Image Processing

    Week 1

    Digital Image Processing

    Week 1

    Number of bits required to store a digitized image

    for 2 b M N k M N b N k

    When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

    Digital Image Processing

    Week 1

    Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

    Measures line pairs per unit distance dots (pixels) per unit distance

    Image resolution = the largest number of discernible line pairs per unit distance

    (eg 100 line pairs per mm)

    Dots per unit distance are commonly used in printing and publishing

    In US the measure is expressed in dots per inch (dpi)

    (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

    Intensity resolution ndash the smallest discernible change in intensity level

    The number of intensity levels (L) is determined by hardware considerations

    L=2k ndash most common k = 8

    Intensity resolution in practice is given by k (number of bits used to quantize intensity)

    Digital Image Processing

    Week 1

    Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

    150 dpi (lower left) 72 dpi (lower right)

    Digital Image Processing

    Week 1

    Reducing the number of gray levels 256 128 64 32

    Digital Image Processing

    Week 1

    Reducing the number of gray levels 16 8 4 2

    Digital Image Processing

    Week 1

    Image Interpolation - used in zooming shrinking rotating and geometric corrections

    Shrinking zooming ndash image resizing ndash image resampling methods

    Interpolation is the process of using known data to estimate values at unknown locations

    Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

    750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

    same spacing as the original and then shrink it so that it fits exactly over the original

    image The pixel spacing in the 750 times 750 grid will be less than in the original image

    Problem assignment of intensity-level in the new 750 times 750 grid

    Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

    intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

    This technique has the tendency to produce undesirable effects like severe distortion of

    straight edges

    Digital Image Processing

    Week 1

    Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

    where the four coefficients are determined from the 4 equations in 4 unknowns that can

    be written using the 4 nearest neighbors of point (x y)

    Bilinear interpolation gives much better results than nearest neighbor interpolation with a

    modest increase in computational effort

    Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

    nearest neighbors of the point 3 3

    0 0

    ( ) i ji j

    i jv x y c x y

    The coefficients cij are obtained solving a 16x16 linear system

    intensity levels of the 16 nearest neighbors of 3 3

    0 0

    ( )i ji j

    i jc x y x y

    Digital Image Processing

    Week 1

    Generally bicubic interpolation does a better job of preserving fine detail than the

    bilinear technique Bicubic interpolation is the standard used in commercial image editing

    programs such as Adobe Photoshop and Corel Photopaint

    Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

    the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

    then zooming the reduced image back to its original size To generate Fig 1(d) nearest

    neighbor interpolation was used (both for shrinking and zooming)

    Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

    interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

    from 1250 dpi to 150 dpi (instead of 72 dpi)

    Digital Image Processing

    Week 1

    Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

    Digital Image Processing

    Week 1

    Neighbors of a Pixel

    A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

    This set of pixels called the 4-neighbors of p denoted by N4 (p)

    The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

    and are denoted ND(p)

    The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

    N8 (p)

    If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

    fall outside the image

    Digital Image Processing

    Week 1

    Adjacency Connectivity Regions Boundaries

    Denote by V the set of intensity levels used to define adjacency

    - in a binary image V 01 (V=0 V=1)

    - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

    We consider 3 types of adjacency

    (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

    (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

    (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

    m-adjacent if

    4( )q N p or

    ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

    Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

    ambiguities that often arise when 8-adjacency is used Consider the example

    Digital Image Processing

    Week 1

    binary image

    0 1 1 0 1 1 0 1 1

    1 0 1 0 0 1 0 0 1 0

    0 0 1 0 0 1 0 0 1

    V

    The three pixels at the top (first line) in the above example show multiple (ambiguous)

    8-adjacency as indicated by the dashed lines This ambiguity is removed by using

    m-adjacency

    Digital Image Processing

    Week 1

    A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

    is a sequence of distinct pixels with coordinates

    and are adjacent 0 0 1 1

    1 1

    ( ) ( ) ( ) ( ) ( )( ) ( ) 12

    n n

    i i i i

    x y x y x y x y s tx y x y i n

    The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

    Depending on the type of adjacency considered the paths are 4- 8- or m-paths

    Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

    in S if there exists a path between them consisting only of pixels from S

    S is a connected set if there is a path in S between any 2 pixels in S

    Let R be a subset of pixels in an image R is a region of the image if R is a connected set

    Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

    that are not adjacent are said to be disjoint When referring to regions only 4- and

    8-adjacency are considered

    Digital Image Processing

    Week 1

    Suppose that an image contains K disjoint regions 1 kR k K none of which

    touches the image border

    the complement of 1

    ( )K

    cu k u u

    k

    R R R R

    We call all the points in Ru the foreground of the image and the points in ( )cuR the

    background of the image

    The boundary (border or contour) of a region R is the set of points that are adjacent to

    points in the complement of R (R)c The border of an image is the set of pixels in the

    region that have at least one background neighbor This definition is referred to as the

    inner border to distinguish it from the notion of outer border which is the corresponding

    border in the background

    Digital Image Processing

    Week 1

    Distance measures

    For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

    function or metric if

    (a) D(p q) ge 0 D(p q) = 0 iff p=q

    (b) D(p q) = D(q p)

    (c) D(p z) le D(p q) + D(q z)

    The Euclidean distance between p and q is defined as 1

    2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

    The pixels q for which ( )eD p q r are the points contained in a disk of radius r

    centered at (x y)

    Digital Image Processing

    Week 1

    The D4 distance (also called city-block distance) between p and q is defined as

    4( ) | | | |D p q x s y t

    The pixels q for which 4( )D p q r form a diamond centered at (xy)

    4

    22 1 2

    2 2 1 0 1 22 1 2

    2

    D

    The pixels with D4 = 1 are the 4-neighbors of (x y)

    The D8 distance (called the chessboard distance) between p and q is defined as

    8( ) max| | | |D p q x s y t

    The pixels q for which 8( )D p q r form a square centered at (x y)

    Digital Image Processing

    Week 1

    8

    2 2 2 2 22 1 1 1 2

    2 2 1 0 1 22 1 1 1 22 2 2 2 2

    D

    The pixels with D8 = 1 are the 8-neighbors of (x y)

    D4 and D8 distances are independent of any paths that might exist between p and q

    because these distances involve only the coordinates of the point

    Digital Image Processing

    Week 1

    Array versus Matrix Operations

    An array operation involving one or more images is carried out on a pixel-by-pixel basis

    11 12 11 12

    21 22 21 22

    a a b ba a b b

    Array product

    11 12 11 12 11 11 12 12

    21 22 21 22 21 21 22 21

    a a b b a b a ba a b b a b a b

    Matrix product

    11 12 11 12 11 11 12 21 11 12 12 21

    21 22 21 22 21 11 22 21 21 12 22 22

    a a b b a b a b a b a ba a b b a b a b a b a b

    We assume array operations unless stated otherwise

    Digital Image Processing

    Week 1

    Linear versus Nonlinear Operations

    One of the most important classifications of image-processing methods is whether it is

    linear or nonlinear

    ( ) ( )H f x y g x y

    H is said to be a linear operator if

    images1 2 1 2

    1 2

    ( ) ( ) ( ) ( )

    H a f x y b f x y a H f x y b H f x y

    a b f f

    Example of nonlinear operator

    the maximum value of the pixels of image max ( )H f f x y f

    1 2

    0 2 6 5 1 1

    2 3 4 7f f a b

    Digital Image Processing

    Week 1

    1 2

    0 2 6 5 6 3max max 1 ( 1) max 2

    2 3 4 7 2 4a f b f

    0 2 6 51 max ( 1) max 3 ( 1)7 4

    2 3 4 7

    Arithmetic Operations in Image Processing

    Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

    f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

    For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

    value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

    The two random variables are uncorrelated when their covariance is 0

    Digital Image Processing

    Week 1

    Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

    used in image enhancement)

    1

    1( ) ( )K

    ii

    g x y g x yK

    If the noise satisfies the properties stated above we have

    2 2( ) ( )

    1( ) ( ) g x y x yE g x y f x yK

    ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

    and g respectively The standard deviation (square root of the variance) at any point in

    the average image is

    ( ) ( )1

    g x y x yK

    Digital Image Processing

    Week 1

    As K increases the variability (as measured by the variance or the standard deviation) of

    the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

    means that ( )g x y approaches f(x y) as the number of noisy images used in the

    averaging process increases

    An important application of image averaging is in the field of astronomy where imaging

    under very low light levels frequently causes sensor noise to render single images

    virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

    was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

    64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

    images respectively

    Digital Image Processing

    Week 1

    Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

    100 noisy images

    a b c d e f

    Digital Image Processing

    Week 1

    A frequent application of image subtraction is in the enhancement of differences between

    images

    (a) (b) (c)

    Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

    significant bit of each pixel (c) the difference between the two images

    Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

    Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

    images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

    difference between images (a) and (b)

    Digital Image Processing

    Week 1

    Mask mode radiography ( ) ( ) ( )g x y f x y h x y

    h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

    intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

    source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

    bloodstream taking a series of images called live images (denoted f(x y)) of the same

    anatomical region as h(x y) and subtracting the mask from the series of incoming live

    images after injection of the contrast medium

    In g(x y) we can find the differences between h and f as enhanced detail

    Images being captured at TV rates we obtain a movie showing how the contrast medium

    propagates through the various arteries in the area being observed

    Digital Image Processing

    Week 1

    a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

    Digital Image Processing

    Week 1

    An important application of image multiplication (and division) is shading correction

    Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

    f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

    When the shading function is known

    ( )( )( )

    g x yf x yh x y

    h(x y) is unknown but we have access to the imaging system we can obtain an

    approximation to the shading function by imaging a target of constant intensity When the

    sensor is not available often the shading pattern can be estimated from the image

    Digital Image Processing

    Week 1

    (a) (b) (c)

    Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

    Digital Image Processing

    Week 1

    Another use of image multiplication is in masking also called region of interest (ROI)

    operations The process consists of multiplying a given image by a mask image that has

    1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

    image and the shape of the ROI can be arbitrary but usually is a rectangular shape

    (a) (b) (c)

    Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

    Digital Image Processing

    Week 1

    In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

    min( )mf f f

    0 ( 255)max( )

    ms

    m

    ff K K K

    f

    Digital Image Processing

    Week 1

    Spatial Operations

    - are performed directly on the pixels of a given image

    There are three categories of spatial operations

    single-pixel operations

    neighborhood operations

    geometric spatial transformations

    Single-pixel operations

    - change the values of intensity for the individual pixels ( )s T z

    where z is the intensity of a pixel in the original image and s is the intensity of the

    corresponding pixel in the processed image

    Digital Image Processing

    Week 1

    Neighborhood operations

    Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

    in an image f Neighborhood processing generates new intensity level at point (x y)

    based on the values of the intensities of the points in Sxy For example if Sxy is a

    rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

    intensity by computing the average value of the pixels in Sxy

    ( )

    1( ) ( )xyr c S

    g x y f r cm n

    The net effect is to perform local blurring in the original image This type of process is

    used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

    largest region of an image

    Digital Image Processing

    Week 1

    Geometric spatial transformations and image registration

    - modify the spatial relationship between pixels in an image

    - these transformations are often called rubber-sheet transformations (analogous to

    printing an image on a sheet of rubber and then stretching the sheet according to a

    predefined set of rules

    A geometric transformation consists of 2 basic operations

    1 a spatial transformation of coordinates

    2 intensity interpolation that assign intensity values to the spatial transformed

    pixels

    The coordinate system transformation ( ) [( )]x y T v w

    (v w) ndash pixel coordinates in the original image

    (x y) ndash pixel coordinates in the transformed image

    Digital Image Processing

    Week 1

    [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

    Affine transform

    11 1211 21 31

    21 2212 22 33

    31 32

    0[ 1] [ 1] [ 1] 0

    1

    t tx t v t w t

    x y v w T v w t ty t v t w t

    t t

    (AT)

    This transform can scale rotate translate or shear a set of coordinate points depending

    on the elements of the matrix T If we want to resize an image rotate it and move the

    result to some location we simply form a 3x3 matrix equal to the matrix product of the

    scaling rotation and translation matrices from Table 1

    Digital Image Processing

    Week 1

    Affine transformations

    Digital Image Processing

    Week 1

    The preceding transformations relocate pixels on an image to new locations To complete

    the process we have to assign intensity values to those locations This task is done by

    using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

    In practice we can use equation (AT) in two basic ways

    forward mapping scan the pixels of the input image (v w) compute the new spatial

    location (x y) of the corresponding pixel in the new image using (AT) directly

    Problems

    - intensity assignment when 2 or more pixels in the original image are transformed to

    the same location in the output image

    - some output locations have no correspondent in the original image (no intensity

    assignment)

    Digital Image Processing

    Week 1

    inverse mapping scans the output pixel locations and at each location (x y)

    computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

    It then interpolates among the nearest input pixels to determine the intensity of the output

    pixel value

    Inverse mappings are more efficient to implement than forward mappings and are used in

    numerous commercial implementations of spatial transformations (MATLAB for ex)

    Digital Image Processing

    Week 1

    Digital Image Processing

    Week 1

    Image registration ndash align two or more images of the same scene

    In image registration we have available the input and output images but the specific

    transformation that produced the output image from the input is generally unknown

    The problem is to estimate the transformation function and then use it to register the two

    images

    - it may be of interest to align (register) two or more image taken at approximately the

    same time but using different imaging systems (MRI scanner and a PET scanner)

    - align images of a given location taken by the same instrument at different moments

    of time (satellite images)

    Solving the problem using tie points (also called control points) which are

    corresponding points whose locations are known precisely in the input and reference

    image

    Digital Image Processing

    Week 1

    How to select tie points

    - interactively selecting them

    - use of algorithms that try to detect these points

    - some imaging systems have physical artifacts (small metallic objects) embedded in

    the imaging sensors These objects produce a set of known points (called reseau

    marks) directly on all images captured by the system which can be used as guides

    for establishing tie points

    The problem of estimating the transformation is one of modeling Suppose we have a set

    of 4 tie points both on the input image and the reference image A simple model based on

    a bilinear approximation is given by

    1 2 3 4

    5 6 7 8

    x c v c w c v w cy c v c w c v w c

    (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

    Digital Image Processing

    Week 1

    When 4 tie points are insufficient to obtain satisfactory registration an approach used

    frequently is to select a larger number of tie points and using this new set of tie points

    subdivide the image in rectangular regions marked by groups of 4 tie points On the

    subregions marked by 4 tie points we applied the transformation model described above

    The number of tie points and the sophistication of the model required to solve the register

    problem depend on the severity of the geometrical distortion

    Digital Image Processing

    Week 1

    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

    Digital Image Processing

    Week 1

    Probabilistic Methods

    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

    p(zk) = the probability that the intensity level zk occurs in the given image

    ( ) kk

    np zM N

    nk = the number of times that intensity zk occurs in the image (MN is the total number of

    pixels in the image) 1

    0( ) 1

    L

    kk

    p z

    The mean (average) intensity of an image is given by 1

    0( )

    L

    k kk

    m z p z

    Digital Image Processing

    Week 1

    The variance of the intensities is 1

    2 2

    0( ) ( )

    L

    k kk

    z m p z

    The variance is a measure of the spread of the values of z about the mean so it is a

    measure of image contrast Usually for measuring image contrast the standard deviation

    ( ) is used

    The n-th moment of a random variable z about the mean is defined as 1

    0( ) ( ) ( )

    Ln

    n k kk

    z z m p z

    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

    3( ) 0z the intensities are biased to values higher than the mean

    ( 3( ) 0z the intensities are biased to values lower than the mean

    Digital Image Processing

    Week 1

    3( ) 0z the intensities are distributed approximately equally on both side of the

    mean

    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

    Figure 1(a) ndash standard deviation 143 (variance = 2045)

    Figure 1(b) ndash standard deviation 316 (variance = 9986)

    Figure 1(c) ndash standard deviation 492 (variance = 24206)

    Digital Image Processing

    Week 1

    Intensity Transformations and Spatial Filtering

    ( ) ( )g x y T f x y

    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

    neighborhood of (x y)

    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

    and much smaller in size than the image

    Digital Image Processing

    Week 1

    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

    called spatial filter (spatial mask kernel template or window)

    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

    ( )s T r

    s and r are denoting respectively the intensity of g and f at (x y)

    Figure 2 left - T produces an output image of higher contrast than the original by

    darkening the intensity levels below k and brightening the levels above k ndash this technique

    is called contrast stretching

    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

    Digital Image Processing

    Week 1

    Figure 2 right - T produces a binary output image A mapping of this form is called

    thresholding function

    Some Basic Intensity Transformation Functions

    Image Negatives

    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

    - equivalent of a photographic negative

    - technique suited for enhancing white or gray detail embedded in dark regions of an

    image

    Digital Image Processing

    Week 1

    Original Negative image

    Digital Image Processing

    Week 1

    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

    Some basic intensity transformation functions

    Digital Image Processing

    Week 1

    This transformation maps a narrow range of low intensity values in the input into a wider

    range An operator of this type is used to expand the values of dark pixels in an image

    while compressing the higher-level values The opposite is true for the inverse log

    transformation The log functions compress the dynamic range of images with large

    variations in pixel values

    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

    Digital Image Processing

    Week 1

    Power-Law (Gamma) Transformations

    - positive constants( ) ( ( ) )s T r c r c s c r

    Plots of gamma transformation for different values of γ (c=1)

    Digital Image Processing

    Week 1

    Power-law curves with 1 map a narrow range of dark input values into a wider range

    of output values with the opposite being true for higher values of input values The

    curves with 1 have the opposite effect of those generated with values of 1

    1c - identity transformation

    A variety of devices used for image capture printing and display respond according to a

    power law The process used to correct these power-law response phenomena is called

    gamma correction

    Digital Image Processing

    Week 1

    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

    Digital Image Processing

    Week 1

    Piecewise-Linear Transformations Functions

    Contrast stretching

    - a process that expands the range of intensity levels in an image so it spans the full

    intensity range of the recording tool or display device

    a b c d Fig5

    Digital Image Processing

    Week 1

    11

    1

    2 1 1 21 2

    2 1 2 1

    22

    2

    [0 ]

    ( ) ( )( ) [ ]( ) ( )

    ( 1 ) [ 1]( 1 )

    s r r rrs r r s r rT r r r r

    r r r rs L r r r L

    L r

    Digital Image Processing

    Week 1

    1 1 2 2r s r s identity transformation (no change)

    1 2 1 2 0 1r r s s L thresholding function

    Figure 5(b) shows an 8-bit image with low contrast

    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

    in the image respectively Thus the transformation function stretched the levels linearly

    from their original range to the full range [0 L-1]

    Figure 5(d) - the thresholding function was used with 1 1 0r s m

    2 2 1r s m L where m is the mean gray level in the image

    The original image on which these results are based is a scanning electron microscope

    image of pollen magnified approximately 700 times

    Digital Image Processing

    Week 1

    Intensity-level slicing

    - highlighting a specific range of intensities in an image

    There are two approaches for intensity-level slicing

    1 display in one value (white for example) all the values in the range of interest and in

    another (say black) all other intensities (Figure 311 (a))

    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

    intensities in the image (Figure 311 (b))

    Digital Image Processing

    Week 1

    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

    highlight the major blood vessels that appear brighter as a result of injecting a contrast

    medium Figure 6(middle) shows the result of applying the first technique for a band near

    the top of the scale of intensities This type of enhancement produces a binary image

    Highlights intensity range [A B] and reduces all other intensities to a lower level

    Highlights range [A B] and preserves all other intensities

    Digital Image Processing

    Week 1

    which is useful for studying the shape of the flow of the contrast substance (to detect

    blockageshellip)

    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

    image around the mean intensity was set to black the other intensities remain unchanged

    Fig 6 - Aortic angiogram and intensity sliced versions

    Digital Image Processing

    Week 1

    Bit-plane slicing

    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

    This technique highlights the contribution made to the whole image appearances by each

    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

    Digital Image Processing

    Week 1

    Digital Image Processing

    Week 1

    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

    • DIP 1 2017
    • DIP 02 (2017)

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Bibliography

      bull RC Gonzales RE Woods Digital Image Processing Prentice Hall2008 3rd ed

      bull RC Gonzales RE Woods SL Eddins Digital Image Processing Using MATLAB Prentice Hall 2003

      bull httpwwwimageprocessingplacecom

      bullM Petrou C Petrou Image Processing the fundamentals John Wiley 2010 2nd ed

      bull W Burger MJ Burge Digital Image Processing An Algorithmic Introduction Using Java Springer 2008

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      bull Image Processing Toolbox (httpwwwmathworkscomproductsimage)

      bull C Solomon T Breckon Fundamentals of Digital Image Processing A Practical Approach with Examples in Matlab Wiley-Blackwell 2011

      bullWK Pratt Digital Image Processing Wiley-Interscience 2007

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Meet LenaThe First Lady of the Internet

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Lenna Soderberg (Sjoumloumlblom) and Jeff Seideman taken in May 1997Imaging Science amp Technology Conference

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      What is Digital Image Processing

      f(xy) = intensity gray level of the image at spatial point (xy)

      x y f(xy) ndash finite discrete quantities -gt digital image

      Digital Image Processing = processing digital images by means of a digital computer

      A digital image is composed of a finite number of elements (location value of intensity)

      These elements are called picture elements image elements pels pixels

      ( )i j ijx y f

      3 f D

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Image processing is not limited to the visual band of the electromagnetic (EM) spectrum

      Image processing gamma to radio waves ultrasound electron microscopy computer-generated images

      image processing image analysis computer vision

      Image processing = discipline in which both the input and the output of a process are images

      Computer Vision = use computer to emulate human vision (AI) ndashlearning making inferences and take actions based on visual inputs

      Image analysis (image understanding) = segmentation partitioning images into regions or objects

      (link between image processing and image analysis)

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Distinction between image processing image analysis computer vision

      low-level mid-level high-level processes

      Low-level processes image preprocessing to reduce noise contrast enhancement image sharpening both input and output are images

      Mid-level processes segmentation partitioning images into regions or objects description of the objects for computer processing classificationrecognition of individual objects inputs are generally images outputs are attributes extracted from the input image (eg edges contours identity of individual objects)

      High-level processes ldquomaking senserdquo of a set of recognized objects performing the cognitive functions associated with vision

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Digital Image Processing (Gonzalez + Woods) =

      processes whose inputs and outputs are images +

      processes that extract attributes from images recognition of individual objects

      (low- and mid-level processes)

      Example

      automated analysis of text = acquiring an image containing text preprocessing the image (enhancement sharpening) extracting (segmenting) the individual characaters describing the characters in a form suitable for computer processing recognition of individual characters

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      The Origins of DIP

      Newspaper industry pictures were sent by submarine cable between London and New York

      Before Bartlane cable picture transmission system (early 1920s) ndash 1 week

      With Bartlane system less than 3 hours

      Specialized printing equipment coded pictures for cable transmission and reconstructed them at the receiving end (1920s -5 distict levels of gray 1929 ndash 15 levels)

      This example is not DIP the computer is not involved

      DIP is linked and devolps in the same rhythm as digital computers (data storage display and transmission)

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      A digital pictureproduced in 1921from a coded tapeby a telegraph printer withspecial type faces(McFarlane)

      A digital picturemade in 1922from a tapepunched after the signals hadcrossed the Atlantic twice(McFarlane)

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      1964 Jet Propulsion Laboratory (Pasadena California) processed pictures of the moon transmitted by Ranger 7 (corrected image distortions)

      The first picture of the moon by a US spacecraft Ranger 7 took this image July 31 1964 about 17 minutes before impacting the lunar surface(Courtesy of NASA)

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

      1970s ndash invention of CAT (computerized axial tomography)

      CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      loz geographers use DIP to study pollution patterns from aerial and satellite imagery

      loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

      loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

      loz astronomy biology nuclear medicine law enforcement industry

      DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

      loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Examples of Fields that Use DIP

      Images can be classified according to their sources (visual X-ray hellip)

      Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Electromagnetic waves can be thought as propagating sinusoidal

      waves of different wavelength or as a stream of massless particles

      each moving in a wavelike pattern with the speed of light Each

      massless particle contains a certain amount (bundle) of energy Each

      bundle of energy is called a photon If spectral bands are grouped

      according to energy per photon we obtain the spectrum shown in the

      image above ranging from gamma-rays (highest energy) to radio

      waves (lowest energy)

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Gamma-Ray Imaging

      Nuclear medicine astronomical observations

      Nuclear medicine

      the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

      Images are produced from the emissions collected by gamma-ray detectors

      Images of this sort are used to locate sites of bone pathology (infections tumors)

      PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Examples of gamma-ray imaging

      Bone scan PET image

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      X-ray imaging

      Medical diagnosticindustry astronomy

      A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

      The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Angiography = contrast-enhancement radiography

      Angiograms = images of blood vessels

      A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

      X-rays are used in CAT (computerized axial tomography)

      X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

      Industrial CAT scans are useful when the parts can be penetreted by X-rays

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Examples of X-ray imaging

      Chest X-rayAortic angiogram

      Head CT Cygnus LoopCircuit boards

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Imaging in the Ultraviolet Band

      Litography industrial inspection microscopy biological imaging astronomical observations

      Ultraviolet light is used in fluorescence microscopy

      Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

      other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

      and then it separates the much weaker radiating fluorescent light from the brighter excitation light

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Imaging in the Visible and Infrared Bands

      Light microscopy astronomy remote sensing industry law enforcement

      LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

      Weather observations and prediction produce major applications of multispectral image from satellites

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Satellite images of Washington DC area in spectral bands of the Table 1

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Examples of light microscopy

      Taxol (anticancer agent)magnified 250X

      Cholesterol(40X)

      Microprocessor(60X)

      Nickel oxidethin film(600X)

      Surface of audio CD(1750X)

      Organicsuperconductor(450X)

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Automated visual inspection of manufactured goods

      a bc de f

      a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Imaging in the Microwave Band

      The dominant aplication of imaging in the microwave band ndash radar

      bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

      bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

      bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

      An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Spaceborne radar image of mountains in southeast Tibet

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Imaging in the Radio Band

      medicine astronomy

      MRI = Magnetic Resonance Imaging

      This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

      Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

      The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      MRI images of a human knee (left) and spine (right)

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Images of the Crab Pulsar covering the electromagnetic spectrum

      Gamma X-ray Optical Infrared Radio

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Other Imaging Modalities

      acoustic imaging electron microscopy synthetic (computer-generated) imaging

      Imaging using sound geological explorations industry medicine

      Mineral and oil exploration

      For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Biometry - iris

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Biometry - fingerprint

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Face detection and recognition

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Gender identification

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Image morphing

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Fundamental Steps in DIP

      methods whose input and output are images

      methods whose inputs are images but whose outputs are attributes extracted from those images

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Outputs are images

      bull image acquisition

      bull image filtering and enhancement

      bull image restoration

      bull color image processing

      bull wavelets and multiresolution processing

      bull compression

      bull morphological processing

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Outputs are attributes

      bull morphological processing

      bull segmentation

      bull representation and description

      bull object recognition

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Image acquisition - may involve preprocessing such as scaling

      Image enhancement

      bull manipulating an image so that the result is more suitable than the original for a specific operation

      bull enhancement is problem oriented

      bull there is no general sbquotheoryrsquo of image enhancement

      bull enhancement use subjective methods for image emprovement

      bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Image restoration

      bull improving the appearance of an image

      bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

      Color image processing

      bull fundamental concept in color models

      bull basic color processing in a digital domain

      Wavelets and multiresolution processing

      representing images in various degree of resolution

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Compression

      reducing the storage required to save an image or the bandwidth required to transmit it

      Morphological processing

      bull tools for extracting image components that are useful in the representation and description of shape

      bull a transition from processes that output images to processes that outputimage attributes

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Segmentation

      bull partitioning an image into its constituents parts or objects

      bull autonomous segmentation is one of the most difficult tasks of DIP

      bull the more accurate the segmentation the more likley recognition is to succeed

      Representation and description (almost always follows segmentation)

      bull segmentation produces either the boundary of a region or all the poits in the region itself

      bull converting the data produced by segmentation to a form suitable for computer processing

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      bull boundary representation the focus is on external shape characteristics such as corners or inflections

      bull complete region the focus is on internal properties such as texture or skeletal shape

      bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

      Object recognition

      the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

      Knowledge database

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Simplified diagramof a cross sectionof the human eye

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

      The cornea is a tough transparent tissue that covers the anterior surface of the eye

      Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

      The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

      The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

      Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

      Fovea = the place where the image of the object of interest falls on

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

      Blind spot region without receptors

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Image formation in the eye

      Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

      Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

      distance between lens and retina along visual axix = 17 mm

      range of focal length = 14 mm to 17 mm

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Optical illusions

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

      gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

      quantities that describe the quality of a chromatic light source radiance

      the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

      measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

      brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      the physical meaning is determined by the source of the image

      ( )f D f x y

      Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

      f(xy) ndash characterized by two components

      i(xy) = illumination component the amount of source illumination incident on the scene being viewed

      r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

      ( ) ( ) ( )

      0 ( ) 0 ( ) 1

      f x y i x y r x y

      i x y r x y

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      r(xy)=0 - total absorption r(xy)=1 - total reflectance

      i(xy) ndash determined by the illumination source

      r(xy) ndash determined by the characteristics of the imaged objects

      is called gray (or intensity) scale

      In practice

      min 0 0 max min min min max max max( ) L l f x y L L i r L i r

      indoor values without additional illuminationmin max10 1000L L

      black whitemin max0 1 0 1 0 1L L L L l l L

      min maxL L

      Digital Image ProcessingDigital Image Processing

      Week 1Week 1

      Digital Image Processing

      Week 1

      Image Sampling and Quantization

      - the output of the sensors is a continuous voltage waveform related to the sensed

      scene

      converting a continuous image f to digital form

      - digitizing (x y) is called sampling

      - digitizing f(x y) is called quantization

      Digital Image Processing

      Week 1

      Digital Image Processing

      Week 1

      Continuous image projected onto a sensor array Result of image sampling and quantization

      Digital Image Processing

      Week 1

      Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

      (00) (01) (0 1)(10) (11) (1 1)

      ( )

      ( 10) ( 11) ( 1 1)

      f f f Nf f f N

      f x y

      f M f M f M N

      image element pixel

      00 01 0 1

      10 11 1 1

      10 11 1 1

      ( ) ( )

      N

      i jN M N

      i j

      M M M N

      a a aa f x i y j f i ja a a

      Aa

      a a a

      f(00) ndash the upper left corner of the image

      Digital Image Processing

      Week 1

      M N ge 0 L=2k

      [0 1]i j i ja a L

      Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

      Digital Image Processing

      Week 1

      Digital Image Processing

      Week 1

      Number of bits required to store a digitized image

      for 2 b M N k M N b N k

      When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

      Digital Image Processing

      Week 1

      Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

      Measures line pairs per unit distance dots (pixels) per unit distance

      Image resolution = the largest number of discernible line pairs per unit distance

      (eg 100 line pairs per mm)

      Dots per unit distance are commonly used in printing and publishing

      In US the measure is expressed in dots per inch (dpi)

      (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

      Intensity resolution ndash the smallest discernible change in intensity level

      The number of intensity levels (L) is determined by hardware considerations

      L=2k ndash most common k = 8

      Intensity resolution in practice is given by k (number of bits used to quantize intensity)

      Digital Image Processing

      Week 1

      Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

      150 dpi (lower left) 72 dpi (lower right)

      Digital Image Processing

      Week 1

      Reducing the number of gray levels 256 128 64 32

      Digital Image Processing

      Week 1

      Reducing the number of gray levels 16 8 4 2

      Digital Image Processing

      Week 1

      Image Interpolation - used in zooming shrinking rotating and geometric corrections

      Shrinking zooming ndash image resizing ndash image resampling methods

      Interpolation is the process of using known data to estimate values at unknown locations

      Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

      750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

      same spacing as the original and then shrink it so that it fits exactly over the original

      image The pixel spacing in the 750 times 750 grid will be less than in the original image

      Problem assignment of intensity-level in the new 750 times 750 grid

      Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

      intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

      This technique has the tendency to produce undesirable effects like severe distortion of

      straight edges

      Digital Image Processing

      Week 1

      Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

      where the four coefficients are determined from the 4 equations in 4 unknowns that can

      be written using the 4 nearest neighbors of point (x y)

      Bilinear interpolation gives much better results than nearest neighbor interpolation with a

      modest increase in computational effort

      Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

      nearest neighbors of the point 3 3

      0 0

      ( ) i ji j

      i jv x y c x y

      The coefficients cij are obtained solving a 16x16 linear system

      intensity levels of the 16 nearest neighbors of 3 3

      0 0

      ( )i ji j

      i jc x y x y

      Digital Image Processing

      Week 1

      Generally bicubic interpolation does a better job of preserving fine detail than the

      bilinear technique Bicubic interpolation is the standard used in commercial image editing

      programs such as Adobe Photoshop and Corel Photopaint

      Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

      the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

      then zooming the reduced image back to its original size To generate Fig 1(d) nearest

      neighbor interpolation was used (both for shrinking and zooming)

      Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

      interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

      from 1250 dpi to 150 dpi (instead of 72 dpi)

      Digital Image Processing

      Week 1

      Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

      Digital Image Processing

      Week 1

      Neighbors of a Pixel

      A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

      This set of pixels called the 4-neighbors of p denoted by N4 (p)

      The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

      and are denoted ND(p)

      The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

      N8 (p)

      If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

      fall outside the image

      Digital Image Processing

      Week 1

      Adjacency Connectivity Regions Boundaries

      Denote by V the set of intensity levels used to define adjacency

      - in a binary image V 01 (V=0 V=1)

      - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

      We consider 3 types of adjacency

      (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

      (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

      (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

      m-adjacent if

      4( )q N p or

      ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

      Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

      ambiguities that often arise when 8-adjacency is used Consider the example

      Digital Image Processing

      Week 1

      binary image

      0 1 1 0 1 1 0 1 1

      1 0 1 0 0 1 0 0 1 0

      0 0 1 0 0 1 0 0 1

      V

      The three pixels at the top (first line) in the above example show multiple (ambiguous)

      8-adjacency as indicated by the dashed lines This ambiguity is removed by using

      m-adjacency

      Digital Image Processing

      Week 1

      A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

      is a sequence of distinct pixels with coordinates

      and are adjacent 0 0 1 1

      1 1

      ( ) ( ) ( ) ( ) ( )( ) ( ) 12

      n n

      i i i i

      x y x y x y x y s tx y x y i n

      The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

      Depending on the type of adjacency considered the paths are 4- 8- or m-paths

      Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

      in S if there exists a path between them consisting only of pixels from S

      S is a connected set if there is a path in S between any 2 pixels in S

      Let R be a subset of pixels in an image R is a region of the image if R is a connected set

      Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

      that are not adjacent are said to be disjoint When referring to regions only 4- and

      8-adjacency are considered

      Digital Image Processing

      Week 1

      Suppose that an image contains K disjoint regions 1 kR k K none of which

      touches the image border

      the complement of 1

      ( )K

      cu k u u

      k

      R R R R

      We call all the points in Ru the foreground of the image and the points in ( )cuR the

      background of the image

      The boundary (border or contour) of a region R is the set of points that are adjacent to

      points in the complement of R (R)c The border of an image is the set of pixels in the

      region that have at least one background neighbor This definition is referred to as the

      inner border to distinguish it from the notion of outer border which is the corresponding

      border in the background

      Digital Image Processing

      Week 1

      Distance measures

      For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

      function or metric if

      (a) D(p q) ge 0 D(p q) = 0 iff p=q

      (b) D(p q) = D(q p)

      (c) D(p z) le D(p q) + D(q z)

      The Euclidean distance between p and q is defined as 1

      2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

      The pixels q for which ( )eD p q r are the points contained in a disk of radius r

      centered at (x y)

      Digital Image Processing

      Week 1

      The D4 distance (also called city-block distance) between p and q is defined as

      4( ) | | | |D p q x s y t

      The pixels q for which 4( )D p q r form a diamond centered at (xy)

      4

      22 1 2

      2 2 1 0 1 22 1 2

      2

      D

      The pixels with D4 = 1 are the 4-neighbors of (x y)

      The D8 distance (called the chessboard distance) between p and q is defined as

      8( ) max| | | |D p q x s y t

      The pixels q for which 8( )D p q r form a square centered at (x y)

      Digital Image Processing

      Week 1

      8

      2 2 2 2 22 1 1 1 2

      2 2 1 0 1 22 1 1 1 22 2 2 2 2

      D

      The pixels with D8 = 1 are the 8-neighbors of (x y)

      D4 and D8 distances are independent of any paths that might exist between p and q

      because these distances involve only the coordinates of the point

      Digital Image Processing

      Week 1

      Array versus Matrix Operations

      An array operation involving one or more images is carried out on a pixel-by-pixel basis

      11 12 11 12

      21 22 21 22

      a a b ba a b b

      Array product

      11 12 11 12 11 11 12 12

      21 22 21 22 21 21 22 21

      a a b b a b a ba a b b a b a b

      Matrix product

      11 12 11 12 11 11 12 21 11 12 12 21

      21 22 21 22 21 11 22 21 21 12 22 22

      a a b b a b a b a b a ba a b b a b a b a b a b

      We assume array operations unless stated otherwise

      Digital Image Processing

      Week 1

      Linear versus Nonlinear Operations

      One of the most important classifications of image-processing methods is whether it is

      linear or nonlinear

      ( ) ( )H f x y g x y

      H is said to be a linear operator if

      images1 2 1 2

      1 2

      ( ) ( ) ( ) ( )

      H a f x y b f x y a H f x y b H f x y

      a b f f

      Example of nonlinear operator

      the maximum value of the pixels of image max ( )H f f x y f

      1 2

      0 2 6 5 1 1

      2 3 4 7f f a b

      Digital Image Processing

      Week 1

      1 2

      0 2 6 5 6 3max max 1 ( 1) max 2

      2 3 4 7 2 4a f b f

      0 2 6 51 max ( 1) max 3 ( 1)7 4

      2 3 4 7

      Arithmetic Operations in Image Processing

      Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

      f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

      For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

      value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

      The two random variables are uncorrelated when their covariance is 0

      Digital Image Processing

      Week 1

      Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

      used in image enhancement)

      1

      1( ) ( )K

      ii

      g x y g x yK

      If the noise satisfies the properties stated above we have

      2 2( ) ( )

      1( ) ( ) g x y x yE g x y f x yK

      ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

      and g respectively The standard deviation (square root of the variance) at any point in

      the average image is

      ( ) ( )1

      g x y x yK

      Digital Image Processing

      Week 1

      As K increases the variability (as measured by the variance or the standard deviation) of

      the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

      means that ( )g x y approaches f(x y) as the number of noisy images used in the

      averaging process increases

      An important application of image averaging is in the field of astronomy where imaging

      under very low light levels frequently causes sensor noise to render single images

      virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

      was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

      64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

      images respectively

      Digital Image Processing

      Week 1

      Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

      100 noisy images

      a b c d e f

      Digital Image Processing

      Week 1

      A frequent application of image subtraction is in the enhancement of differences between

      images

      (a) (b) (c)

      Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

      significant bit of each pixel (c) the difference between the two images

      Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

      Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

      images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

      difference between images (a) and (b)

      Digital Image Processing

      Week 1

      Mask mode radiography ( ) ( ) ( )g x y f x y h x y

      h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

      intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

      source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

      bloodstream taking a series of images called live images (denoted f(x y)) of the same

      anatomical region as h(x y) and subtracting the mask from the series of incoming live

      images after injection of the contrast medium

      In g(x y) we can find the differences between h and f as enhanced detail

      Images being captured at TV rates we obtain a movie showing how the contrast medium

      propagates through the various arteries in the area being observed

      Digital Image Processing

      Week 1

      a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

      Digital Image Processing

      Week 1

      An important application of image multiplication (and division) is shading correction

      Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

      f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

      When the shading function is known

      ( )( )( )

      g x yf x yh x y

      h(x y) is unknown but we have access to the imaging system we can obtain an

      approximation to the shading function by imaging a target of constant intensity When the

      sensor is not available often the shading pattern can be estimated from the image

      Digital Image Processing

      Week 1

      (a) (b) (c)

      Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

      Digital Image Processing

      Week 1

      Another use of image multiplication is in masking also called region of interest (ROI)

      operations The process consists of multiplying a given image by a mask image that has

      1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

      image and the shape of the ROI can be arbitrary but usually is a rectangular shape

      (a) (b) (c)

      Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

      Digital Image Processing

      Week 1

      In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

      min( )mf f f

      0 ( 255)max( )

      ms

      m

      ff K K K

      f

      Digital Image Processing

      Week 1

      Spatial Operations

      - are performed directly on the pixels of a given image

      There are three categories of spatial operations

      single-pixel operations

      neighborhood operations

      geometric spatial transformations

      Single-pixel operations

      - change the values of intensity for the individual pixels ( )s T z

      where z is the intensity of a pixel in the original image and s is the intensity of the

      corresponding pixel in the processed image

      Digital Image Processing

      Week 1

      Neighborhood operations

      Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

      in an image f Neighborhood processing generates new intensity level at point (x y)

      based on the values of the intensities of the points in Sxy For example if Sxy is a

      rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

      intensity by computing the average value of the pixels in Sxy

      ( )

      1( ) ( )xyr c S

      g x y f r cm n

      The net effect is to perform local blurring in the original image This type of process is

      used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

      largest region of an image

      Digital Image Processing

      Week 1

      Geometric spatial transformations and image registration

      - modify the spatial relationship between pixels in an image

      - these transformations are often called rubber-sheet transformations (analogous to

      printing an image on a sheet of rubber and then stretching the sheet according to a

      predefined set of rules

      A geometric transformation consists of 2 basic operations

      1 a spatial transformation of coordinates

      2 intensity interpolation that assign intensity values to the spatial transformed

      pixels

      The coordinate system transformation ( ) [( )]x y T v w

      (v w) ndash pixel coordinates in the original image

      (x y) ndash pixel coordinates in the transformed image

      Digital Image Processing

      Week 1

      [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

      Affine transform

      11 1211 21 31

      21 2212 22 33

      31 32

      0[ 1] [ 1] [ 1] 0

      1

      t tx t v t w t

      x y v w T v w t ty t v t w t

      t t

      (AT)

      This transform can scale rotate translate or shear a set of coordinate points depending

      on the elements of the matrix T If we want to resize an image rotate it and move the

      result to some location we simply form a 3x3 matrix equal to the matrix product of the

      scaling rotation and translation matrices from Table 1

      Digital Image Processing

      Week 1

      Affine transformations

      Digital Image Processing

      Week 1

      The preceding transformations relocate pixels on an image to new locations To complete

      the process we have to assign intensity values to those locations This task is done by

      using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

      In practice we can use equation (AT) in two basic ways

      forward mapping scan the pixels of the input image (v w) compute the new spatial

      location (x y) of the corresponding pixel in the new image using (AT) directly

      Problems

      - intensity assignment when 2 or more pixels in the original image are transformed to

      the same location in the output image

      - some output locations have no correspondent in the original image (no intensity

      assignment)

      Digital Image Processing

      Week 1

      inverse mapping scans the output pixel locations and at each location (x y)

      computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

      It then interpolates among the nearest input pixels to determine the intensity of the output

      pixel value

      Inverse mappings are more efficient to implement than forward mappings and are used in

      numerous commercial implementations of spatial transformations (MATLAB for ex)

      Digital Image Processing

      Week 1

      Digital Image Processing

      Week 1

      Image registration ndash align two or more images of the same scene

      In image registration we have available the input and output images but the specific

      transformation that produced the output image from the input is generally unknown

      The problem is to estimate the transformation function and then use it to register the two

      images

      - it may be of interest to align (register) two or more image taken at approximately the

      same time but using different imaging systems (MRI scanner and a PET scanner)

      - align images of a given location taken by the same instrument at different moments

      of time (satellite images)

      Solving the problem using tie points (also called control points) which are

      corresponding points whose locations are known precisely in the input and reference

      image

      Digital Image Processing

      Week 1

      How to select tie points

      - interactively selecting them

      - use of algorithms that try to detect these points

      - some imaging systems have physical artifacts (small metallic objects) embedded in

      the imaging sensors These objects produce a set of known points (called reseau

      marks) directly on all images captured by the system which can be used as guides

      for establishing tie points

      The problem of estimating the transformation is one of modeling Suppose we have a set

      of 4 tie points both on the input image and the reference image A simple model based on

      a bilinear approximation is given by

      1 2 3 4

      5 6 7 8

      x c v c w c v w cy c v c w c v w c

      (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

      Digital Image Processing

      Week 1

      When 4 tie points are insufficient to obtain satisfactory registration an approach used

      frequently is to select a larger number of tie points and using this new set of tie points

      subdivide the image in rectangular regions marked by groups of 4 tie points On the

      subregions marked by 4 tie points we applied the transformation model described above

      The number of tie points and the sophistication of the model required to solve the register

      problem depend on the severity of the geometrical distortion

      Digital Image Processing

      Week 1

      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

      Digital Image Processing

      Week 1

      Probabilistic Methods

      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

      p(zk) = the probability that the intensity level zk occurs in the given image

      ( ) kk

      np zM N

      nk = the number of times that intensity zk occurs in the image (MN is the total number of

      pixels in the image) 1

      0( ) 1

      L

      kk

      p z

      The mean (average) intensity of an image is given by 1

      0( )

      L

      k kk

      m z p z

      Digital Image Processing

      Week 1

      The variance of the intensities is 1

      2 2

      0( ) ( )

      L

      k kk

      z m p z

      The variance is a measure of the spread of the values of z about the mean so it is a

      measure of image contrast Usually for measuring image contrast the standard deviation

      ( ) is used

      The n-th moment of a random variable z about the mean is defined as 1

      0( ) ( ) ( )

      Ln

      n k kk

      z z m p z

      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

      3( ) 0z the intensities are biased to values higher than the mean

      ( 3( ) 0z the intensities are biased to values lower than the mean

      Digital Image Processing

      Week 1

      3( ) 0z the intensities are distributed approximately equally on both side of the

      mean

      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

      Figure 1(a) ndash standard deviation 143 (variance = 2045)

      Figure 1(b) ndash standard deviation 316 (variance = 9986)

      Figure 1(c) ndash standard deviation 492 (variance = 24206)

      Digital Image Processing

      Week 1

      Intensity Transformations and Spatial Filtering

      ( ) ( )g x y T f x y

      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

      neighborhood of (x y)

      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

      and much smaller in size than the image

      Digital Image Processing

      Week 1

      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

      called spatial filter (spatial mask kernel template or window)

      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

      ( )s T r

      s and r are denoting respectively the intensity of g and f at (x y)

      Figure 2 left - T produces an output image of higher contrast than the original by

      darkening the intensity levels below k and brightening the levels above k ndash this technique

      is called contrast stretching

      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

      Digital Image Processing

      Week 1

      Figure 2 right - T produces a binary output image A mapping of this form is called

      thresholding function

      Some Basic Intensity Transformation Functions

      Image Negatives

      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

      - equivalent of a photographic negative

      - technique suited for enhancing white or gray detail embedded in dark regions of an

      image

      Digital Image Processing

      Week 1

      Original Negative image

      Digital Image Processing

      Week 1

      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

      Some basic intensity transformation functions

      Digital Image Processing

      Week 1

      This transformation maps a narrow range of low intensity values in the input into a wider

      range An operator of this type is used to expand the values of dark pixels in an image

      while compressing the higher-level values The opposite is true for the inverse log

      transformation The log functions compress the dynamic range of images with large

      variations in pixel values

      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

      Digital Image Processing

      Week 1

      Power-Law (Gamma) Transformations

      - positive constants( ) ( ( ) )s T r c r c s c r

      Plots of gamma transformation for different values of γ (c=1)

      Digital Image Processing

      Week 1

      Power-law curves with 1 map a narrow range of dark input values into a wider range

      of output values with the opposite being true for higher values of input values The

      curves with 1 have the opposite effect of those generated with values of 1

      1c - identity transformation

      A variety of devices used for image capture printing and display respond according to a

      power law The process used to correct these power-law response phenomena is called

      gamma correction

      Digital Image Processing

      Week 1

      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

      Digital Image Processing

      Week 1

      Piecewise-Linear Transformations Functions

      Contrast stretching

      - a process that expands the range of intensity levels in an image so it spans the full

      intensity range of the recording tool or display device

      a b c d Fig5

      Digital Image Processing

      Week 1

      11

      1

      2 1 1 21 2

      2 1 2 1

      22

      2

      [0 ]

      ( ) ( )( ) [ ]( ) ( )

      ( 1 ) [ 1]( 1 )

      s r r rrs r r s r rT r r r r

      r r r rs L r r r L

      L r

      Digital Image Processing

      Week 1

      1 1 2 2r s r s identity transformation (no change)

      1 2 1 2 0 1r r s s L thresholding function

      Figure 5(b) shows an 8-bit image with low contrast

      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

      in the image respectively Thus the transformation function stretched the levels linearly

      from their original range to the full range [0 L-1]

      Figure 5(d) - the thresholding function was used with 1 1 0r s m

      2 2 1r s m L where m is the mean gray level in the image

      The original image on which these results are based is a scanning electron microscope

      image of pollen magnified approximately 700 times

      Digital Image Processing

      Week 1

      Intensity-level slicing

      - highlighting a specific range of intensities in an image

      There are two approaches for intensity-level slicing

      1 display in one value (white for example) all the values in the range of interest and in

      another (say black) all other intensities (Figure 311 (a))

      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

      intensities in the image (Figure 311 (b))

      Digital Image Processing

      Week 1

      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

      highlight the major blood vessels that appear brighter as a result of injecting a contrast

      medium Figure 6(middle) shows the result of applying the first technique for a band near

      the top of the scale of intensities This type of enhancement produces a binary image

      Highlights intensity range [A B] and reduces all other intensities to a lower level

      Highlights range [A B] and preserves all other intensities

      Digital Image Processing

      Week 1

      which is useful for studying the shape of the flow of the contrast substance (to detect

      blockageshellip)

      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

      image around the mean intensity was set to black the other intensities remain unchanged

      Fig 6 - Aortic angiogram and intensity sliced versions

      Digital Image Processing

      Week 1

      Bit-plane slicing

      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

      This technique highlights the contribution made to the whole image appearances by each

      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

      Digital Image Processing

      Week 1

      Digital Image Processing

      Week 1

      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

      • DIP 1 2017
      • DIP 02 (2017)

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        bull Image Processing Toolbox (httpwwwmathworkscomproductsimage)

        bull C Solomon T Breckon Fundamentals of Digital Image Processing A Practical Approach with Examples in Matlab Wiley-Blackwell 2011

        bullWK Pratt Digital Image Processing Wiley-Interscience 2007

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Meet LenaThe First Lady of the Internet

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Lenna Soderberg (Sjoumloumlblom) and Jeff Seideman taken in May 1997Imaging Science amp Technology Conference

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        What is Digital Image Processing

        f(xy) = intensity gray level of the image at spatial point (xy)

        x y f(xy) ndash finite discrete quantities -gt digital image

        Digital Image Processing = processing digital images by means of a digital computer

        A digital image is composed of a finite number of elements (location value of intensity)

        These elements are called picture elements image elements pels pixels

        ( )i j ijx y f

        3 f D

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Image processing is not limited to the visual band of the electromagnetic (EM) spectrum

        Image processing gamma to radio waves ultrasound electron microscopy computer-generated images

        image processing image analysis computer vision

        Image processing = discipline in which both the input and the output of a process are images

        Computer Vision = use computer to emulate human vision (AI) ndashlearning making inferences and take actions based on visual inputs

        Image analysis (image understanding) = segmentation partitioning images into regions or objects

        (link between image processing and image analysis)

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Distinction between image processing image analysis computer vision

        low-level mid-level high-level processes

        Low-level processes image preprocessing to reduce noise contrast enhancement image sharpening both input and output are images

        Mid-level processes segmentation partitioning images into regions or objects description of the objects for computer processing classificationrecognition of individual objects inputs are generally images outputs are attributes extracted from the input image (eg edges contours identity of individual objects)

        High-level processes ldquomaking senserdquo of a set of recognized objects performing the cognitive functions associated with vision

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Digital Image Processing (Gonzalez + Woods) =

        processes whose inputs and outputs are images +

        processes that extract attributes from images recognition of individual objects

        (low- and mid-level processes)

        Example

        automated analysis of text = acquiring an image containing text preprocessing the image (enhancement sharpening) extracting (segmenting) the individual characaters describing the characters in a form suitable for computer processing recognition of individual characters

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        The Origins of DIP

        Newspaper industry pictures were sent by submarine cable between London and New York

        Before Bartlane cable picture transmission system (early 1920s) ndash 1 week

        With Bartlane system less than 3 hours

        Specialized printing equipment coded pictures for cable transmission and reconstructed them at the receiving end (1920s -5 distict levels of gray 1929 ndash 15 levels)

        This example is not DIP the computer is not involved

        DIP is linked and devolps in the same rhythm as digital computers (data storage display and transmission)

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        A digital pictureproduced in 1921from a coded tapeby a telegraph printer withspecial type faces(McFarlane)

        A digital picturemade in 1922from a tapepunched after the signals hadcrossed the Atlantic twice(McFarlane)

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        1964 Jet Propulsion Laboratory (Pasadena California) processed pictures of the moon transmitted by Ranger 7 (corrected image distortions)

        The first picture of the moon by a US spacecraft Ranger 7 took this image July 31 1964 about 17 minutes before impacting the lunar surface(Courtesy of NASA)

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

        1970s ndash invention of CAT (computerized axial tomography)

        CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        loz geographers use DIP to study pollution patterns from aerial and satellite imagery

        loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

        loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

        loz astronomy biology nuclear medicine law enforcement industry

        DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

        loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Examples of Fields that Use DIP

        Images can be classified according to their sources (visual X-ray hellip)

        Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Electromagnetic waves can be thought as propagating sinusoidal

        waves of different wavelength or as a stream of massless particles

        each moving in a wavelike pattern with the speed of light Each

        massless particle contains a certain amount (bundle) of energy Each

        bundle of energy is called a photon If spectral bands are grouped

        according to energy per photon we obtain the spectrum shown in the

        image above ranging from gamma-rays (highest energy) to radio

        waves (lowest energy)

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Gamma-Ray Imaging

        Nuclear medicine astronomical observations

        Nuclear medicine

        the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

        Images are produced from the emissions collected by gamma-ray detectors

        Images of this sort are used to locate sites of bone pathology (infections tumors)

        PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Examples of gamma-ray imaging

        Bone scan PET image

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        X-ray imaging

        Medical diagnosticindustry astronomy

        A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

        The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Angiography = contrast-enhancement radiography

        Angiograms = images of blood vessels

        A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

        X-rays are used in CAT (computerized axial tomography)

        X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

        Industrial CAT scans are useful when the parts can be penetreted by X-rays

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Examples of X-ray imaging

        Chest X-rayAortic angiogram

        Head CT Cygnus LoopCircuit boards

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Imaging in the Ultraviolet Band

        Litography industrial inspection microscopy biological imaging astronomical observations

        Ultraviolet light is used in fluorescence microscopy

        Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

        other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

        and then it separates the much weaker radiating fluorescent light from the brighter excitation light

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Imaging in the Visible and Infrared Bands

        Light microscopy astronomy remote sensing industry law enforcement

        LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

        Weather observations and prediction produce major applications of multispectral image from satellites

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Satellite images of Washington DC area in spectral bands of the Table 1

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Examples of light microscopy

        Taxol (anticancer agent)magnified 250X

        Cholesterol(40X)

        Microprocessor(60X)

        Nickel oxidethin film(600X)

        Surface of audio CD(1750X)

        Organicsuperconductor(450X)

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Automated visual inspection of manufactured goods

        a bc de f

        a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Imaging in the Microwave Band

        The dominant aplication of imaging in the microwave band ndash radar

        bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

        bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

        bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

        An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Spaceborne radar image of mountains in southeast Tibet

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Imaging in the Radio Band

        medicine astronomy

        MRI = Magnetic Resonance Imaging

        This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

        Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

        The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        MRI images of a human knee (left) and spine (right)

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Images of the Crab Pulsar covering the electromagnetic spectrum

        Gamma X-ray Optical Infrared Radio

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Other Imaging Modalities

        acoustic imaging electron microscopy synthetic (computer-generated) imaging

        Imaging using sound geological explorations industry medicine

        Mineral and oil exploration

        For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Biometry - iris

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Biometry - fingerprint

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Face detection and recognition

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Gender identification

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Image morphing

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Fundamental Steps in DIP

        methods whose input and output are images

        methods whose inputs are images but whose outputs are attributes extracted from those images

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Outputs are images

        bull image acquisition

        bull image filtering and enhancement

        bull image restoration

        bull color image processing

        bull wavelets and multiresolution processing

        bull compression

        bull morphological processing

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Outputs are attributes

        bull morphological processing

        bull segmentation

        bull representation and description

        bull object recognition

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Image acquisition - may involve preprocessing such as scaling

        Image enhancement

        bull manipulating an image so that the result is more suitable than the original for a specific operation

        bull enhancement is problem oriented

        bull there is no general sbquotheoryrsquo of image enhancement

        bull enhancement use subjective methods for image emprovement

        bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Image restoration

        bull improving the appearance of an image

        bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

        Color image processing

        bull fundamental concept in color models

        bull basic color processing in a digital domain

        Wavelets and multiresolution processing

        representing images in various degree of resolution

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Compression

        reducing the storage required to save an image or the bandwidth required to transmit it

        Morphological processing

        bull tools for extracting image components that are useful in the representation and description of shape

        bull a transition from processes that output images to processes that outputimage attributes

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Segmentation

        bull partitioning an image into its constituents parts or objects

        bull autonomous segmentation is one of the most difficult tasks of DIP

        bull the more accurate the segmentation the more likley recognition is to succeed

        Representation and description (almost always follows segmentation)

        bull segmentation produces either the boundary of a region or all the poits in the region itself

        bull converting the data produced by segmentation to a form suitable for computer processing

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        bull boundary representation the focus is on external shape characteristics such as corners or inflections

        bull complete region the focus is on internal properties such as texture or skeletal shape

        bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

        Object recognition

        the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

        Knowledge database

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Simplified diagramof a cross sectionof the human eye

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

        The cornea is a tough transparent tissue that covers the anterior surface of the eye

        Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

        The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

        The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

        Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

        Fovea = the place where the image of the object of interest falls on

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

        Blind spot region without receptors

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Image formation in the eye

        Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

        Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

        distance between lens and retina along visual axix = 17 mm

        range of focal length = 14 mm to 17 mm

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Optical illusions

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

        gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

        quantities that describe the quality of a chromatic light source radiance

        the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

        measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

        brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        the physical meaning is determined by the source of the image

        ( )f D f x y

        Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

        f(xy) ndash characterized by two components

        i(xy) = illumination component the amount of source illumination incident on the scene being viewed

        r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

        ( ) ( ) ( )

        0 ( ) 0 ( ) 1

        f x y i x y r x y

        i x y r x y

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        r(xy)=0 - total absorption r(xy)=1 - total reflectance

        i(xy) ndash determined by the illumination source

        r(xy) ndash determined by the characteristics of the imaged objects

        is called gray (or intensity) scale

        In practice

        min 0 0 max min min min max max max( ) L l f x y L L i r L i r

        indoor values without additional illuminationmin max10 1000L L

        black whitemin max0 1 0 1 0 1L L L L l l L

        min maxL L

        Digital Image ProcessingDigital Image Processing

        Week 1Week 1

        Digital Image Processing

        Week 1

        Image Sampling and Quantization

        - the output of the sensors is a continuous voltage waveform related to the sensed

        scene

        converting a continuous image f to digital form

        - digitizing (x y) is called sampling

        - digitizing f(x y) is called quantization

        Digital Image Processing

        Week 1

        Digital Image Processing

        Week 1

        Continuous image projected onto a sensor array Result of image sampling and quantization

        Digital Image Processing

        Week 1

        Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

        (00) (01) (0 1)(10) (11) (1 1)

        ( )

        ( 10) ( 11) ( 1 1)

        f f f Nf f f N

        f x y

        f M f M f M N

        image element pixel

        00 01 0 1

        10 11 1 1

        10 11 1 1

        ( ) ( )

        N

        i jN M N

        i j

        M M M N

        a a aa f x i y j f i ja a a

        Aa

        a a a

        f(00) ndash the upper left corner of the image

        Digital Image Processing

        Week 1

        M N ge 0 L=2k

        [0 1]i j i ja a L

        Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

        Digital Image Processing

        Week 1

        Digital Image Processing

        Week 1

        Number of bits required to store a digitized image

        for 2 b M N k M N b N k

        When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

        Digital Image Processing

        Week 1

        Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

        Measures line pairs per unit distance dots (pixels) per unit distance

        Image resolution = the largest number of discernible line pairs per unit distance

        (eg 100 line pairs per mm)

        Dots per unit distance are commonly used in printing and publishing

        In US the measure is expressed in dots per inch (dpi)

        (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

        Intensity resolution ndash the smallest discernible change in intensity level

        The number of intensity levels (L) is determined by hardware considerations

        L=2k ndash most common k = 8

        Intensity resolution in practice is given by k (number of bits used to quantize intensity)

        Digital Image Processing

        Week 1

        Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

        150 dpi (lower left) 72 dpi (lower right)

        Digital Image Processing

        Week 1

        Reducing the number of gray levels 256 128 64 32

        Digital Image Processing

        Week 1

        Reducing the number of gray levels 16 8 4 2

        Digital Image Processing

        Week 1

        Image Interpolation - used in zooming shrinking rotating and geometric corrections

        Shrinking zooming ndash image resizing ndash image resampling methods

        Interpolation is the process of using known data to estimate values at unknown locations

        Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

        750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

        same spacing as the original and then shrink it so that it fits exactly over the original

        image The pixel spacing in the 750 times 750 grid will be less than in the original image

        Problem assignment of intensity-level in the new 750 times 750 grid

        Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

        intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

        This technique has the tendency to produce undesirable effects like severe distortion of

        straight edges

        Digital Image Processing

        Week 1

        Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

        where the four coefficients are determined from the 4 equations in 4 unknowns that can

        be written using the 4 nearest neighbors of point (x y)

        Bilinear interpolation gives much better results than nearest neighbor interpolation with a

        modest increase in computational effort

        Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

        nearest neighbors of the point 3 3

        0 0

        ( ) i ji j

        i jv x y c x y

        The coefficients cij are obtained solving a 16x16 linear system

        intensity levels of the 16 nearest neighbors of 3 3

        0 0

        ( )i ji j

        i jc x y x y

        Digital Image Processing

        Week 1

        Generally bicubic interpolation does a better job of preserving fine detail than the

        bilinear technique Bicubic interpolation is the standard used in commercial image editing

        programs such as Adobe Photoshop and Corel Photopaint

        Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

        the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

        then zooming the reduced image back to its original size To generate Fig 1(d) nearest

        neighbor interpolation was used (both for shrinking and zooming)

        Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

        interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

        from 1250 dpi to 150 dpi (instead of 72 dpi)

        Digital Image Processing

        Week 1

        Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

        Digital Image Processing

        Week 1

        Neighbors of a Pixel

        A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

        This set of pixels called the 4-neighbors of p denoted by N4 (p)

        The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

        and are denoted ND(p)

        The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

        N8 (p)

        If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

        fall outside the image

        Digital Image Processing

        Week 1

        Adjacency Connectivity Regions Boundaries

        Denote by V the set of intensity levels used to define adjacency

        - in a binary image V 01 (V=0 V=1)

        - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

        We consider 3 types of adjacency

        (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

        (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

        (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

        m-adjacent if

        4( )q N p or

        ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

        Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

        ambiguities that often arise when 8-adjacency is used Consider the example

        Digital Image Processing

        Week 1

        binary image

        0 1 1 0 1 1 0 1 1

        1 0 1 0 0 1 0 0 1 0

        0 0 1 0 0 1 0 0 1

        V

        The three pixels at the top (first line) in the above example show multiple (ambiguous)

        8-adjacency as indicated by the dashed lines This ambiguity is removed by using

        m-adjacency

        Digital Image Processing

        Week 1

        A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

        is a sequence of distinct pixels with coordinates

        and are adjacent 0 0 1 1

        1 1

        ( ) ( ) ( ) ( ) ( )( ) ( ) 12

        n n

        i i i i

        x y x y x y x y s tx y x y i n

        The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

        Depending on the type of adjacency considered the paths are 4- 8- or m-paths

        Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

        in S if there exists a path between them consisting only of pixels from S

        S is a connected set if there is a path in S between any 2 pixels in S

        Let R be a subset of pixels in an image R is a region of the image if R is a connected set

        Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

        that are not adjacent are said to be disjoint When referring to regions only 4- and

        8-adjacency are considered

        Digital Image Processing

        Week 1

        Suppose that an image contains K disjoint regions 1 kR k K none of which

        touches the image border

        the complement of 1

        ( )K

        cu k u u

        k

        R R R R

        We call all the points in Ru the foreground of the image and the points in ( )cuR the

        background of the image

        The boundary (border or contour) of a region R is the set of points that are adjacent to

        points in the complement of R (R)c The border of an image is the set of pixels in the

        region that have at least one background neighbor This definition is referred to as the

        inner border to distinguish it from the notion of outer border which is the corresponding

        border in the background

        Digital Image Processing

        Week 1

        Distance measures

        For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

        function or metric if

        (a) D(p q) ge 0 D(p q) = 0 iff p=q

        (b) D(p q) = D(q p)

        (c) D(p z) le D(p q) + D(q z)

        The Euclidean distance between p and q is defined as 1

        2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

        The pixels q for which ( )eD p q r are the points contained in a disk of radius r

        centered at (x y)

        Digital Image Processing

        Week 1

        The D4 distance (also called city-block distance) between p and q is defined as

        4( ) | | | |D p q x s y t

        The pixels q for which 4( )D p q r form a diamond centered at (xy)

        4

        22 1 2

        2 2 1 0 1 22 1 2

        2

        D

        The pixels with D4 = 1 are the 4-neighbors of (x y)

        The D8 distance (called the chessboard distance) between p and q is defined as

        8( ) max| | | |D p q x s y t

        The pixels q for which 8( )D p q r form a square centered at (x y)

        Digital Image Processing

        Week 1

        8

        2 2 2 2 22 1 1 1 2

        2 2 1 0 1 22 1 1 1 22 2 2 2 2

        D

        The pixels with D8 = 1 are the 8-neighbors of (x y)

        D4 and D8 distances are independent of any paths that might exist between p and q

        because these distances involve only the coordinates of the point

        Digital Image Processing

        Week 1

        Array versus Matrix Operations

        An array operation involving one or more images is carried out on a pixel-by-pixel basis

        11 12 11 12

        21 22 21 22

        a a b ba a b b

        Array product

        11 12 11 12 11 11 12 12

        21 22 21 22 21 21 22 21

        a a b b a b a ba a b b a b a b

        Matrix product

        11 12 11 12 11 11 12 21 11 12 12 21

        21 22 21 22 21 11 22 21 21 12 22 22

        a a b b a b a b a b a ba a b b a b a b a b a b

        We assume array operations unless stated otherwise

        Digital Image Processing

        Week 1

        Linear versus Nonlinear Operations

        One of the most important classifications of image-processing methods is whether it is

        linear or nonlinear

        ( ) ( )H f x y g x y

        H is said to be a linear operator if

        images1 2 1 2

        1 2

        ( ) ( ) ( ) ( )

        H a f x y b f x y a H f x y b H f x y

        a b f f

        Example of nonlinear operator

        the maximum value of the pixels of image max ( )H f f x y f

        1 2

        0 2 6 5 1 1

        2 3 4 7f f a b

        Digital Image Processing

        Week 1

        1 2

        0 2 6 5 6 3max max 1 ( 1) max 2

        2 3 4 7 2 4a f b f

        0 2 6 51 max ( 1) max 3 ( 1)7 4

        2 3 4 7

        Arithmetic Operations in Image Processing

        Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

        f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

        For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

        value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

        The two random variables are uncorrelated when their covariance is 0

        Digital Image Processing

        Week 1

        Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

        used in image enhancement)

        1

        1( ) ( )K

        ii

        g x y g x yK

        If the noise satisfies the properties stated above we have

        2 2( ) ( )

        1( ) ( ) g x y x yE g x y f x yK

        ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

        and g respectively The standard deviation (square root of the variance) at any point in

        the average image is

        ( ) ( )1

        g x y x yK

        Digital Image Processing

        Week 1

        As K increases the variability (as measured by the variance or the standard deviation) of

        the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

        means that ( )g x y approaches f(x y) as the number of noisy images used in the

        averaging process increases

        An important application of image averaging is in the field of astronomy where imaging

        under very low light levels frequently causes sensor noise to render single images

        virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

        was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

        64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

        images respectively

        Digital Image Processing

        Week 1

        Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

        100 noisy images

        a b c d e f

        Digital Image Processing

        Week 1

        A frequent application of image subtraction is in the enhancement of differences between

        images

        (a) (b) (c)

        Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

        significant bit of each pixel (c) the difference between the two images

        Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

        Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

        images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

        difference between images (a) and (b)

        Digital Image Processing

        Week 1

        Mask mode radiography ( ) ( ) ( )g x y f x y h x y

        h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

        intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

        source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

        bloodstream taking a series of images called live images (denoted f(x y)) of the same

        anatomical region as h(x y) and subtracting the mask from the series of incoming live

        images after injection of the contrast medium

        In g(x y) we can find the differences between h and f as enhanced detail

        Images being captured at TV rates we obtain a movie showing how the contrast medium

        propagates through the various arteries in the area being observed

        Digital Image Processing

        Week 1

        a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

        Digital Image Processing

        Week 1

        An important application of image multiplication (and division) is shading correction

        Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

        f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

        When the shading function is known

        ( )( )( )

        g x yf x yh x y

        h(x y) is unknown but we have access to the imaging system we can obtain an

        approximation to the shading function by imaging a target of constant intensity When the

        sensor is not available often the shading pattern can be estimated from the image

        Digital Image Processing

        Week 1

        (a) (b) (c)

        Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

        Digital Image Processing

        Week 1

        Another use of image multiplication is in masking also called region of interest (ROI)

        operations The process consists of multiplying a given image by a mask image that has

        1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

        image and the shape of the ROI can be arbitrary but usually is a rectangular shape

        (a) (b) (c)

        Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

        Digital Image Processing

        Week 1

        In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

        min( )mf f f

        0 ( 255)max( )

        ms

        m

        ff K K K

        f

        Digital Image Processing

        Week 1

        Spatial Operations

        - are performed directly on the pixels of a given image

        There are three categories of spatial operations

        single-pixel operations

        neighborhood operations

        geometric spatial transformations

        Single-pixel operations

        - change the values of intensity for the individual pixels ( )s T z

        where z is the intensity of a pixel in the original image and s is the intensity of the

        corresponding pixel in the processed image

        Digital Image Processing

        Week 1

        Neighborhood operations

        Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

        in an image f Neighborhood processing generates new intensity level at point (x y)

        based on the values of the intensities of the points in Sxy For example if Sxy is a

        rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

        intensity by computing the average value of the pixels in Sxy

        ( )

        1( ) ( )xyr c S

        g x y f r cm n

        The net effect is to perform local blurring in the original image This type of process is

        used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

        largest region of an image

        Digital Image Processing

        Week 1

        Geometric spatial transformations and image registration

        - modify the spatial relationship between pixels in an image

        - these transformations are often called rubber-sheet transformations (analogous to

        printing an image on a sheet of rubber and then stretching the sheet according to a

        predefined set of rules

        A geometric transformation consists of 2 basic operations

        1 a spatial transformation of coordinates

        2 intensity interpolation that assign intensity values to the spatial transformed

        pixels

        The coordinate system transformation ( ) [( )]x y T v w

        (v w) ndash pixel coordinates in the original image

        (x y) ndash pixel coordinates in the transformed image

        Digital Image Processing

        Week 1

        [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

        Affine transform

        11 1211 21 31

        21 2212 22 33

        31 32

        0[ 1] [ 1] [ 1] 0

        1

        t tx t v t w t

        x y v w T v w t ty t v t w t

        t t

        (AT)

        This transform can scale rotate translate or shear a set of coordinate points depending

        on the elements of the matrix T If we want to resize an image rotate it and move the

        result to some location we simply form a 3x3 matrix equal to the matrix product of the

        scaling rotation and translation matrices from Table 1

        Digital Image Processing

        Week 1

        Affine transformations

        Digital Image Processing

        Week 1

        The preceding transformations relocate pixels on an image to new locations To complete

        the process we have to assign intensity values to those locations This task is done by

        using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

        In practice we can use equation (AT) in two basic ways

        forward mapping scan the pixels of the input image (v w) compute the new spatial

        location (x y) of the corresponding pixel in the new image using (AT) directly

        Problems

        - intensity assignment when 2 or more pixels in the original image are transformed to

        the same location in the output image

        - some output locations have no correspondent in the original image (no intensity

        assignment)

        Digital Image Processing

        Week 1

        inverse mapping scans the output pixel locations and at each location (x y)

        computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

        It then interpolates among the nearest input pixels to determine the intensity of the output

        pixel value

        Inverse mappings are more efficient to implement than forward mappings and are used in

        numerous commercial implementations of spatial transformations (MATLAB for ex)

        Digital Image Processing

        Week 1

        Digital Image Processing

        Week 1

        Image registration ndash align two or more images of the same scene

        In image registration we have available the input and output images but the specific

        transformation that produced the output image from the input is generally unknown

        The problem is to estimate the transformation function and then use it to register the two

        images

        - it may be of interest to align (register) two or more image taken at approximately the

        same time but using different imaging systems (MRI scanner and a PET scanner)

        - align images of a given location taken by the same instrument at different moments

        of time (satellite images)

        Solving the problem using tie points (also called control points) which are

        corresponding points whose locations are known precisely in the input and reference

        image

        Digital Image Processing

        Week 1

        How to select tie points

        - interactively selecting them

        - use of algorithms that try to detect these points

        - some imaging systems have physical artifacts (small metallic objects) embedded in

        the imaging sensors These objects produce a set of known points (called reseau

        marks) directly on all images captured by the system which can be used as guides

        for establishing tie points

        The problem of estimating the transformation is one of modeling Suppose we have a set

        of 4 tie points both on the input image and the reference image A simple model based on

        a bilinear approximation is given by

        1 2 3 4

        5 6 7 8

        x c v c w c v w cy c v c w c v w c

        (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

        Digital Image Processing

        Week 1

        When 4 tie points are insufficient to obtain satisfactory registration an approach used

        frequently is to select a larger number of tie points and using this new set of tie points

        subdivide the image in rectangular regions marked by groups of 4 tie points On the

        subregions marked by 4 tie points we applied the transformation model described above

        The number of tie points and the sophistication of the model required to solve the register

        problem depend on the severity of the geometrical distortion

        Digital Image Processing

        Week 1

        a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

        Digital Image Processing

        Week 1

        Probabilistic Methods

        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

        p(zk) = the probability that the intensity level zk occurs in the given image

        ( ) kk

        np zM N

        nk = the number of times that intensity zk occurs in the image (MN is the total number of

        pixels in the image) 1

        0( ) 1

        L

        kk

        p z

        The mean (average) intensity of an image is given by 1

        0( )

        L

        k kk

        m z p z

        Digital Image Processing

        Week 1

        The variance of the intensities is 1

        2 2

        0( ) ( )

        L

        k kk

        z m p z

        The variance is a measure of the spread of the values of z about the mean so it is a

        measure of image contrast Usually for measuring image contrast the standard deviation

        ( ) is used

        The n-th moment of a random variable z about the mean is defined as 1

        0( ) ( ) ( )

        Ln

        n k kk

        z z m p z

        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

        3( ) 0z the intensities are biased to values higher than the mean

        ( 3( ) 0z the intensities are biased to values lower than the mean

        Digital Image Processing

        Week 1

        3( ) 0z the intensities are distributed approximately equally on both side of the

        mean

        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

        Figure 1(a) ndash standard deviation 143 (variance = 2045)

        Figure 1(b) ndash standard deviation 316 (variance = 9986)

        Figure 1(c) ndash standard deviation 492 (variance = 24206)

        Digital Image Processing

        Week 1

        Intensity Transformations and Spatial Filtering

        ( ) ( )g x y T f x y

        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

        neighborhood of (x y)

        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

        and much smaller in size than the image

        Digital Image Processing

        Week 1

        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

        called spatial filter (spatial mask kernel template or window)

        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

        ( )s T r

        s and r are denoting respectively the intensity of g and f at (x y)

        Figure 2 left - T produces an output image of higher contrast than the original by

        darkening the intensity levels below k and brightening the levels above k ndash this technique

        is called contrast stretching

        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

        Digital Image Processing

        Week 1

        Figure 2 right - T produces a binary output image A mapping of this form is called

        thresholding function

        Some Basic Intensity Transformation Functions

        Image Negatives

        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

        - equivalent of a photographic negative

        - technique suited for enhancing white or gray detail embedded in dark regions of an

        image

        Digital Image Processing

        Week 1

        Original Negative image

        Digital Image Processing

        Week 1

        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

        Some basic intensity transformation functions

        Digital Image Processing

        Week 1

        This transformation maps a narrow range of low intensity values in the input into a wider

        range An operator of this type is used to expand the values of dark pixels in an image

        while compressing the higher-level values The opposite is true for the inverse log

        transformation The log functions compress the dynamic range of images with large

        variations in pixel values

        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

        Digital Image Processing

        Week 1

        Power-Law (Gamma) Transformations

        - positive constants( ) ( ( ) )s T r c r c s c r

        Plots of gamma transformation for different values of γ (c=1)

        Digital Image Processing

        Week 1

        Power-law curves with 1 map a narrow range of dark input values into a wider range

        of output values with the opposite being true for higher values of input values The

        curves with 1 have the opposite effect of those generated with values of 1

        1c - identity transformation

        A variety of devices used for image capture printing and display respond according to a

        power law The process used to correct these power-law response phenomena is called

        gamma correction

        Digital Image Processing

        Week 1

        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

        Digital Image Processing

        Week 1

        Piecewise-Linear Transformations Functions

        Contrast stretching

        - a process that expands the range of intensity levels in an image so it spans the full

        intensity range of the recording tool or display device

        a b c d Fig5

        Digital Image Processing

        Week 1

        11

        1

        2 1 1 21 2

        2 1 2 1

        22

        2

        [0 ]

        ( ) ( )( ) [ ]( ) ( )

        ( 1 ) [ 1]( 1 )

        s r r rrs r r s r rT r r r r

        r r r rs L r r r L

        L r

        Digital Image Processing

        Week 1

        1 1 2 2r s r s identity transformation (no change)

        1 2 1 2 0 1r r s s L thresholding function

        Figure 5(b) shows an 8-bit image with low contrast

        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

        in the image respectively Thus the transformation function stretched the levels linearly

        from their original range to the full range [0 L-1]

        Figure 5(d) - the thresholding function was used with 1 1 0r s m

        2 2 1r s m L where m is the mean gray level in the image

        The original image on which these results are based is a scanning electron microscope

        image of pollen magnified approximately 700 times

        Digital Image Processing

        Week 1

        Intensity-level slicing

        - highlighting a specific range of intensities in an image

        There are two approaches for intensity-level slicing

        1 display in one value (white for example) all the values in the range of interest and in

        another (say black) all other intensities (Figure 311 (a))

        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

        intensities in the image (Figure 311 (b))

        Digital Image Processing

        Week 1

        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

        highlight the major blood vessels that appear brighter as a result of injecting a contrast

        medium Figure 6(middle) shows the result of applying the first technique for a band near

        the top of the scale of intensities This type of enhancement produces a binary image

        Highlights intensity range [A B] and reduces all other intensities to a lower level

        Highlights range [A B] and preserves all other intensities

        Digital Image Processing

        Week 1

        which is useful for studying the shape of the flow of the contrast substance (to detect

        blockageshellip)

        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

        image around the mean intensity was set to black the other intensities remain unchanged

        Fig 6 - Aortic angiogram and intensity sliced versions

        Digital Image Processing

        Week 1

        Bit-plane slicing

        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

        This technique highlights the contribution made to the whole image appearances by each

        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

        Digital Image Processing

        Week 1

        Digital Image Processing

        Week 1

        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

        • DIP 1 2017
        • DIP 02 (2017)

          Digital Image ProcessingDigital Image Processing

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          Meet LenaThe First Lady of the Internet

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          Lenna Soderberg (Sjoumloumlblom) and Jeff Seideman taken in May 1997Imaging Science amp Technology Conference

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          What is Digital Image Processing

          f(xy) = intensity gray level of the image at spatial point (xy)

          x y f(xy) ndash finite discrete quantities -gt digital image

          Digital Image Processing = processing digital images by means of a digital computer

          A digital image is composed of a finite number of elements (location value of intensity)

          These elements are called picture elements image elements pels pixels

          ( )i j ijx y f

          3 f D

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Image processing is not limited to the visual band of the electromagnetic (EM) spectrum

          Image processing gamma to radio waves ultrasound electron microscopy computer-generated images

          image processing image analysis computer vision

          Image processing = discipline in which both the input and the output of a process are images

          Computer Vision = use computer to emulate human vision (AI) ndashlearning making inferences and take actions based on visual inputs

          Image analysis (image understanding) = segmentation partitioning images into regions or objects

          (link between image processing and image analysis)

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Distinction between image processing image analysis computer vision

          low-level mid-level high-level processes

          Low-level processes image preprocessing to reduce noise contrast enhancement image sharpening both input and output are images

          Mid-level processes segmentation partitioning images into regions or objects description of the objects for computer processing classificationrecognition of individual objects inputs are generally images outputs are attributes extracted from the input image (eg edges contours identity of individual objects)

          High-level processes ldquomaking senserdquo of a set of recognized objects performing the cognitive functions associated with vision

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Digital Image Processing (Gonzalez + Woods) =

          processes whose inputs and outputs are images +

          processes that extract attributes from images recognition of individual objects

          (low- and mid-level processes)

          Example

          automated analysis of text = acquiring an image containing text preprocessing the image (enhancement sharpening) extracting (segmenting) the individual characaters describing the characters in a form suitable for computer processing recognition of individual characters

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          The Origins of DIP

          Newspaper industry pictures were sent by submarine cable between London and New York

          Before Bartlane cable picture transmission system (early 1920s) ndash 1 week

          With Bartlane system less than 3 hours

          Specialized printing equipment coded pictures for cable transmission and reconstructed them at the receiving end (1920s -5 distict levels of gray 1929 ndash 15 levels)

          This example is not DIP the computer is not involved

          DIP is linked and devolps in the same rhythm as digital computers (data storage display and transmission)

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          A digital pictureproduced in 1921from a coded tapeby a telegraph printer withspecial type faces(McFarlane)

          A digital picturemade in 1922from a tapepunched after the signals hadcrossed the Atlantic twice(McFarlane)

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          1964 Jet Propulsion Laboratory (Pasadena California) processed pictures of the moon transmitted by Ranger 7 (corrected image distortions)

          The first picture of the moon by a US spacecraft Ranger 7 took this image July 31 1964 about 17 minutes before impacting the lunar surface(Courtesy of NASA)

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

          1970s ndash invention of CAT (computerized axial tomography)

          CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          loz geographers use DIP to study pollution patterns from aerial and satellite imagery

          loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

          loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

          loz astronomy biology nuclear medicine law enforcement industry

          DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

          loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Examples of Fields that Use DIP

          Images can be classified according to their sources (visual X-ray hellip)

          Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Electromagnetic waves can be thought as propagating sinusoidal

          waves of different wavelength or as a stream of massless particles

          each moving in a wavelike pattern with the speed of light Each

          massless particle contains a certain amount (bundle) of energy Each

          bundle of energy is called a photon If spectral bands are grouped

          according to energy per photon we obtain the spectrum shown in the

          image above ranging from gamma-rays (highest energy) to radio

          waves (lowest energy)

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Gamma-Ray Imaging

          Nuclear medicine astronomical observations

          Nuclear medicine

          the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

          Images are produced from the emissions collected by gamma-ray detectors

          Images of this sort are used to locate sites of bone pathology (infections tumors)

          PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Examples of gamma-ray imaging

          Bone scan PET image

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          X-ray imaging

          Medical diagnosticindustry astronomy

          A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

          The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Angiography = contrast-enhancement radiography

          Angiograms = images of blood vessels

          A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

          X-rays are used in CAT (computerized axial tomography)

          X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

          Industrial CAT scans are useful when the parts can be penetreted by X-rays

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Examples of X-ray imaging

          Chest X-rayAortic angiogram

          Head CT Cygnus LoopCircuit boards

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Imaging in the Ultraviolet Band

          Litography industrial inspection microscopy biological imaging astronomical observations

          Ultraviolet light is used in fluorescence microscopy

          Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

          other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

          and then it separates the much weaker radiating fluorescent light from the brighter excitation light

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Imaging in the Visible and Infrared Bands

          Light microscopy astronomy remote sensing industry law enforcement

          LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

          Weather observations and prediction produce major applications of multispectral image from satellites

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Satellite images of Washington DC area in spectral bands of the Table 1

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Examples of light microscopy

          Taxol (anticancer agent)magnified 250X

          Cholesterol(40X)

          Microprocessor(60X)

          Nickel oxidethin film(600X)

          Surface of audio CD(1750X)

          Organicsuperconductor(450X)

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Automated visual inspection of manufactured goods

          a bc de f

          a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Imaging in the Microwave Band

          The dominant aplication of imaging in the microwave band ndash radar

          bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

          bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

          bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

          An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Spaceborne radar image of mountains in southeast Tibet

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Imaging in the Radio Band

          medicine astronomy

          MRI = Magnetic Resonance Imaging

          This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

          Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

          The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          MRI images of a human knee (left) and spine (right)

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Images of the Crab Pulsar covering the electromagnetic spectrum

          Gamma X-ray Optical Infrared Radio

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Other Imaging Modalities

          acoustic imaging electron microscopy synthetic (computer-generated) imaging

          Imaging using sound geological explorations industry medicine

          Mineral and oil exploration

          For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Digital Image ProcessingDigital Image Processing

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          Biometry - iris

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Biometry - fingerprint

          Digital Image ProcessingDigital Image Processing

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          Face detection and recognition

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Gender identification

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Image morphing

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Fundamental Steps in DIP

          methods whose input and output are images

          methods whose inputs are images but whose outputs are attributes extracted from those images

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Outputs are images

          bull image acquisition

          bull image filtering and enhancement

          bull image restoration

          bull color image processing

          bull wavelets and multiresolution processing

          bull compression

          bull morphological processing

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Outputs are attributes

          bull morphological processing

          bull segmentation

          bull representation and description

          bull object recognition

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Image acquisition - may involve preprocessing such as scaling

          Image enhancement

          bull manipulating an image so that the result is more suitable than the original for a specific operation

          bull enhancement is problem oriented

          bull there is no general sbquotheoryrsquo of image enhancement

          bull enhancement use subjective methods for image emprovement

          bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Image restoration

          bull improving the appearance of an image

          bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

          Color image processing

          bull fundamental concept in color models

          bull basic color processing in a digital domain

          Wavelets and multiresolution processing

          representing images in various degree of resolution

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Compression

          reducing the storage required to save an image or the bandwidth required to transmit it

          Morphological processing

          bull tools for extracting image components that are useful in the representation and description of shape

          bull a transition from processes that output images to processes that outputimage attributes

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Segmentation

          bull partitioning an image into its constituents parts or objects

          bull autonomous segmentation is one of the most difficult tasks of DIP

          bull the more accurate the segmentation the more likley recognition is to succeed

          Representation and description (almost always follows segmentation)

          bull segmentation produces either the boundary of a region or all the poits in the region itself

          bull converting the data produced by segmentation to a form suitable for computer processing

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          bull boundary representation the focus is on external shape characteristics such as corners or inflections

          bull complete region the focus is on internal properties such as texture or skeletal shape

          bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

          Object recognition

          the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

          Knowledge database

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Simplified diagramof a cross sectionof the human eye

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

          The cornea is a tough transparent tissue that covers the anterior surface of the eye

          Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

          The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

          The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

          Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

          Fovea = the place where the image of the object of interest falls on

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

          Blind spot region without receptors

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Image formation in the eye

          Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

          Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

          distance between lens and retina along visual axix = 17 mm

          range of focal length = 14 mm to 17 mm

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Digital Image ProcessingDigital Image Processing

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          Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Optical illusions

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

          gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

          quantities that describe the quality of a chromatic light source radiance

          the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

          measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

          brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          the physical meaning is determined by the source of the image

          ( )f D f x y

          Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

          f(xy) ndash characterized by two components

          i(xy) = illumination component the amount of source illumination incident on the scene being viewed

          r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

          ( ) ( ) ( )

          0 ( ) 0 ( ) 1

          f x y i x y r x y

          i x y r x y

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          r(xy)=0 - total absorption r(xy)=1 - total reflectance

          i(xy) ndash determined by the illumination source

          r(xy) ndash determined by the characteristics of the imaged objects

          is called gray (or intensity) scale

          In practice

          min 0 0 max min min min max max max( ) L l f x y L L i r L i r

          indoor values without additional illuminationmin max10 1000L L

          black whitemin max0 1 0 1 0 1L L L L l l L

          min maxL L

          Digital Image ProcessingDigital Image Processing

          Week 1Week 1

          Digital Image Processing

          Week 1

          Image Sampling and Quantization

          - the output of the sensors is a continuous voltage waveform related to the sensed

          scene

          converting a continuous image f to digital form

          - digitizing (x y) is called sampling

          - digitizing f(x y) is called quantization

          Digital Image Processing

          Week 1

          Digital Image Processing

          Week 1

          Continuous image projected onto a sensor array Result of image sampling and quantization

          Digital Image Processing

          Week 1

          Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

          (00) (01) (0 1)(10) (11) (1 1)

          ( )

          ( 10) ( 11) ( 1 1)

          f f f Nf f f N

          f x y

          f M f M f M N

          image element pixel

          00 01 0 1

          10 11 1 1

          10 11 1 1

          ( ) ( )

          N

          i jN M N

          i j

          M M M N

          a a aa f x i y j f i ja a a

          Aa

          a a a

          f(00) ndash the upper left corner of the image

          Digital Image Processing

          Week 1

          M N ge 0 L=2k

          [0 1]i j i ja a L

          Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

          Digital Image Processing

          Week 1

          Digital Image Processing

          Week 1

          Number of bits required to store a digitized image

          for 2 b M N k M N b N k

          When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

          Digital Image Processing

          Week 1

          Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

          Measures line pairs per unit distance dots (pixels) per unit distance

          Image resolution = the largest number of discernible line pairs per unit distance

          (eg 100 line pairs per mm)

          Dots per unit distance are commonly used in printing and publishing

          In US the measure is expressed in dots per inch (dpi)

          (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

          Intensity resolution ndash the smallest discernible change in intensity level

          The number of intensity levels (L) is determined by hardware considerations

          L=2k ndash most common k = 8

          Intensity resolution in practice is given by k (number of bits used to quantize intensity)

          Digital Image Processing

          Week 1

          Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

          150 dpi (lower left) 72 dpi (lower right)

          Digital Image Processing

          Week 1

          Reducing the number of gray levels 256 128 64 32

          Digital Image Processing

          Week 1

          Reducing the number of gray levels 16 8 4 2

          Digital Image Processing

          Week 1

          Image Interpolation - used in zooming shrinking rotating and geometric corrections

          Shrinking zooming ndash image resizing ndash image resampling methods

          Interpolation is the process of using known data to estimate values at unknown locations

          Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

          750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

          same spacing as the original and then shrink it so that it fits exactly over the original

          image The pixel spacing in the 750 times 750 grid will be less than in the original image

          Problem assignment of intensity-level in the new 750 times 750 grid

          Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

          intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

          This technique has the tendency to produce undesirable effects like severe distortion of

          straight edges

          Digital Image Processing

          Week 1

          Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

          where the four coefficients are determined from the 4 equations in 4 unknowns that can

          be written using the 4 nearest neighbors of point (x y)

          Bilinear interpolation gives much better results than nearest neighbor interpolation with a

          modest increase in computational effort

          Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

          nearest neighbors of the point 3 3

          0 0

          ( ) i ji j

          i jv x y c x y

          The coefficients cij are obtained solving a 16x16 linear system

          intensity levels of the 16 nearest neighbors of 3 3

          0 0

          ( )i ji j

          i jc x y x y

          Digital Image Processing

          Week 1

          Generally bicubic interpolation does a better job of preserving fine detail than the

          bilinear technique Bicubic interpolation is the standard used in commercial image editing

          programs such as Adobe Photoshop and Corel Photopaint

          Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

          the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

          then zooming the reduced image back to its original size To generate Fig 1(d) nearest

          neighbor interpolation was used (both for shrinking and zooming)

          Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

          interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

          from 1250 dpi to 150 dpi (instead of 72 dpi)

          Digital Image Processing

          Week 1

          Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

          Digital Image Processing

          Week 1

          Neighbors of a Pixel

          A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

          This set of pixels called the 4-neighbors of p denoted by N4 (p)

          The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

          and are denoted ND(p)

          The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

          N8 (p)

          If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

          fall outside the image

          Digital Image Processing

          Week 1

          Adjacency Connectivity Regions Boundaries

          Denote by V the set of intensity levels used to define adjacency

          - in a binary image V 01 (V=0 V=1)

          - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

          We consider 3 types of adjacency

          (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

          (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

          (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

          m-adjacent if

          4( )q N p or

          ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

          Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

          ambiguities that often arise when 8-adjacency is used Consider the example

          Digital Image Processing

          Week 1

          binary image

          0 1 1 0 1 1 0 1 1

          1 0 1 0 0 1 0 0 1 0

          0 0 1 0 0 1 0 0 1

          V

          The three pixels at the top (first line) in the above example show multiple (ambiguous)

          8-adjacency as indicated by the dashed lines This ambiguity is removed by using

          m-adjacency

          Digital Image Processing

          Week 1

          A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

          is a sequence of distinct pixels with coordinates

          and are adjacent 0 0 1 1

          1 1

          ( ) ( ) ( ) ( ) ( )( ) ( ) 12

          n n

          i i i i

          x y x y x y x y s tx y x y i n

          The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

          Depending on the type of adjacency considered the paths are 4- 8- or m-paths

          Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

          in S if there exists a path between them consisting only of pixels from S

          S is a connected set if there is a path in S between any 2 pixels in S

          Let R be a subset of pixels in an image R is a region of the image if R is a connected set

          Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

          that are not adjacent are said to be disjoint When referring to regions only 4- and

          8-adjacency are considered

          Digital Image Processing

          Week 1

          Suppose that an image contains K disjoint regions 1 kR k K none of which

          touches the image border

          the complement of 1

          ( )K

          cu k u u

          k

          R R R R

          We call all the points in Ru the foreground of the image and the points in ( )cuR the

          background of the image

          The boundary (border or contour) of a region R is the set of points that are adjacent to

          points in the complement of R (R)c The border of an image is the set of pixels in the

          region that have at least one background neighbor This definition is referred to as the

          inner border to distinguish it from the notion of outer border which is the corresponding

          border in the background

          Digital Image Processing

          Week 1

          Distance measures

          For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

          function or metric if

          (a) D(p q) ge 0 D(p q) = 0 iff p=q

          (b) D(p q) = D(q p)

          (c) D(p z) le D(p q) + D(q z)

          The Euclidean distance between p and q is defined as 1

          2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

          The pixels q for which ( )eD p q r are the points contained in a disk of radius r

          centered at (x y)

          Digital Image Processing

          Week 1

          The D4 distance (also called city-block distance) between p and q is defined as

          4( ) | | | |D p q x s y t

          The pixels q for which 4( )D p q r form a diamond centered at (xy)

          4

          22 1 2

          2 2 1 0 1 22 1 2

          2

          D

          The pixels with D4 = 1 are the 4-neighbors of (x y)

          The D8 distance (called the chessboard distance) between p and q is defined as

          8( ) max| | | |D p q x s y t

          The pixels q for which 8( )D p q r form a square centered at (x y)

          Digital Image Processing

          Week 1

          8

          2 2 2 2 22 1 1 1 2

          2 2 1 0 1 22 1 1 1 22 2 2 2 2

          D

          The pixels with D8 = 1 are the 8-neighbors of (x y)

          D4 and D8 distances are independent of any paths that might exist between p and q

          because these distances involve only the coordinates of the point

          Digital Image Processing

          Week 1

          Array versus Matrix Operations

          An array operation involving one or more images is carried out on a pixel-by-pixel basis

          11 12 11 12

          21 22 21 22

          a a b ba a b b

          Array product

          11 12 11 12 11 11 12 12

          21 22 21 22 21 21 22 21

          a a b b a b a ba a b b a b a b

          Matrix product

          11 12 11 12 11 11 12 21 11 12 12 21

          21 22 21 22 21 11 22 21 21 12 22 22

          a a b b a b a b a b a ba a b b a b a b a b a b

          We assume array operations unless stated otherwise

          Digital Image Processing

          Week 1

          Linear versus Nonlinear Operations

          One of the most important classifications of image-processing methods is whether it is

          linear or nonlinear

          ( ) ( )H f x y g x y

          H is said to be a linear operator if

          images1 2 1 2

          1 2

          ( ) ( ) ( ) ( )

          H a f x y b f x y a H f x y b H f x y

          a b f f

          Example of nonlinear operator

          the maximum value of the pixels of image max ( )H f f x y f

          1 2

          0 2 6 5 1 1

          2 3 4 7f f a b

          Digital Image Processing

          Week 1

          1 2

          0 2 6 5 6 3max max 1 ( 1) max 2

          2 3 4 7 2 4a f b f

          0 2 6 51 max ( 1) max 3 ( 1)7 4

          2 3 4 7

          Arithmetic Operations in Image Processing

          Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

          f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

          For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

          value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

          The two random variables are uncorrelated when their covariance is 0

          Digital Image Processing

          Week 1

          Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

          used in image enhancement)

          1

          1( ) ( )K

          ii

          g x y g x yK

          If the noise satisfies the properties stated above we have

          2 2( ) ( )

          1( ) ( ) g x y x yE g x y f x yK

          ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

          and g respectively The standard deviation (square root of the variance) at any point in

          the average image is

          ( ) ( )1

          g x y x yK

          Digital Image Processing

          Week 1

          As K increases the variability (as measured by the variance or the standard deviation) of

          the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

          means that ( )g x y approaches f(x y) as the number of noisy images used in the

          averaging process increases

          An important application of image averaging is in the field of astronomy where imaging

          under very low light levels frequently causes sensor noise to render single images

          virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

          was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

          64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

          images respectively

          Digital Image Processing

          Week 1

          Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

          100 noisy images

          a b c d e f

          Digital Image Processing

          Week 1

          A frequent application of image subtraction is in the enhancement of differences between

          images

          (a) (b) (c)

          Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

          significant bit of each pixel (c) the difference between the two images

          Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

          Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

          images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

          difference between images (a) and (b)

          Digital Image Processing

          Week 1

          Mask mode radiography ( ) ( ) ( )g x y f x y h x y

          h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

          intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

          source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

          bloodstream taking a series of images called live images (denoted f(x y)) of the same

          anatomical region as h(x y) and subtracting the mask from the series of incoming live

          images after injection of the contrast medium

          In g(x y) we can find the differences between h and f as enhanced detail

          Images being captured at TV rates we obtain a movie showing how the contrast medium

          propagates through the various arteries in the area being observed

          Digital Image Processing

          Week 1

          a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

          Digital Image Processing

          Week 1

          An important application of image multiplication (and division) is shading correction

          Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

          f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

          When the shading function is known

          ( )( )( )

          g x yf x yh x y

          h(x y) is unknown but we have access to the imaging system we can obtain an

          approximation to the shading function by imaging a target of constant intensity When the

          sensor is not available often the shading pattern can be estimated from the image

          Digital Image Processing

          Week 1

          (a) (b) (c)

          Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

          Digital Image Processing

          Week 1

          Another use of image multiplication is in masking also called region of interest (ROI)

          operations The process consists of multiplying a given image by a mask image that has

          1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

          image and the shape of the ROI can be arbitrary but usually is a rectangular shape

          (a) (b) (c)

          Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

          Digital Image Processing

          Week 1

          In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

          min( )mf f f

          0 ( 255)max( )

          ms

          m

          ff K K K

          f

          Digital Image Processing

          Week 1

          Spatial Operations

          - are performed directly on the pixels of a given image

          There are three categories of spatial operations

          single-pixel operations

          neighborhood operations

          geometric spatial transformations

          Single-pixel operations

          - change the values of intensity for the individual pixels ( )s T z

          where z is the intensity of a pixel in the original image and s is the intensity of the

          corresponding pixel in the processed image

          Digital Image Processing

          Week 1

          Neighborhood operations

          Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

          in an image f Neighborhood processing generates new intensity level at point (x y)

          based on the values of the intensities of the points in Sxy For example if Sxy is a

          rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

          intensity by computing the average value of the pixels in Sxy

          ( )

          1( ) ( )xyr c S

          g x y f r cm n

          The net effect is to perform local blurring in the original image This type of process is

          used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

          largest region of an image

          Digital Image Processing

          Week 1

          Geometric spatial transformations and image registration

          - modify the spatial relationship between pixels in an image

          - these transformations are often called rubber-sheet transformations (analogous to

          printing an image on a sheet of rubber and then stretching the sheet according to a

          predefined set of rules

          A geometric transformation consists of 2 basic operations

          1 a spatial transformation of coordinates

          2 intensity interpolation that assign intensity values to the spatial transformed

          pixels

          The coordinate system transformation ( ) [( )]x y T v w

          (v w) ndash pixel coordinates in the original image

          (x y) ndash pixel coordinates in the transformed image

          Digital Image Processing

          Week 1

          [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

          Affine transform

          11 1211 21 31

          21 2212 22 33

          31 32

          0[ 1] [ 1] [ 1] 0

          1

          t tx t v t w t

          x y v w T v w t ty t v t w t

          t t

          (AT)

          This transform can scale rotate translate or shear a set of coordinate points depending

          on the elements of the matrix T If we want to resize an image rotate it and move the

          result to some location we simply form a 3x3 matrix equal to the matrix product of the

          scaling rotation and translation matrices from Table 1

          Digital Image Processing

          Week 1

          Affine transformations

          Digital Image Processing

          Week 1

          The preceding transformations relocate pixels on an image to new locations To complete

          the process we have to assign intensity values to those locations This task is done by

          using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

          In practice we can use equation (AT) in two basic ways

          forward mapping scan the pixels of the input image (v w) compute the new spatial

          location (x y) of the corresponding pixel in the new image using (AT) directly

          Problems

          - intensity assignment when 2 or more pixels in the original image are transformed to

          the same location in the output image

          - some output locations have no correspondent in the original image (no intensity

          assignment)

          Digital Image Processing

          Week 1

          inverse mapping scans the output pixel locations and at each location (x y)

          computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

          It then interpolates among the nearest input pixels to determine the intensity of the output

          pixel value

          Inverse mappings are more efficient to implement than forward mappings and are used in

          numerous commercial implementations of spatial transformations (MATLAB for ex)

          Digital Image Processing

          Week 1

          Digital Image Processing

          Week 1

          Image registration ndash align two or more images of the same scene

          In image registration we have available the input and output images but the specific

          transformation that produced the output image from the input is generally unknown

          The problem is to estimate the transformation function and then use it to register the two

          images

          - it may be of interest to align (register) two or more image taken at approximately the

          same time but using different imaging systems (MRI scanner and a PET scanner)

          - align images of a given location taken by the same instrument at different moments

          of time (satellite images)

          Solving the problem using tie points (also called control points) which are

          corresponding points whose locations are known precisely in the input and reference

          image

          Digital Image Processing

          Week 1

          How to select tie points

          - interactively selecting them

          - use of algorithms that try to detect these points

          - some imaging systems have physical artifacts (small metallic objects) embedded in

          the imaging sensors These objects produce a set of known points (called reseau

          marks) directly on all images captured by the system which can be used as guides

          for establishing tie points

          The problem of estimating the transformation is one of modeling Suppose we have a set

          of 4 tie points both on the input image and the reference image A simple model based on

          a bilinear approximation is given by

          1 2 3 4

          5 6 7 8

          x c v c w c v w cy c v c w c v w c

          (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

          Digital Image Processing

          Week 1

          When 4 tie points are insufficient to obtain satisfactory registration an approach used

          frequently is to select a larger number of tie points and using this new set of tie points

          subdivide the image in rectangular regions marked by groups of 4 tie points On the

          subregions marked by 4 tie points we applied the transformation model described above

          The number of tie points and the sophistication of the model required to solve the register

          problem depend on the severity of the geometrical distortion

          Digital Image Processing

          Week 1

          a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

          Digital Image Processing

          Week 1

          Probabilistic Methods

          zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

          p(zk) = the probability that the intensity level zk occurs in the given image

          ( ) kk

          np zM N

          nk = the number of times that intensity zk occurs in the image (MN is the total number of

          pixels in the image) 1

          0( ) 1

          L

          kk

          p z

          The mean (average) intensity of an image is given by 1

          0( )

          L

          k kk

          m z p z

          Digital Image Processing

          Week 1

          The variance of the intensities is 1

          2 2

          0( ) ( )

          L

          k kk

          z m p z

          The variance is a measure of the spread of the values of z about the mean so it is a

          measure of image contrast Usually for measuring image contrast the standard deviation

          ( ) is used

          The n-th moment of a random variable z about the mean is defined as 1

          0( ) ( ) ( )

          Ln

          n k kk

          z z m p z

          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

          3( ) 0z the intensities are biased to values higher than the mean

          ( 3( ) 0z the intensities are biased to values lower than the mean

          Digital Image Processing

          Week 1

          3( ) 0z the intensities are distributed approximately equally on both side of the

          mean

          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

          Figure 1(a) ndash standard deviation 143 (variance = 2045)

          Figure 1(b) ndash standard deviation 316 (variance = 9986)

          Figure 1(c) ndash standard deviation 492 (variance = 24206)

          Digital Image Processing

          Week 1

          Intensity Transformations and Spatial Filtering

          ( ) ( )g x y T f x y

          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

          neighborhood of (x y)

          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

          and much smaller in size than the image

          Digital Image Processing

          Week 1

          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

          called spatial filter (spatial mask kernel template or window)

          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

          ( )s T r

          s and r are denoting respectively the intensity of g and f at (x y)

          Figure 2 left - T produces an output image of higher contrast than the original by

          darkening the intensity levels below k and brightening the levels above k ndash this technique

          is called contrast stretching

          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

          Digital Image Processing

          Week 1

          Figure 2 right - T produces a binary output image A mapping of this form is called

          thresholding function

          Some Basic Intensity Transformation Functions

          Image Negatives

          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

          - equivalent of a photographic negative

          - technique suited for enhancing white or gray detail embedded in dark regions of an

          image

          Digital Image Processing

          Week 1

          Original Negative image

          Digital Image Processing

          Week 1

          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

          Some basic intensity transformation functions

          Digital Image Processing

          Week 1

          This transformation maps a narrow range of low intensity values in the input into a wider

          range An operator of this type is used to expand the values of dark pixels in an image

          while compressing the higher-level values The opposite is true for the inverse log

          transformation The log functions compress the dynamic range of images with large

          variations in pixel values

          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

          Digital Image Processing

          Week 1

          Power-Law (Gamma) Transformations

          - positive constants( ) ( ( ) )s T r c r c s c r

          Plots of gamma transformation for different values of γ (c=1)

          Digital Image Processing

          Week 1

          Power-law curves with 1 map a narrow range of dark input values into a wider range

          of output values with the opposite being true for higher values of input values The

          curves with 1 have the opposite effect of those generated with values of 1

          1c - identity transformation

          A variety of devices used for image capture printing and display respond according to a

          power law The process used to correct these power-law response phenomena is called

          gamma correction

          Digital Image Processing

          Week 1

          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

          Digital Image Processing

          Week 1

          Piecewise-Linear Transformations Functions

          Contrast stretching

          - a process that expands the range of intensity levels in an image so it spans the full

          intensity range of the recording tool or display device

          a b c d Fig5

          Digital Image Processing

          Week 1

          11

          1

          2 1 1 21 2

          2 1 2 1

          22

          2

          [0 ]

          ( ) ( )( ) [ ]( ) ( )

          ( 1 ) [ 1]( 1 )

          s r r rrs r r s r rT r r r r

          r r r rs L r r r L

          L r

          Digital Image Processing

          Week 1

          1 1 2 2r s r s identity transformation (no change)

          1 2 1 2 0 1r r s s L thresholding function

          Figure 5(b) shows an 8-bit image with low contrast

          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

          in the image respectively Thus the transformation function stretched the levels linearly

          from their original range to the full range [0 L-1]

          Figure 5(d) - the thresholding function was used with 1 1 0r s m

          2 2 1r s m L where m is the mean gray level in the image

          The original image on which these results are based is a scanning electron microscope

          image of pollen magnified approximately 700 times

          Digital Image Processing

          Week 1

          Intensity-level slicing

          - highlighting a specific range of intensities in an image

          There are two approaches for intensity-level slicing

          1 display in one value (white for example) all the values in the range of interest and in

          another (say black) all other intensities (Figure 311 (a))

          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

          intensities in the image (Figure 311 (b))

          Digital Image Processing

          Week 1

          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

          highlight the major blood vessels that appear brighter as a result of injecting a contrast

          medium Figure 6(middle) shows the result of applying the first technique for a band near

          the top of the scale of intensities This type of enhancement produces a binary image

          Highlights intensity range [A B] and reduces all other intensities to a lower level

          Highlights range [A B] and preserves all other intensities

          Digital Image Processing

          Week 1

          which is useful for studying the shape of the flow of the contrast substance (to detect

          blockageshellip)

          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

          image around the mean intensity was set to black the other intensities remain unchanged

          Fig 6 - Aortic angiogram and intensity sliced versions

          Digital Image Processing

          Week 1

          Bit-plane slicing

          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

          This technique highlights the contribution made to the whole image appearances by each

          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

          Digital Image Processing

          Week 1

          Digital Image Processing

          Week 1

          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

          • DIP 1 2017
          • DIP 02 (2017)

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Meet LenaThe First Lady of the Internet

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Lenna Soderberg (Sjoumloumlblom) and Jeff Seideman taken in May 1997Imaging Science amp Technology Conference

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            What is Digital Image Processing

            f(xy) = intensity gray level of the image at spatial point (xy)

            x y f(xy) ndash finite discrete quantities -gt digital image

            Digital Image Processing = processing digital images by means of a digital computer

            A digital image is composed of a finite number of elements (location value of intensity)

            These elements are called picture elements image elements pels pixels

            ( )i j ijx y f

            3 f D

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Image processing is not limited to the visual band of the electromagnetic (EM) spectrum

            Image processing gamma to radio waves ultrasound electron microscopy computer-generated images

            image processing image analysis computer vision

            Image processing = discipline in which both the input and the output of a process are images

            Computer Vision = use computer to emulate human vision (AI) ndashlearning making inferences and take actions based on visual inputs

            Image analysis (image understanding) = segmentation partitioning images into regions or objects

            (link between image processing and image analysis)

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Distinction between image processing image analysis computer vision

            low-level mid-level high-level processes

            Low-level processes image preprocessing to reduce noise contrast enhancement image sharpening both input and output are images

            Mid-level processes segmentation partitioning images into regions or objects description of the objects for computer processing classificationrecognition of individual objects inputs are generally images outputs are attributes extracted from the input image (eg edges contours identity of individual objects)

            High-level processes ldquomaking senserdquo of a set of recognized objects performing the cognitive functions associated with vision

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Digital Image Processing (Gonzalez + Woods) =

            processes whose inputs and outputs are images +

            processes that extract attributes from images recognition of individual objects

            (low- and mid-level processes)

            Example

            automated analysis of text = acquiring an image containing text preprocessing the image (enhancement sharpening) extracting (segmenting) the individual characaters describing the characters in a form suitable for computer processing recognition of individual characters

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            The Origins of DIP

            Newspaper industry pictures were sent by submarine cable between London and New York

            Before Bartlane cable picture transmission system (early 1920s) ndash 1 week

            With Bartlane system less than 3 hours

            Specialized printing equipment coded pictures for cable transmission and reconstructed them at the receiving end (1920s -5 distict levels of gray 1929 ndash 15 levels)

            This example is not DIP the computer is not involved

            DIP is linked and devolps in the same rhythm as digital computers (data storage display and transmission)

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            A digital pictureproduced in 1921from a coded tapeby a telegraph printer withspecial type faces(McFarlane)

            A digital picturemade in 1922from a tapepunched after the signals hadcrossed the Atlantic twice(McFarlane)

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            1964 Jet Propulsion Laboratory (Pasadena California) processed pictures of the moon transmitted by Ranger 7 (corrected image distortions)

            The first picture of the moon by a US spacecraft Ranger 7 took this image July 31 1964 about 17 minutes before impacting the lunar surface(Courtesy of NASA)

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

            1970s ndash invention of CAT (computerized axial tomography)

            CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            loz geographers use DIP to study pollution patterns from aerial and satellite imagery

            loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

            loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

            loz astronomy biology nuclear medicine law enforcement industry

            DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

            loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Examples of Fields that Use DIP

            Images can be classified according to their sources (visual X-ray hellip)

            Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Electromagnetic waves can be thought as propagating sinusoidal

            waves of different wavelength or as a stream of massless particles

            each moving in a wavelike pattern with the speed of light Each

            massless particle contains a certain amount (bundle) of energy Each

            bundle of energy is called a photon If spectral bands are grouped

            according to energy per photon we obtain the spectrum shown in the

            image above ranging from gamma-rays (highest energy) to radio

            waves (lowest energy)

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Gamma-Ray Imaging

            Nuclear medicine astronomical observations

            Nuclear medicine

            the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

            Images are produced from the emissions collected by gamma-ray detectors

            Images of this sort are used to locate sites of bone pathology (infections tumors)

            PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Examples of gamma-ray imaging

            Bone scan PET image

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            X-ray imaging

            Medical diagnosticindustry astronomy

            A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

            The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Angiography = contrast-enhancement radiography

            Angiograms = images of blood vessels

            A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

            X-rays are used in CAT (computerized axial tomography)

            X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

            Industrial CAT scans are useful when the parts can be penetreted by X-rays

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Examples of X-ray imaging

            Chest X-rayAortic angiogram

            Head CT Cygnus LoopCircuit boards

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Imaging in the Ultraviolet Band

            Litography industrial inspection microscopy biological imaging astronomical observations

            Ultraviolet light is used in fluorescence microscopy

            Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

            other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

            and then it separates the much weaker radiating fluorescent light from the brighter excitation light

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Imaging in the Visible and Infrared Bands

            Light microscopy astronomy remote sensing industry law enforcement

            LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

            Weather observations and prediction produce major applications of multispectral image from satellites

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Satellite images of Washington DC area in spectral bands of the Table 1

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Examples of light microscopy

            Taxol (anticancer agent)magnified 250X

            Cholesterol(40X)

            Microprocessor(60X)

            Nickel oxidethin film(600X)

            Surface of audio CD(1750X)

            Organicsuperconductor(450X)

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Automated visual inspection of manufactured goods

            a bc de f

            a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Imaging in the Microwave Band

            The dominant aplication of imaging in the microwave band ndash radar

            bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

            bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

            bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

            An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Spaceborne radar image of mountains in southeast Tibet

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Imaging in the Radio Band

            medicine astronomy

            MRI = Magnetic Resonance Imaging

            This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

            Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

            The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            MRI images of a human knee (left) and spine (right)

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Images of the Crab Pulsar covering the electromagnetic spectrum

            Gamma X-ray Optical Infrared Radio

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Other Imaging Modalities

            acoustic imaging electron microscopy synthetic (computer-generated) imaging

            Imaging using sound geological explorations industry medicine

            Mineral and oil exploration

            For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Biometry - iris

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Biometry - fingerprint

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Face detection and recognition

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Gender identification

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Image morphing

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Fundamental Steps in DIP

            methods whose input and output are images

            methods whose inputs are images but whose outputs are attributes extracted from those images

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Outputs are images

            bull image acquisition

            bull image filtering and enhancement

            bull image restoration

            bull color image processing

            bull wavelets and multiresolution processing

            bull compression

            bull morphological processing

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Outputs are attributes

            bull morphological processing

            bull segmentation

            bull representation and description

            bull object recognition

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Image acquisition - may involve preprocessing such as scaling

            Image enhancement

            bull manipulating an image so that the result is more suitable than the original for a specific operation

            bull enhancement is problem oriented

            bull there is no general sbquotheoryrsquo of image enhancement

            bull enhancement use subjective methods for image emprovement

            bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Image restoration

            bull improving the appearance of an image

            bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

            Color image processing

            bull fundamental concept in color models

            bull basic color processing in a digital domain

            Wavelets and multiresolution processing

            representing images in various degree of resolution

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Compression

            reducing the storage required to save an image or the bandwidth required to transmit it

            Morphological processing

            bull tools for extracting image components that are useful in the representation and description of shape

            bull a transition from processes that output images to processes that outputimage attributes

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Segmentation

            bull partitioning an image into its constituents parts or objects

            bull autonomous segmentation is one of the most difficult tasks of DIP

            bull the more accurate the segmentation the more likley recognition is to succeed

            Representation and description (almost always follows segmentation)

            bull segmentation produces either the boundary of a region or all the poits in the region itself

            bull converting the data produced by segmentation to a form suitable for computer processing

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            bull boundary representation the focus is on external shape characteristics such as corners or inflections

            bull complete region the focus is on internal properties such as texture or skeletal shape

            bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

            Object recognition

            the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

            Knowledge database

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Simplified diagramof a cross sectionof the human eye

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

            The cornea is a tough transparent tissue that covers the anterior surface of the eye

            Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

            The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

            The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

            Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

            Fovea = the place where the image of the object of interest falls on

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

            Blind spot region without receptors

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Image formation in the eye

            Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

            Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

            distance between lens and retina along visual axix = 17 mm

            range of focal length = 14 mm to 17 mm

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Optical illusions

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

            gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

            quantities that describe the quality of a chromatic light source radiance

            the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

            measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

            brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            the physical meaning is determined by the source of the image

            ( )f D f x y

            Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

            f(xy) ndash characterized by two components

            i(xy) = illumination component the amount of source illumination incident on the scene being viewed

            r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

            ( ) ( ) ( )

            0 ( ) 0 ( ) 1

            f x y i x y r x y

            i x y r x y

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            r(xy)=0 - total absorption r(xy)=1 - total reflectance

            i(xy) ndash determined by the illumination source

            r(xy) ndash determined by the characteristics of the imaged objects

            is called gray (or intensity) scale

            In practice

            min 0 0 max min min min max max max( ) L l f x y L L i r L i r

            indoor values without additional illuminationmin max10 1000L L

            black whitemin max0 1 0 1 0 1L L L L l l L

            min maxL L

            Digital Image ProcessingDigital Image Processing

            Week 1Week 1

            Digital Image Processing

            Week 1

            Image Sampling and Quantization

            - the output of the sensors is a continuous voltage waveform related to the sensed

            scene

            converting a continuous image f to digital form

            - digitizing (x y) is called sampling

            - digitizing f(x y) is called quantization

            Digital Image Processing

            Week 1

            Digital Image Processing

            Week 1

            Continuous image projected onto a sensor array Result of image sampling and quantization

            Digital Image Processing

            Week 1

            Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

            (00) (01) (0 1)(10) (11) (1 1)

            ( )

            ( 10) ( 11) ( 1 1)

            f f f Nf f f N

            f x y

            f M f M f M N

            image element pixel

            00 01 0 1

            10 11 1 1

            10 11 1 1

            ( ) ( )

            N

            i jN M N

            i j

            M M M N

            a a aa f x i y j f i ja a a

            Aa

            a a a

            f(00) ndash the upper left corner of the image

            Digital Image Processing

            Week 1

            M N ge 0 L=2k

            [0 1]i j i ja a L

            Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

            Digital Image Processing

            Week 1

            Digital Image Processing

            Week 1

            Number of bits required to store a digitized image

            for 2 b M N k M N b N k

            When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

            Digital Image Processing

            Week 1

            Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

            Measures line pairs per unit distance dots (pixels) per unit distance

            Image resolution = the largest number of discernible line pairs per unit distance

            (eg 100 line pairs per mm)

            Dots per unit distance are commonly used in printing and publishing

            In US the measure is expressed in dots per inch (dpi)

            (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

            Intensity resolution ndash the smallest discernible change in intensity level

            The number of intensity levels (L) is determined by hardware considerations

            L=2k ndash most common k = 8

            Intensity resolution in practice is given by k (number of bits used to quantize intensity)

            Digital Image Processing

            Week 1

            Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

            150 dpi (lower left) 72 dpi (lower right)

            Digital Image Processing

            Week 1

            Reducing the number of gray levels 256 128 64 32

            Digital Image Processing

            Week 1

            Reducing the number of gray levels 16 8 4 2

            Digital Image Processing

            Week 1

            Image Interpolation - used in zooming shrinking rotating and geometric corrections

            Shrinking zooming ndash image resizing ndash image resampling methods

            Interpolation is the process of using known data to estimate values at unknown locations

            Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

            750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

            same spacing as the original and then shrink it so that it fits exactly over the original

            image The pixel spacing in the 750 times 750 grid will be less than in the original image

            Problem assignment of intensity-level in the new 750 times 750 grid

            Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

            intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

            This technique has the tendency to produce undesirable effects like severe distortion of

            straight edges

            Digital Image Processing

            Week 1

            Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

            where the four coefficients are determined from the 4 equations in 4 unknowns that can

            be written using the 4 nearest neighbors of point (x y)

            Bilinear interpolation gives much better results than nearest neighbor interpolation with a

            modest increase in computational effort

            Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

            nearest neighbors of the point 3 3

            0 0

            ( ) i ji j

            i jv x y c x y

            The coefficients cij are obtained solving a 16x16 linear system

            intensity levels of the 16 nearest neighbors of 3 3

            0 0

            ( )i ji j

            i jc x y x y

            Digital Image Processing

            Week 1

            Generally bicubic interpolation does a better job of preserving fine detail than the

            bilinear technique Bicubic interpolation is the standard used in commercial image editing

            programs such as Adobe Photoshop and Corel Photopaint

            Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

            the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

            then zooming the reduced image back to its original size To generate Fig 1(d) nearest

            neighbor interpolation was used (both for shrinking and zooming)

            Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

            interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

            from 1250 dpi to 150 dpi (instead of 72 dpi)

            Digital Image Processing

            Week 1

            Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

            Digital Image Processing

            Week 1

            Neighbors of a Pixel

            A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

            This set of pixels called the 4-neighbors of p denoted by N4 (p)

            The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

            and are denoted ND(p)

            The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

            N8 (p)

            If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

            fall outside the image

            Digital Image Processing

            Week 1

            Adjacency Connectivity Regions Boundaries

            Denote by V the set of intensity levels used to define adjacency

            - in a binary image V 01 (V=0 V=1)

            - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

            We consider 3 types of adjacency

            (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

            (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

            (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

            m-adjacent if

            4( )q N p or

            ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

            Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

            ambiguities that often arise when 8-adjacency is used Consider the example

            Digital Image Processing

            Week 1

            binary image

            0 1 1 0 1 1 0 1 1

            1 0 1 0 0 1 0 0 1 0

            0 0 1 0 0 1 0 0 1

            V

            The three pixels at the top (first line) in the above example show multiple (ambiguous)

            8-adjacency as indicated by the dashed lines This ambiguity is removed by using

            m-adjacency

            Digital Image Processing

            Week 1

            A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

            is a sequence of distinct pixels with coordinates

            and are adjacent 0 0 1 1

            1 1

            ( ) ( ) ( ) ( ) ( )( ) ( ) 12

            n n

            i i i i

            x y x y x y x y s tx y x y i n

            The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

            Depending on the type of adjacency considered the paths are 4- 8- or m-paths

            Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

            in S if there exists a path between them consisting only of pixels from S

            S is a connected set if there is a path in S between any 2 pixels in S

            Let R be a subset of pixels in an image R is a region of the image if R is a connected set

            Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

            that are not adjacent are said to be disjoint When referring to regions only 4- and

            8-adjacency are considered

            Digital Image Processing

            Week 1

            Suppose that an image contains K disjoint regions 1 kR k K none of which

            touches the image border

            the complement of 1

            ( )K

            cu k u u

            k

            R R R R

            We call all the points in Ru the foreground of the image and the points in ( )cuR the

            background of the image

            The boundary (border or contour) of a region R is the set of points that are adjacent to

            points in the complement of R (R)c The border of an image is the set of pixels in the

            region that have at least one background neighbor This definition is referred to as the

            inner border to distinguish it from the notion of outer border which is the corresponding

            border in the background

            Digital Image Processing

            Week 1

            Distance measures

            For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

            function or metric if

            (a) D(p q) ge 0 D(p q) = 0 iff p=q

            (b) D(p q) = D(q p)

            (c) D(p z) le D(p q) + D(q z)

            The Euclidean distance between p and q is defined as 1

            2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

            The pixels q for which ( )eD p q r are the points contained in a disk of radius r

            centered at (x y)

            Digital Image Processing

            Week 1

            The D4 distance (also called city-block distance) between p and q is defined as

            4( ) | | | |D p q x s y t

            The pixels q for which 4( )D p q r form a diamond centered at (xy)

            4

            22 1 2

            2 2 1 0 1 22 1 2

            2

            D

            The pixels with D4 = 1 are the 4-neighbors of (x y)

            The D8 distance (called the chessboard distance) between p and q is defined as

            8( ) max| | | |D p q x s y t

            The pixels q for which 8( )D p q r form a square centered at (x y)

            Digital Image Processing

            Week 1

            8

            2 2 2 2 22 1 1 1 2

            2 2 1 0 1 22 1 1 1 22 2 2 2 2

            D

            The pixels with D8 = 1 are the 8-neighbors of (x y)

            D4 and D8 distances are independent of any paths that might exist between p and q

            because these distances involve only the coordinates of the point

            Digital Image Processing

            Week 1

            Array versus Matrix Operations

            An array operation involving one or more images is carried out on a pixel-by-pixel basis

            11 12 11 12

            21 22 21 22

            a a b ba a b b

            Array product

            11 12 11 12 11 11 12 12

            21 22 21 22 21 21 22 21

            a a b b a b a ba a b b a b a b

            Matrix product

            11 12 11 12 11 11 12 21 11 12 12 21

            21 22 21 22 21 11 22 21 21 12 22 22

            a a b b a b a b a b a ba a b b a b a b a b a b

            We assume array operations unless stated otherwise

            Digital Image Processing

            Week 1

            Linear versus Nonlinear Operations

            One of the most important classifications of image-processing methods is whether it is

            linear or nonlinear

            ( ) ( )H f x y g x y

            H is said to be a linear operator if

            images1 2 1 2

            1 2

            ( ) ( ) ( ) ( )

            H a f x y b f x y a H f x y b H f x y

            a b f f

            Example of nonlinear operator

            the maximum value of the pixels of image max ( )H f f x y f

            1 2

            0 2 6 5 1 1

            2 3 4 7f f a b

            Digital Image Processing

            Week 1

            1 2

            0 2 6 5 6 3max max 1 ( 1) max 2

            2 3 4 7 2 4a f b f

            0 2 6 51 max ( 1) max 3 ( 1)7 4

            2 3 4 7

            Arithmetic Operations in Image Processing

            Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

            f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

            For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

            value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

            The two random variables are uncorrelated when their covariance is 0

            Digital Image Processing

            Week 1

            Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

            used in image enhancement)

            1

            1( ) ( )K

            ii

            g x y g x yK

            If the noise satisfies the properties stated above we have

            2 2( ) ( )

            1( ) ( ) g x y x yE g x y f x yK

            ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

            and g respectively The standard deviation (square root of the variance) at any point in

            the average image is

            ( ) ( )1

            g x y x yK

            Digital Image Processing

            Week 1

            As K increases the variability (as measured by the variance or the standard deviation) of

            the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

            means that ( )g x y approaches f(x y) as the number of noisy images used in the

            averaging process increases

            An important application of image averaging is in the field of astronomy where imaging

            under very low light levels frequently causes sensor noise to render single images

            virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

            was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

            64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

            images respectively

            Digital Image Processing

            Week 1

            Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

            100 noisy images

            a b c d e f

            Digital Image Processing

            Week 1

            A frequent application of image subtraction is in the enhancement of differences between

            images

            (a) (b) (c)

            Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

            significant bit of each pixel (c) the difference between the two images

            Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

            Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

            images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

            difference between images (a) and (b)

            Digital Image Processing

            Week 1

            Mask mode radiography ( ) ( ) ( )g x y f x y h x y

            h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

            intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

            source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

            bloodstream taking a series of images called live images (denoted f(x y)) of the same

            anatomical region as h(x y) and subtracting the mask from the series of incoming live

            images after injection of the contrast medium

            In g(x y) we can find the differences between h and f as enhanced detail

            Images being captured at TV rates we obtain a movie showing how the contrast medium

            propagates through the various arteries in the area being observed

            Digital Image Processing

            Week 1

            a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

            Digital Image Processing

            Week 1

            An important application of image multiplication (and division) is shading correction

            Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

            f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

            When the shading function is known

            ( )( )( )

            g x yf x yh x y

            h(x y) is unknown but we have access to the imaging system we can obtain an

            approximation to the shading function by imaging a target of constant intensity When the

            sensor is not available often the shading pattern can be estimated from the image

            Digital Image Processing

            Week 1

            (a) (b) (c)

            Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

            Digital Image Processing

            Week 1

            Another use of image multiplication is in masking also called region of interest (ROI)

            operations The process consists of multiplying a given image by a mask image that has

            1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

            image and the shape of the ROI can be arbitrary but usually is a rectangular shape

            (a) (b) (c)

            Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

            Digital Image Processing

            Week 1

            In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

            min( )mf f f

            0 ( 255)max( )

            ms

            m

            ff K K K

            f

            Digital Image Processing

            Week 1

            Spatial Operations

            - are performed directly on the pixels of a given image

            There are three categories of spatial operations

            single-pixel operations

            neighborhood operations

            geometric spatial transformations

            Single-pixel operations

            - change the values of intensity for the individual pixels ( )s T z

            where z is the intensity of a pixel in the original image and s is the intensity of the

            corresponding pixel in the processed image

            Digital Image Processing

            Week 1

            Neighborhood operations

            Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

            in an image f Neighborhood processing generates new intensity level at point (x y)

            based on the values of the intensities of the points in Sxy For example if Sxy is a

            rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

            intensity by computing the average value of the pixels in Sxy

            ( )

            1( ) ( )xyr c S

            g x y f r cm n

            The net effect is to perform local blurring in the original image This type of process is

            used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

            largest region of an image

            Digital Image Processing

            Week 1

            Geometric spatial transformations and image registration

            - modify the spatial relationship between pixels in an image

            - these transformations are often called rubber-sheet transformations (analogous to

            printing an image on a sheet of rubber and then stretching the sheet according to a

            predefined set of rules

            A geometric transformation consists of 2 basic operations

            1 a spatial transformation of coordinates

            2 intensity interpolation that assign intensity values to the spatial transformed

            pixels

            The coordinate system transformation ( ) [( )]x y T v w

            (v w) ndash pixel coordinates in the original image

            (x y) ndash pixel coordinates in the transformed image

            Digital Image Processing

            Week 1

            [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

            Affine transform

            11 1211 21 31

            21 2212 22 33

            31 32

            0[ 1] [ 1] [ 1] 0

            1

            t tx t v t w t

            x y v w T v w t ty t v t w t

            t t

            (AT)

            This transform can scale rotate translate or shear a set of coordinate points depending

            on the elements of the matrix T If we want to resize an image rotate it and move the

            result to some location we simply form a 3x3 matrix equal to the matrix product of the

            scaling rotation and translation matrices from Table 1

            Digital Image Processing

            Week 1

            Affine transformations

            Digital Image Processing

            Week 1

            The preceding transformations relocate pixels on an image to new locations To complete

            the process we have to assign intensity values to those locations This task is done by

            using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

            In practice we can use equation (AT) in two basic ways

            forward mapping scan the pixels of the input image (v w) compute the new spatial

            location (x y) of the corresponding pixel in the new image using (AT) directly

            Problems

            - intensity assignment when 2 or more pixels in the original image are transformed to

            the same location in the output image

            - some output locations have no correspondent in the original image (no intensity

            assignment)

            Digital Image Processing

            Week 1

            inverse mapping scans the output pixel locations and at each location (x y)

            computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

            It then interpolates among the nearest input pixels to determine the intensity of the output

            pixel value

            Inverse mappings are more efficient to implement than forward mappings and are used in

            numerous commercial implementations of spatial transformations (MATLAB for ex)

            Digital Image Processing

            Week 1

            Digital Image Processing

            Week 1

            Image registration ndash align two or more images of the same scene

            In image registration we have available the input and output images but the specific

            transformation that produced the output image from the input is generally unknown

            The problem is to estimate the transformation function and then use it to register the two

            images

            - it may be of interest to align (register) two or more image taken at approximately the

            same time but using different imaging systems (MRI scanner and a PET scanner)

            - align images of a given location taken by the same instrument at different moments

            of time (satellite images)

            Solving the problem using tie points (also called control points) which are

            corresponding points whose locations are known precisely in the input and reference

            image

            Digital Image Processing

            Week 1

            How to select tie points

            - interactively selecting them

            - use of algorithms that try to detect these points

            - some imaging systems have physical artifacts (small metallic objects) embedded in

            the imaging sensors These objects produce a set of known points (called reseau

            marks) directly on all images captured by the system which can be used as guides

            for establishing tie points

            The problem of estimating the transformation is one of modeling Suppose we have a set

            of 4 tie points both on the input image and the reference image A simple model based on

            a bilinear approximation is given by

            1 2 3 4

            5 6 7 8

            x c v c w c v w cy c v c w c v w c

            (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

            Digital Image Processing

            Week 1

            When 4 tie points are insufficient to obtain satisfactory registration an approach used

            frequently is to select a larger number of tie points and using this new set of tie points

            subdivide the image in rectangular regions marked by groups of 4 tie points On the

            subregions marked by 4 tie points we applied the transformation model described above

            The number of tie points and the sophistication of the model required to solve the register

            problem depend on the severity of the geometrical distortion

            Digital Image Processing

            Week 1

            a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

            Digital Image Processing

            Week 1

            Probabilistic Methods

            zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

            p(zk) = the probability that the intensity level zk occurs in the given image

            ( ) kk

            np zM N

            nk = the number of times that intensity zk occurs in the image (MN is the total number of

            pixels in the image) 1

            0( ) 1

            L

            kk

            p z

            The mean (average) intensity of an image is given by 1

            0( )

            L

            k kk

            m z p z

            Digital Image Processing

            Week 1

            The variance of the intensities is 1

            2 2

            0( ) ( )

            L

            k kk

            z m p z

            The variance is a measure of the spread of the values of z about the mean so it is a

            measure of image contrast Usually for measuring image contrast the standard deviation

            ( ) is used

            The n-th moment of a random variable z about the mean is defined as 1

            0( ) ( ) ( )

            Ln

            n k kk

            z z m p z

            ( 20 1 2( ) 1 ( ) 0 ( )z z z )

            3( ) 0z the intensities are biased to values higher than the mean

            ( 3( ) 0z the intensities are biased to values lower than the mean

            Digital Image Processing

            Week 1

            3( ) 0z the intensities are distributed approximately equally on both side of the

            mean

            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

            Figure 1(a) ndash standard deviation 143 (variance = 2045)

            Figure 1(b) ndash standard deviation 316 (variance = 9986)

            Figure 1(c) ndash standard deviation 492 (variance = 24206)

            Digital Image Processing

            Week 1

            Intensity Transformations and Spatial Filtering

            ( ) ( )g x y T f x y

            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

            neighborhood of (x y)

            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

            and much smaller in size than the image

            Digital Image Processing

            Week 1

            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

            called spatial filter (spatial mask kernel template or window)

            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

            ( )s T r

            s and r are denoting respectively the intensity of g and f at (x y)

            Figure 2 left - T produces an output image of higher contrast than the original by

            darkening the intensity levels below k and brightening the levels above k ndash this technique

            is called contrast stretching

            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

            Digital Image Processing

            Week 1

            Figure 2 right - T produces a binary output image A mapping of this form is called

            thresholding function

            Some Basic Intensity Transformation Functions

            Image Negatives

            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

            - equivalent of a photographic negative

            - technique suited for enhancing white or gray detail embedded in dark regions of an

            image

            Digital Image Processing

            Week 1

            Original Negative image

            Digital Image Processing

            Week 1

            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

            Some basic intensity transformation functions

            Digital Image Processing

            Week 1

            This transformation maps a narrow range of low intensity values in the input into a wider

            range An operator of this type is used to expand the values of dark pixels in an image

            while compressing the higher-level values The opposite is true for the inverse log

            transformation The log functions compress the dynamic range of images with large

            variations in pixel values

            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

            Digital Image Processing

            Week 1

            Power-Law (Gamma) Transformations

            - positive constants( ) ( ( ) )s T r c r c s c r

            Plots of gamma transformation for different values of γ (c=1)

            Digital Image Processing

            Week 1

            Power-law curves with 1 map a narrow range of dark input values into a wider range

            of output values with the opposite being true for higher values of input values The

            curves with 1 have the opposite effect of those generated with values of 1

            1c - identity transformation

            A variety of devices used for image capture printing and display respond according to a

            power law The process used to correct these power-law response phenomena is called

            gamma correction

            Digital Image Processing

            Week 1

            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

            Digital Image Processing

            Week 1

            Piecewise-Linear Transformations Functions

            Contrast stretching

            - a process that expands the range of intensity levels in an image so it spans the full

            intensity range of the recording tool or display device

            a b c d Fig5

            Digital Image Processing

            Week 1

            11

            1

            2 1 1 21 2

            2 1 2 1

            22

            2

            [0 ]

            ( ) ( )( ) [ ]( ) ( )

            ( 1 ) [ 1]( 1 )

            s r r rrs r r s r rT r r r r

            r r r rs L r r r L

            L r

            Digital Image Processing

            Week 1

            1 1 2 2r s r s identity transformation (no change)

            1 2 1 2 0 1r r s s L thresholding function

            Figure 5(b) shows an 8-bit image with low contrast

            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

            in the image respectively Thus the transformation function stretched the levels linearly

            from their original range to the full range [0 L-1]

            Figure 5(d) - the thresholding function was used with 1 1 0r s m

            2 2 1r s m L where m is the mean gray level in the image

            The original image on which these results are based is a scanning electron microscope

            image of pollen magnified approximately 700 times

            Digital Image Processing

            Week 1

            Intensity-level slicing

            - highlighting a specific range of intensities in an image

            There are two approaches for intensity-level slicing

            1 display in one value (white for example) all the values in the range of interest and in

            another (say black) all other intensities (Figure 311 (a))

            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

            intensities in the image (Figure 311 (b))

            Digital Image Processing

            Week 1

            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

            highlight the major blood vessels that appear brighter as a result of injecting a contrast

            medium Figure 6(middle) shows the result of applying the first technique for a band near

            the top of the scale of intensities This type of enhancement produces a binary image

            Highlights intensity range [A B] and reduces all other intensities to a lower level

            Highlights range [A B] and preserves all other intensities

            Digital Image Processing

            Week 1

            which is useful for studying the shape of the flow of the contrast substance (to detect

            blockageshellip)

            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

            image around the mean intensity was set to black the other intensities remain unchanged

            Fig 6 - Aortic angiogram and intensity sliced versions

            Digital Image Processing

            Week 1

            Bit-plane slicing

            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

            This technique highlights the contribution made to the whole image appearances by each

            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

            Digital Image Processing

            Week 1

            Digital Image Processing

            Week 1

            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

            • DIP 1 2017
            • DIP 02 (2017)

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Lenna Soderberg (Sjoumloumlblom) and Jeff Seideman taken in May 1997Imaging Science amp Technology Conference

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              What is Digital Image Processing

              f(xy) = intensity gray level of the image at spatial point (xy)

              x y f(xy) ndash finite discrete quantities -gt digital image

              Digital Image Processing = processing digital images by means of a digital computer

              A digital image is composed of a finite number of elements (location value of intensity)

              These elements are called picture elements image elements pels pixels

              ( )i j ijx y f

              3 f D

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Image processing is not limited to the visual band of the electromagnetic (EM) spectrum

              Image processing gamma to radio waves ultrasound electron microscopy computer-generated images

              image processing image analysis computer vision

              Image processing = discipline in which both the input and the output of a process are images

              Computer Vision = use computer to emulate human vision (AI) ndashlearning making inferences and take actions based on visual inputs

              Image analysis (image understanding) = segmentation partitioning images into regions or objects

              (link between image processing and image analysis)

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Distinction between image processing image analysis computer vision

              low-level mid-level high-level processes

              Low-level processes image preprocessing to reduce noise contrast enhancement image sharpening both input and output are images

              Mid-level processes segmentation partitioning images into regions or objects description of the objects for computer processing classificationrecognition of individual objects inputs are generally images outputs are attributes extracted from the input image (eg edges contours identity of individual objects)

              High-level processes ldquomaking senserdquo of a set of recognized objects performing the cognitive functions associated with vision

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Digital Image Processing (Gonzalez + Woods) =

              processes whose inputs and outputs are images +

              processes that extract attributes from images recognition of individual objects

              (low- and mid-level processes)

              Example

              automated analysis of text = acquiring an image containing text preprocessing the image (enhancement sharpening) extracting (segmenting) the individual characaters describing the characters in a form suitable for computer processing recognition of individual characters

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              The Origins of DIP

              Newspaper industry pictures were sent by submarine cable between London and New York

              Before Bartlane cable picture transmission system (early 1920s) ndash 1 week

              With Bartlane system less than 3 hours

              Specialized printing equipment coded pictures for cable transmission and reconstructed them at the receiving end (1920s -5 distict levels of gray 1929 ndash 15 levels)

              This example is not DIP the computer is not involved

              DIP is linked and devolps in the same rhythm as digital computers (data storage display and transmission)

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              A digital pictureproduced in 1921from a coded tapeby a telegraph printer withspecial type faces(McFarlane)

              A digital picturemade in 1922from a tapepunched after the signals hadcrossed the Atlantic twice(McFarlane)

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              1964 Jet Propulsion Laboratory (Pasadena California) processed pictures of the moon transmitted by Ranger 7 (corrected image distortions)

              The first picture of the moon by a US spacecraft Ranger 7 took this image July 31 1964 about 17 minutes before impacting the lunar surface(Courtesy of NASA)

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

              1970s ndash invention of CAT (computerized axial tomography)

              CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              loz geographers use DIP to study pollution patterns from aerial and satellite imagery

              loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

              loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

              loz astronomy biology nuclear medicine law enforcement industry

              DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

              loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Examples of Fields that Use DIP

              Images can be classified according to their sources (visual X-ray hellip)

              Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Electromagnetic waves can be thought as propagating sinusoidal

              waves of different wavelength or as a stream of massless particles

              each moving in a wavelike pattern with the speed of light Each

              massless particle contains a certain amount (bundle) of energy Each

              bundle of energy is called a photon If spectral bands are grouped

              according to energy per photon we obtain the spectrum shown in the

              image above ranging from gamma-rays (highest energy) to radio

              waves (lowest energy)

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Gamma-Ray Imaging

              Nuclear medicine astronomical observations

              Nuclear medicine

              the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

              Images are produced from the emissions collected by gamma-ray detectors

              Images of this sort are used to locate sites of bone pathology (infections tumors)

              PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Examples of gamma-ray imaging

              Bone scan PET image

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              X-ray imaging

              Medical diagnosticindustry astronomy

              A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

              The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Angiography = contrast-enhancement radiography

              Angiograms = images of blood vessels

              A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

              X-rays are used in CAT (computerized axial tomography)

              X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

              Industrial CAT scans are useful when the parts can be penetreted by X-rays

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Examples of X-ray imaging

              Chest X-rayAortic angiogram

              Head CT Cygnus LoopCircuit boards

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Imaging in the Ultraviolet Band

              Litography industrial inspection microscopy biological imaging astronomical observations

              Ultraviolet light is used in fluorescence microscopy

              Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

              other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

              and then it separates the much weaker radiating fluorescent light from the brighter excitation light

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Imaging in the Visible and Infrared Bands

              Light microscopy astronomy remote sensing industry law enforcement

              LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

              Weather observations and prediction produce major applications of multispectral image from satellites

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Satellite images of Washington DC area in spectral bands of the Table 1

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Examples of light microscopy

              Taxol (anticancer agent)magnified 250X

              Cholesterol(40X)

              Microprocessor(60X)

              Nickel oxidethin film(600X)

              Surface of audio CD(1750X)

              Organicsuperconductor(450X)

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Automated visual inspection of manufactured goods

              a bc de f

              a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Imaging in the Microwave Band

              The dominant aplication of imaging in the microwave band ndash radar

              bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

              bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

              bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

              An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Spaceborne radar image of mountains in southeast Tibet

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Imaging in the Radio Band

              medicine astronomy

              MRI = Magnetic Resonance Imaging

              This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

              Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

              The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              MRI images of a human knee (left) and spine (right)

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Images of the Crab Pulsar covering the electromagnetic spectrum

              Gamma X-ray Optical Infrared Radio

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Other Imaging Modalities

              acoustic imaging electron microscopy synthetic (computer-generated) imaging

              Imaging using sound geological explorations industry medicine

              Mineral and oil exploration

              For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Biometry - iris

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Biometry - fingerprint

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Face detection and recognition

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Gender identification

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Image morphing

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Fundamental Steps in DIP

              methods whose input and output are images

              methods whose inputs are images but whose outputs are attributes extracted from those images

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Outputs are images

              bull image acquisition

              bull image filtering and enhancement

              bull image restoration

              bull color image processing

              bull wavelets and multiresolution processing

              bull compression

              bull morphological processing

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Outputs are attributes

              bull morphological processing

              bull segmentation

              bull representation and description

              bull object recognition

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Image acquisition - may involve preprocessing such as scaling

              Image enhancement

              bull manipulating an image so that the result is more suitable than the original for a specific operation

              bull enhancement is problem oriented

              bull there is no general sbquotheoryrsquo of image enhancement

              bull enhancement use subjective methods for image emprovement

              bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Image restoration

              bull improving the appearance of an image

              bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

              Color image processing

              bull fundamental concept in color models

              bull basic color processing in a digital domain

              Wavelets and multiresolution processing

              representing images in various degree of resolution

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Compression

              reducing the storage required to save an image or the bandwidth required to transmit it

              Morphological processing

              bull tools for extracting image components that are useful in the representation and description of shape

              bull a transition from processes that output images to processes that outputimage attributes

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Segmentation

              bull partitioning an image into its constituents parts or objects

              bull autonomous segmentation is one of the most difficult tasks of DIP

              bull the more accurate the segmentation the more likley recognition is to succeed

              Representation and description (almost always follows segmentation)

              bull segmentation produces either the boundary of a region or all the poits in the region itself

              bull converting the data produced by segmentation to a form suitable for computer processing

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              bull boundary representation the focus is on external shape characteristics such as corners or inflections

              bull complete region the focus is on internal properties such as texture or skeletal shape

              bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

              Object recognition

              the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

              Knowledge database

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Simplified diagramof a cross sectionof the human eye

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

              The cornea is a tough transparent tissue that covers the anterior surface of the eye

              Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

              The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

              The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

              Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

              Fovea = the place where the image of the object of interest falls on

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

              Blind spot region without receptors

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Image formation in the eye

              Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

              Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

              distance between lens and retina along visual axix = 17 mm

              range of focal length = 14 mm to 17 mm

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Optical illusions

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

              gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

              quantities that describe the quality of a chromatic light source radiance

              the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

              measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

              brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              the physical meaning is determined by the source of the image

              ( )f D f x y

              Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

              f(xy) ndash characterized by two components

              i(xy) = illumination component the amount of source illumination incident on the scene being viewed

              r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

              ( ) ( ) ( )

              0 ( ) 0 ( ) 1

              f x y i x y r x y

              i x y r x y

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              r(xy)=0 - total absorption r(xy)=1 - total reflectance

              i(xy) ndash determined by the illumination source

              r(xy) ndash determined by the characteristics of the imaged objects

              is called gray (or intensity) scale

              In practice

              min 0 0 max min min min max max max( ) L l f x y L L i r L i r

              indoor values without additional illuminationmin max10 1000L L

              black whitemin max0 1 0 1 0 1L L L L l l L

              min maxL L

              Digital Image ProcessingDigital Image Processing

              Week 1Week 1

              Digital Image Processing

              Week 1

              Image Sampling and Quantization

              - the output of the sensors is a continuous voltage waveform related to the sensed

              scene

              converting a continuous image f to digital form

              - digitizing (x y) is called sampling

              - digitizing f(x y) is called quantization

              Digital Image Processing

              Week 1

              Digital Image Processing

              Week 1

              Continuous image projected onto a sensor array Result of image sampling and quantization

              Digital Image Processing

              Week 1

              Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

              (00) (01) (0 1)(10) (11) (1 1)

              ( )

              ( 10) ( 11) ( 1 1)

              f f f Nf f f N

              f x y

              f M f M f M N

              image element pixel

              00 01 0 1

              10 11 1 1

              10 11 1 1

              ( ) ( )

              N

              i jN M N

              i j

              M M M N

              a a aa f x i y j f i ja a a

              Aa

              a a a

              f(00) ndash the upper left corner of the image

              Digital Image Processing

              Week 1

              M N ge 0 L=2k

              [0 1]i j i ja a L

              Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

              Digital Image Processing

              Week 1

              Digital Image Processing

              Week 1

              Number of bits required to store a digitized image

              for 2 b M N k M N b N k

              When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

              Digital Image Processing

              Week 1

              Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

              Measures line pairs per unit distance dots (pixels) per unit distance

              Image resolution = the largest number of discernible line pairs per unit distance

              (eg 100 line pairs per mm)

              Dots per unit distance are commonly used in printing and publishing

              In US the measure is expressed in dots per inch (dpi)

              (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

              Intensity resolution ndash the smallest discernible change in intensity level

              The number of intensity levels (L) is determined by hardware considerations

              L=2k ndash most common k = 8

              Intensity resolution in practice is given by k (number of bits used to quantize intensity)

              Digital Image Processing

              Week 1

              Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

              150 dpi (lower left) 72 dpi (lower right)

              Digital Image Processing

              Week 1

              Reducing the number of gray levels 256 128 64 32

              Digital Image Processing

              Week 1

              Reducing the number of gray levels 16 8 4 2

              Digital Image Processing

              Week 1

              Image Interpolation - used in zooming shrinking rotating and geometric corrections

              Shrinking zooming ndash image resizing ndash image resampling methods

              Interpolation is the process of using known data to estimate values at unknown locations

              Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

              750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

              same spacing as the original and then shrink it so that it fits exactly over the original

              image The pixel spacing in the 750 times 750 grid will be less than in the original image

              Problem assignment of intensity-level in the new 750 times 750 grid

              Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

              intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

              This technique has the tendency to produce undesirable effects like severe distortion of

              straight edges

              Digital Image Processing

              Week 1

              Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

              where the four coefficients are determined from the 4 equations in 4 unknowns that can

              be written using the 4 nearest neighbors of point (x y)

              Bilinear interpolation gives much better results than nearest neighbor interpolation with a

              modest increase in computational effort

              Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

              nearest neighbors of the point 3 3

              0 0

              ( ) i ji j

              i jv x y c x y

              The coefficients cij are obtained solving a 16x16 linear system

              intensity levels of the 16 nearest neighbors of 3 3

              0 0

              ( )i ji j

              i jc x y x y

              Digital Image Processing

              Week 1

              Generally bicubic interpolation does a better job of preserving fine detail than the

              bilinear technique Bicubic interpolation is the standard used in commercial image editing

              programs such as Adobe Photoshop and Corel Photopaint

              Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

              the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

              then zooming the reduced image back to its original size To generate Fig 1(d) nearest

              neighbor interpolation was used (both for shrinking and zooming)

              Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

              interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

              from 1250 dpi to 150 dpi (instead of 72 dpi)

              Digital Image Processing

              Week 1

              Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

              Digital Image Processing

              Week 1

              Neighbors of a Pixel

              A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

              This set of pixels called the 4-neighbors of p denoted by N4 (p)

              The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

              and are denoted ND(p)

              The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

              N8 (p)

              If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

              fall outside the image

              Digital Image Processing

              Week 1

              Adjacency Connectivity Regions Boundaries

              Denote by V the set of intensity levels used to define adjacency

              - in a binary image V 01 (V=0 V=1)

              - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

              We consider 3 types of adjacency

              (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

              (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

              (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

              m-adjacent if

              4( )q N p or

              ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

              Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

              ambiguities that often arise when 8-adjacency is used Consider the example

              Digital Image Processing

              Week 1

              binary image

              0 1 1 0 1 1 0 1 1

              1 0 1 0 0 1 0 0 1 0

              0 0 1 0 0 1 0 0 1

              V

              The three pixels at the top (first line) in the above example show multiple (ambiguous)

              8-adjacency as indicated by the dashed lines This ambiguity is removed by using

              m-adjacency

              Digital Image Processing

              Week 1

              A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

              is a sequence of distinct pixels with coordinates

              and are adjacent 0 0 1 1

              1 1

              ( ) ( ) ( ) ( ) ( )( ) ( ) 12

              n n

              i i i i

              x y x y x y x y s tx y x y i n

              The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

              Depending on the type of adjacency considered the paths are 4- 8- or m-paths

              Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

              in S if there exists a path between them consisting only of pixels from S

              S is a connected set if there is a path in S between any 2 pixels in S

              Let R be a subset of pixels in an image R is a region of the image if R is a connected set

              Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

              that are not adjacent are said to be disjoint When referring to regions only 4- and

              8-adjacency are considered

              Digital Image Processing

              Week 1

              Suppose that an image contains K disjoint regions 1 kR k K none of which

              touches the image border

              the complement of 1

              ( )K

              cu k u u

              k

              R R R R

              We call all the points in Ru the foreground of the image and the points in ( )cuR the

              background of the image

              The boundary (border or contour) of a region R is the set of points that are adjacent to

              points in the complement of R (R)c The border of an image is the set of pixels in the

              region that have at least one background neighbor This definition is referred to as the

              inner border to distinguish it from the notion of outer border which is the corresponding

              border in the background

              Digital Image Processing

              Week 1

              Distance measures

              For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

              function or metric if

              (a) D(p q) ge 0 D(p q) = 0 iff p=q

              (b) D(p q) = D(q p)

              (c) D(p z) le D(p q) + D(q z)

              The Euclidean distance between p and q is defined as 1

              2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

              The pixels q for which ( )eD p q r are the points contained in a disk of radius r

              centered at (x y)

              Digital Image Processing

              Week 1

              The D4 distance (also called city-block distance) between p and q is defined as

              4( ) | | | |D p q x s y t

              The pixels q for which 4( )D p q r form a diamond centered at (xy)

              4

              22 1 2

              2 2 1 0 1 22 1 2

              2

              D

              The pixels with D4 = 1 are the 4-neighbors of (x y)

              The D8 distance (called the chessboard distance) between p and q is defined as

              8( ) max| | | |D p q x s y t

              The pixels q for which 8( )D p q r form a square centered at (x y)

              Digital Image Processing

              Week 1

              8

              2 2 2 2 22 1 1 1 2

              2 2 1 0 1 22 1 1 1 22 2 2 2 2

              D

              The pixels with D8 = 1 are the 8-neighbors of (x y)

              D4 and D8 distances are independent of any paths that might exist between p and q

              because these distances involve only the coordinates of the point

              Digital Image Processing

              Week 1

              Array versus Matrix Operations

              An array operation involving one or more images is carried out on a pixel-by-pixel basis

              11 12 11 12

              21 22 21 22

              a a b ba a b b

              Array product

              11 12 11 12 11 11 12 12

              21 22 21 22 21 21 22 21

              a a b b a b a ba a b b a b a b

              Matrix product

              11 12 11 12 11 11 12 21 11 12 12 21

              21 22 21 22 21 11 22 21 21 12 22 22

              a a b b a b a b a b a ba a b b a b a b a b a b

              We assume array operations unless stated otherwise

              Digital Image Processing

              Week 1

              Linear versus Nonlinear Operations

              One of the most important classifications of image-processing methods is whether it is

              linear or nonlinear

              ( ) ( )H f x y g x y

              H is said to be a linear operator if

              images1 2 1 2

              1 2

              ( ) ( ) ( ) ( )

              H a f x y b f x y a H f x y b H f x y

              a b f f

              Example of nonlinear operator

              the maximum value of the pixels of image max ( )H f f x y f

              1 2

              0 2 6 5 1 1

              2 3 4 7f f a b

              Digital Image Processing

              Week 1

              1 2

              0 2 6 5 6 3max max 1 ( 1) max 2

              2 3 4 7 2 4a f b f

              0 2 6 51 max ( 1) max 3 ( 1)7 4

              2 3 4 7

              Arithmetic Operations in Image Processing

              Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

              f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

              For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

              value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

              The two random variables are uncorrelated when their covariance is 0

              Digital Image Processing

              Week 1

              Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

              used in image enhancement)

              1

              1( ) ( )K

              ii

              g x y g x yK

              If the noise satisfies the properties stated above we have

              2 2( ) ( )

              1( ) ( ) g x y x yE g x y f x yK

              ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

              and g respectively The standard deviation (square root of the variance) at any point in

              the average image is

              ( ) ( )1

              g x y x yK

              Digital Image Processing

              Week 1

              As K increases the variability (as measured by the variance or the standard deviation) of

              the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

              means that ( )g x y approaches f(x y) as the number of noisy images used in the

              averaging process increases

              An important application of image averaging is in the field of astronomy where imaging

              under very low light levels frequently causes sensor noise to render single images

              virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

              was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

              64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

              images respectively

              Digital Image Processing

              Week 1

              Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

              100 noisy images

              a b c d e f

              Digital Image Processing

              Week 1

              A frequent application of image subtraction is in the enhancement of differences between

              images

              (a) (b) (c)

              Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

              significant bit of each pixel (c) the difference between the two images

              Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

              Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

              images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

              difference between images (a) and (b)

              Digital Image Processing

              Week 1

              Mask mode radiography ( ) ( ) ( )g x y f x y h x y

              h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

              intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

              source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

              bloodstream taking a series of images called live images (denoted f(x y)) of the same

              anatomical region as h(x y) and subtracting the mask from the series of incoming live

              images after injection of the contrast medium

              In g(x y) we can find the differences between h and f as enhanced detail

              Images being captured at TV rates we obtain a movie showing how the contrast medium

              propagates through the various arteries in the area being observed

              Digital Image Processing

              Week 1

              a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

              Digital Image Processing

              Week 1

              An important application of image multiplication (and division) is shading correction

              Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

              f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

              When the shading function is known

              ( )( )( )

              g x yf x yh x y

              h(x y) is unknown but we have access to the imaging system we can obtain an

              approximation to the shading function by imaging a target of constant intensity When the

              sensor is not available often the shading pattern can be estimated from the image

              Digital Image Processing

              Week 1

              (a) (b) (c)

              Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

              Digital Image Processing

              Week 1

              Another use of image multiplication is in masking also called region of interest (ROI)

              operations The process consists of multiplying a given image by a mask image that has

              1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

              image and the shape of the ROI can be arbitrary but usually is a rectangular shape

              (a) (b) (c)

              Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

              Digital Image Processing

              Week 1

              In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

              min( )mf f f

              0 ( 255)max( )

              ms

              m

              ff K K K

              f

              Digital Image Processing

              Week 1

              Spatial Operations

              - are performed directly on the pixels of a given image

              There are three categories of spatial operations

              single-pixel operations

              neighborhood operations

              geometric spatial transformations

              Single-pixel operations

              - change the values of intensity for the individual pixels ( )s T z

              where z is the intensity of a pixel in the original image and s is the intensity of the

              corresponding pixel in the processed image

              Digital Image Processing

              Week 1

              Neighborhood operations

              Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

              in an image f Neighborhood processing generates new intensity level at point (x y)

              based on the values of the intensities of the points in Sxy For example if Sxy is a

              rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

              intensity by computing the average value of the pixels in Sxy

              ( )

              1( ) ( )xyr c S

              g x y f r cm n

              The net effect is to perform local blurring in the original image This type of process is

              used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

              largest region of an image

              Digital Image Processing

              Week 1

              Geometric spatial transformations and image registration

              - modify the spatial relationship between pixels in an image

              - these transformations are often called rubber-sheet transformations (analogous to

              printing an image on a sheet of rubber and then stretching the sheet according to a

              predefined set of rules

              A geometric transformation consists of 2 basic operations

              1 a spatial transformation of coordinates

              2 intensity interpolation that assign intensity values to the spatial transformed

              pixels

              The coordinate system transformation ( ) [( )]x y T v w

              (v w) ndash pixel coordinates in the original image

              (x y) ndash pixel coordinates in the transformed image

              Digital Image Processing

              Week 1

              [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

              Affine transform

              11 1211 21 31

              21 2212 22 33

              31 32

              0[ 1] [ 1] [ 1] 0

              1

              t tx t v t w t

              x y v w T v w t ty t v t w t

              t t

              (AT)

              This transform can scale rotate translate or shear a set of coordinate points depending

              on the elements of the matrix T If we want to resize an image rotate it and move the

              result to some location we simply form a 3x3 matrix equal to the matrix product of the

              scaling rotation and translation matrices from Table 1

              Digital Image Processing

              Week 1

              Affine transformations

              Digital Image Processing

              Week 1

              The preceding transformations relocate pixels on an image to new locations To complete

              the process we have to assign intensity values to those locations This task is done by

              using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

              In practice we can use equation (AT) in two basic ways

              forward mapping scan the pixels of the input image (v w) compute the new spatial

              location (x y) of the corresponding pixel in the new image using (AT) directly

              Problems

              - intensity assignment when 2 or more pixels in the original image are transformed to

              the same location in the output image

              - some output locations have no correspondent in the original image (no intensity

              assignment)

              Digital Image Processing

              Week 1

              inverse mapping scans the output pixel locations and at each location (x y)

              computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

              It then interpolates among the nearest input pixels to determine the intensity of the output

              pixel value

              Inverse mappings are more efficient to implement than forward mappings and are used in

              numerous commercial implementations of spatial transformations (MATLAB for ex)

              Digital Image Processing

              Week 1

              Digital Image Processing

              Week 1

              Image registration ndash align two or more images of the same scene

              In image registration we have available the input and output images but the specific

              transformation that produced the output image from the input is generally unknown

              The problem is to estimate the transformation function and then use it to register the two

              images

              - it may be of interest to align (register) two or more image taken at approximately the

              same time but using different imaging systems (MRI scanner and a PET scanner)

              - align images of a given location taken by the same instrument at different moments

              of time (satellite images)

              Solving the problem using tie points (also called control points) which are

              corresponding points whose locations are known precisely in the input and reference

              image

              Digital Image Processing

              Week 1

              How to select tie points

              - interactively selecting them

              - use of algorithms that try to detect these points

              - some imaging systems have physical artifacts (small metallic objects) embedded in

              the imaging sensors These objects produce a set of known points (called reseau

              marks) directly on all images captured by the system which can be used as guides

              for establishing tie points

              The problem of estimating the transformation is one of modeling Suppose we have a set

              of 4 tie points both on the input image and the reference image A simple model based on

              a bilinear approximation is given by

              1 2 3 4

              5 6 7 8

              x c v c w c v w cy c v c w c v w c

              (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

              Digital Image Processing

              Week 1

              When 4 tie points are insufficient to obtain satisfactory registration an approach used

              frequently is to select a larger number of tie points and using this new set of tie points

              subdivide the image in rectangular regions marked by groups of 4 tie points On the

              subregions marked by 4 tie points we applied the transformation model described above

              The number of tie points and the sophistication of the model required to solve the register

              problem depend on the severity of the geometrical distortion

              Digital Image Processing

              Week 1

              a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

              Digital Image Processing

              Week 1

              Probabilistic Methods

              zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

              p(zk) = the probability that the intensity level zk occurs in the given image

              ( ) kk

              np zM N

              nk = the number of times that intensity zk occurs in the image (MN is the total number of

              pixels in the image) 1

              0( ) 1

              L

              kk

              p z

              The mean (average) intensity of an image is given by 1

              0( )

              L

              k kk

              m z p z

              Digital Image Processing

              Week 1

              The variance of the intensities is 1

              2 2

              0( ) ( )

              L

              k kk

              z m p z

              The variance is a measure of the spread of the values of z about the mean so it is a

              measure of image contrast Usually for measuring image contrast the standard deviation

              ( ) is used

              The n-th moment of a random variable z about the mean is defined as 1

              0( ) ( ) ( )

              Ln

              n k kk

              z z m p z

              ( 20 1 2( ) 1 ( ) 0 ( )z z z )

              3( ) 0z the intensities are biased to values higher than the mean

              ( 3( ) 0z the intensities are biased to values lower than the mean

              Digital Image Processing

              Week 1

              3( ) 0z the intensities are distributed approximately equally on both side of the

              mean

              Fig1 (a) Low contrast (b) medium contrast (c) high contrast

              Figure 1(a) ndash standard deviation 143 (variance = 2045)

              Figure 1(b) ndash standard deviation 316 (variance = 9986)

              Figure 1(c) ndash standard deviation 492 (variance = 24206)

              Digital Image Processing

              Week 1

              Intensity Transformations and Spatial Filtering

              ( ) ( )g x y T f x y

              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

              neighborhood of (x y)

              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

              and much smaller in size than the image

              Digital Image Processing

              Week 1

              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

              called spatial filter (spatial mask kernel template or window)

              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

              ( )s T r

              s and r are denoting respectively the intensity of g and f at (x y)

              Figure 2 left - T produces an output image of higher contrast than the original by

              darkening the intensity levels below k and brightening the levels above k ndash this technique

              is called contrast stretching

              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

              Digital Image Processing

              Week 1

              Figure 2 right - T produces a binary output image A mapping of this form is called

              thresholding function

              Some Basic Intensity Transformation Functions

              Image Negatives

              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

              - equivalent of a photographic negative

              - technique suited for enhancing white or gray detail embedded in dark regions of an

              image

              Digital Image Processing

              Week 1

              Original Negative image

              Digital Image Processing

              Week 1

              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

              Some basic intensity transformation functions

              Digital Image Processing

              Week 1

              This transformation maps a narrow range of low intensity values in the input into a wider

              range An operator of this type is used to expand the values of dark pixels in an image

              while compressing the higher-level values The opposite is true for the inverse log

              transformation The log functions compress the dynamic range of images with large

              variations in pixel values

              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

              Digital Image Processing

              Week 1

              Power-Law (Gamma) Transformations

              - positive constants( ) ( ( ) )s T r c r c s c r

              Plots of gamma transformation for different values of γ (c=1)

              Digital Image Processing

              Week 1

              Power-law curves with 1 map a narrow range of dark input values into a wider range

              of output values with the opposite being true for higher values of input values The

              curves with 1 have the opposite effect of those generated with values of 1

              1c - identity transformation

              A variety of devices used for image capture printing and display respond according to a

              power law The process used to correct these power-law response phenomena is called

              gamma correction

              Digital Image Processing

              Week 1

              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

              Digital Image Processing

              Week 1

              Piecewise-Linear Transformations Functions

              Contrast stretching

              - a process that expands the range of intensity levels in an image so it spans the full

              intensity range of the recording tool or display device

              a b c d Fig5

              Digital Image Processing

              Week 1

              11

              1

              2 1 1 21 2

              2 1 2 1

              22

              2

              [0 ]

              ( ) ( )( ) [ ]( ) ( )

              ( 1 ) [ 1]( 1 )

              s r r rrs r r s r rT r r r r

              r r r rs L r r r L

              L r

              Digital Image Processing

              Week 1

              1 1 2 2r s r s identity transformation (no change)

              1 2 1 2 0 1r r s s L thresholding function

              Figure 5(b) shows an 8-bit image with low contrast

              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

              in the image respectively Thus the transformation function stretched the levels linearly

              from their original range to the full range [0 L-1]

              Figure 5(d) - the thresholding function was used with 1 1 0r s m

              2 2 1r s m L where m is the mean gray level in the image

              The original image on which these results are based is a scanning electron microscope

              image of pollen magnified approximately 700 times

              Digital Image Processing

              Week 1

              Intensity-level slicing

              - highlighting a specific range of intensities in an image

              There are two approaches for intensity-level slicing

              1 display in one value (white for example) all the values in the range of interest and in

              another (say black) all other intensities (Figure 311 (a))

              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

              intensities in the image (Figure 311 (b))

              Digital Image Processing

              Week 1

              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

              highlight the major blood vessels that appear brighter as a result of injecting a contrast

              medium Figure 6(middle) shows the result of applying the first technique for a band near

              the top of the scale of intensities This type of enhancement produces a binary image

              Highlights intensity range [A B] and reduces all other intensities to a lower level

              Highlights range [A B] and preserves all other intensities

              Digital Image Processing

              Week 1

              which is useful for studying the shape of the flow of the contrast substance (to detect

              blockageshellip)

              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

              image around the mean intensity was set to black the other intensities remain unchanged

              Fig 6 - Aortic angiogram and intensity sliced versions

              Digital Image Processing

              Week 1

              Bit-plane slicing

              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

              This technique highlights the contribution made to the whole image appearances by each

              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

              Digital Image Processing

              Week 1

              Digital Image Processing

              Week 1

              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

              • DIP 1 2017
              • DIP 02 (2017)

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                What is Digital Image Processing

                f(xy) = intensity gray level of the image at spatial point (xy)

                x y f(xy) ndash finite discrete quantities -gt digital image

                Digital Image Processing = processing digital images by means of a digital computer

                A digital image is composed of a finite number of elements (location value of intensity)

                These elements are called picture elements image elements pels pixels

                ( )i j ijx y f

                3 f D

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Image processing is not limited to the visual band of the electromagnetic (EM) spectrum

                Image processing gamma to radio waves ultrasound electron microscopy computer-generated images

                image processing image analysis computer vision

                Image processing = discipline in which both the input and the output of a process are images

                Computer Vision = use computer to emulate human vision (AI) ndashlearning making inferences and take actions based on visual inputs

                Image analysis (image understanding) = segmentation partitioning images into regions or objects

                (link between image processing and image analysis)

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Distinction between image processing image analysis computer vision

                low-level mid-level high-level processes

                Low-level processes image preprocessing to reduce noise contrast enhancement image sharpening both input and output are images

                Mid-level processes segmentation partitioning images into regions or objects description of the objects for computer processing classificationrecognition of individual objects inputs are generally images outputs are attributes extracted from the input image (eg edges contours identity of individual objects)

                High-level processes ldquomaking senserdquo of a set of recognized objects performing the cognitive functions associated with vision

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Digital Image Processing (Gonzalez + Woods) =

                processes whose inputs and outputs are images +

                processes that extract attributes from images recognition of individual objects

                (low- and mid-level processes)

                Example

                automated analysis of text = acquiring an image containing text preprocessing the image (enhancement sharpening) extracting (segmenting) the individual characaters describing the characters in a form suitable for computer processing recognition of individual characters

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                The Origins of DIP

                Newspaper industry pictures were sent by submarine cable between London and New York

                Before Bartlane cable picture transmission system (early 1920s) ndash 1 week

                With Bartlane system less than 3 hours

                Specialized printing equipment coded pictures for cable transmission and reconstructed them at the receiving end (1920s -5 distict levels of gray 1929 ndash 15 levels)

                This example is not DIP the computer is not involved

                DIP is linked and devolps in the same rhythm as digital computers (data storage display and transmission)

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                A digital pictureproduced in 1921from a coded tapeby a telegraph printer withspecial type faces(McFarlane)

                A digital picturemade in 1922from a tapepunched after the signals hadcrossed the Atlantic twice(McFarlane)

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                1964 Jet Propulsion Laboratory (Pasadena California) processed pictures of the moon transmitted by Ranger 7 (corrected image distortions)

                The first picture of the moon by a US spacecraft Ranger 7 took this image July 31 1964 about 17 minutes before impacting the lunar surface(Courtesy of NASA)

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

                1970s ndash invention of CAT (computerized axial tomography)

                CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                loz geographers use DIP to study pollution patterns from aerial and satellite imagery

                loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

                loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

                loz astronomy biology nuclear medicine law enforcement industry

                DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

                loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Examples of Fields that Use DIP

                Images can be classified according to their sources (visual X-ray hellip)

                Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Electromagnetic waves can be thought as propagating sinusoidal

                waves of different wavelength or as a stream of massless particles

                each moving in a wavelike pattern with the speed of light Each

                massless particle contains a certain amount (bundle) of energy Each

                bundle of energy is called a photon If spectral bands are grouped

                according to energy per photon we obtain the spectrum shown in the

                image above ranging from gamma-rays (highest energy) to radio

                waves (lowest energy)

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Gamma-Ray Imaging

                Nuclear medicine astronomical observations

                Nuclear medicine

                the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

                Images are produced from the emissions collected by gamma-ray detectors

                Images of this sort are used to locate sites of bone pathology (infections tumors)

                PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Examples of gamma-ray imaging

                Bone scan PET image

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                X-ray imaging

                Medical diagnosticindustry astronomy

                A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Angiography = contrast-enhancement radiography

                Angiograms = images of blood vessels

                A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                X-rays are used in CAT (computerized axial tomography)

                X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                Industrial CAT scans are useful when the parts can be penetreted by X-rays

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Examples of X-ray imaging

                Chest X-rayAortic angiogram

                Head CT Cygnus LoopCircuit boards

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Imaging in the Ultraviolet Band

                Litography industrial inspection microscopy biological imaging astronomical observations

                Ultraviolet light is used in fluorescence microscopy

                Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Imaging in the Visible and Infrared Bands

                Light microscopy astronomy remote sensing industry law enforcement

                LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                Weather observations and prediction produce major applications of multispectral image from satellites

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Satellite images of Washington DC area in spectral bands of the Table 1

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Examples of light microscopy

                Taxol (anticancer agent)magnified 250X

                Cholesterol(40X)

                Microprocessor(60X)

                Nickel oxidethin film(600X)

                Surface of audio CD(1750X)

                Organicsuperconductor(450X)

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Automated visual inspection of manufactured goods

                a bc de f

                a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Imaging in the Microwave Band

                The dominant aplication of imaging in the microwave band ndash radar

                bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Spaceborne radar image of mountains in southeast Tibet

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Imaging in the Radio Band

                medicine astronomy

                MRI = Magnetic Resonance Imaging

                This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                MRI images of a human knee (left) and spine (right)

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Images of the Crab Pulsar covering the electromagnetic spectrum

                Gamma X-ray Optical Infrared Radio

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Other Imaging Modalities

                acoustic imaging electron microscopy synthetic (computer-generated) imaging

                Imaging using sound geological explorations industry medicine

                Mineral and oil exploration

                For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Biometry - iris

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Biometry - fingerprint

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Face detection and recognition

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Gender identification

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Image morphing

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Fundamental Steps in DIP

                methods whose input and output are images

                methods whose inputs are images but whose outputs are attributes extracted from those images

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Outputs are images

                bull image acquisition

                bull image filtering and enhancement

                bull image restoration

                bull color image processing

                bull wavelets and multiresolution processing

                bull compression

                bull morphological processing

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Outputs are attributes

                bull morphological processing

                bull segmentation

                bull representation and description

                bull object recognition

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Image acquisition - may involve preprocessing such as scaling

                Image enhancement

                bull manipulating an image so that the result is more suitable than the original for a specific operation

                bull enhancement is problem oriented

                bull there is no general sbquotheoryrsquo of image enhancement

                bull enhancement use subjective methods for image emprovement

                bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Image restoration

                bull improving the appearance of an image

                bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                Color image processing

                bull fundamental concept in color models

                bull basic color processing in a digital domain

                Wavelets and multiresolution processing

                representing images in various degree of resolution

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Compression

                reducing the storage required to save an image or the bandwidth required to transmit it

                Morphological processing

                bull tools for extracting image components that are useful in the representation and description of shape

                bull a transition from processes that output images to processes that outputimage attributes

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Segmentation

                bull partitioning an image into its constituents parts or objects

                bull autonomous segmentation is one of the most difficult tasks of DIP

                bull the more accurate the segmentation the more likley recognition is to succeed

                Representation and description (almost always follows segmentation)

                bull segmentation produces either the boundary of a region or all the poits in the region itself

                bull converting the data produced by segmentation to a form suitable for computer processing

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                bull boundary representation the focus is on external shape characteristics such as corners or inflections

                bull complete region the focus is on internal properties such as texture or skeletal shape

                bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                Object recognition

                the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                Knowledge database

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Simplified diagramof a cross sectionof the human eye

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                The cornea is a tough transparent tissue that covers the anterior surface of the eye

                Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                Fovea = the place where the image of the object of interest falls on

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                Blind spot region without receptors

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Image formation in the eye

                Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                distance between lens and retina along visual axix = 17 mm

                range of focal length = 14 mm to 17 mm

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Optical illusions

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                quantities that describe the quality of a chromatic light source radiance

                the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                the physical meaning is determined by the source of the image

                ( )f D f x y

                Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                f(xy) ndash characterized by two components

                i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                ( ) ( ) ( )

                0 ( ) 0 ( ) 1

                f x y i x y r x y

                i x y r x y

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                r(xy)=0 - total absorption r(xy)=1 - total reflectance

                i(xy) ndash determined by the illumination source

                r(xy) ndash determined by the characteristics of the imaged objects

                is called gray (or intensity) scale

                In practice

                min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                indoor values without additional illuminationmin max10 1000L L

                black whitemin max0 1 0 1 0 1L L L L l l L

                min maxL L

                Digital Image ProcessingDigital Image Processing

                Week 1Week 1

                Digital Image Processing

                Week 1

                Image Sampling and Quantization

                - the output of the sensors is a continuous voltage waveform related to the sensed

                scene

                converting a continuous image f to digital form

                - digitizing (x y) is called sampling

                - digitizing f(x y) is called quantization

                Digital Image Processing

                Week 1

                Digital Image Processing

                Week 1

                Continuous image projected onto a sensor array Result of image sampling and quantization

                Digital Image Processing

                Week 1

                Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                (00) (01) (0 1)(10) (11) (1 1)

                ( )

                ( 10) ( 11) ( 1 1)

                f f f Nf f f N

                f x y

                f M f M f M N

                image element pixel

                00 01 0 1

                10 11 1 1

                10 11 1 1

                ( ) ( )

                N

                i jN M N

                i j

                M M M N

                a a aa f x i y j f i ja a a

                Aa

                a a a

                f(00) ndash the upper left corner of the image

                Digital Image Processing

                Week 1

                M N ge 0 L=2k

                [0 1]i j i ja a L

                Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                Digital Image Processing

                Week 1

                Digital Image Processing

                Week 1

                Number of bits required to store a digitized image

                for 2 b M N k M N b N k

                When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                Digital Image Processing

                Week 1

                Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                Measures line pairs per unit distance dots (pixels) per unit distance

                Image resolution = the largest number of discernible line pairs per unit distance

                (eg 100 line pairs per mm)

                Dots per unit distance are commonly used in printing and publishing

                In US the measure is expressed in dots per inch (dpi)

                (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                Intensity resolution ndash the smallest discernible change in intensity level

                The number of intensity levels (L) is determined by hardware considerations

                L=2k ndash most common k = 8

                Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                Digital Image Processing

                Week 1

                Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                150 dpi (lower left) 72 dpi (lower right)

                Digital Image Processing

                Week 1

                Reducing the number of gray levels 256 128 64 32

                Digital Image Processing

                Week 1

                Reducing the number of gray levels 16 8 4 2

                Digital Image Processing

                Week 1

                Image Interpolation - used in zooming shrinking rotating and geometric corrections

                Shrinking zooming ndash image resizing ndash image resampling methods

                Interpolation is the process of using known data to estimate values at unknown locations

                Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                same spacing as the original and then shrink it so that it fits exactly over the original

                image The pixel spacing in the 750 times 750 grid will be less than in the original image

                Problem assignment of intensity-level in the new 750 times 750 grid

                Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                This technique has the tendency to produce undesirable effects like severe distortion of

                straight edges

                Digital Image Processing

                Week 1

                Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                where the four coefficients are determined from the 4 equations in 4 unknowns that can

                be written using the 4 nearest neighbors of point (x y)

                Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                modest increase in computational effort

                Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                nearest neighbors of the point 3 3

                0 0

                ( ) i ji j

                i jv x y c x y

                The coefficients cij are obtained solving a 16x16 linear system

                intensity levels of the 16 nearest neighbors of 3 3

                0 0

                ( )i ji j

                i jc x y x y

                Digital Image Processing

                Week 1

                Generally bicubic interpolation does a better job of preserving fine detail than the

                bilinear technique Bicubic interpolation is the standard used in commercial image editing

                programs such as Adobe Photoshop and Corel Photopaint

                Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                neighbor interpolation was used (both for shrinking and zooming)

                Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                from 1250 dpi to 150 dpi (instead of 72 dpi)

                Digital Image Processing

                Week 1

                Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                Digital Image Processing

                Week 1

                Neighbors of a Pixel

                A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                This set of pixels called the 4-neighbors of p denoted by N4 (p)

                The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                and are denoted ND(p)

                The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                N8 (p)

                If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                fall outside the image

                Digital Image Processing

                Week 1

                Adjacency Connectivity Regions Boundaries

                Denote by V the set of intensity levels used to define adjacency

                - in a binary image V 01 (V=0 V=1)

                - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                We consider 3 types of adjacency

                (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                m-adjacent if

                4( )q N p or

                ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                ambiguities that often arise when 8-adjacency is used Consider the example

                Digital Image Processing

                Week 1

                binary image

                0 1 1 0 1 1 0 1 1

                1 0 1 0 0 1 0 0 1 0

                0 0 1 0 0 1 0 0 1

                V

                The three pixels at the top (first line) in the above example show multiple (ambiguous)

                8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                m-adjacency

                Digital Image Processing

                Week 1

                A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                is a sequence of distinct pixels with coordinates

                and are adjacent 0 0 1 1

                1 1

                ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                n n

                i i i i

                x y x y x y x y s tx y x y i n

                The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                in S if there exists a path between them consisting only of pixels from S

                S is a connected set if there is a path in S between any 2 pixels in S

                Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                that are not adjacent are said to be disjoint When referring to regions only 4- and

                8-adjacency are considered

                Digital Image Processing

                Week 1

                Suppose that an image contains K disjoint regions 1 kR k K none of which

                touches the image border

                the complement of 1

                ( )K

                cu k u u

                k

                R R R R

                We call all the points in Ru the foreground of the image and the points in ( )cuR the

                background of the image

                The boundary (border or contour) of a region R is the set of points that are adjacent to

                points in the complement of R (R)c The border of an image is the set of pixels in the

                region that have at least one background neighbor This definition is referred to as the

                inner border to distinguish it from the notion of outer border which is the corresponding

                border in the background

                Digital Image Processing

                Week 1

                Distance measures

                For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                function or metric if

                (a) D(p q) ge 0 D(p q) = 0 iff p=q

                (b) D(p q) = D(q p)

                (c) D(p z) le D(p q) + D(q z)

                The Euclidean distance between p and q is defined as 1

                2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                centered at (x y)

                Digital Image Processing

                Week 1

                The D4 distance (also called city-block distance) between p and q is defined as

                4( ) | | | |D p q x s y t

                The pixels q for which 4( )D p q r form a diamond centered at (xy)

                4

                22 1 2

                2 2 1 0 1 22 1 2

                2

                D

                The pixels with D4 = 1 are the 4-neighbors of (x y)

                The D8 distance (called the chessboard distance) between p and q is defined as

                8( ) max| | | |D p q x s y t

                The pixels q for which 8( )D p q r form a square centered at (x y)

                Digital Image Processing

                Week 1

                8

                2 2 2 2 22 1 1 1 2

                2 2 1 0 1 22 1 1 1 22 2 2 2 2

                D

                The pixels with D8 = 1 are the 8-neighbors of (x y)

                D4 and D8 distances are independent of any paths that might exist between p and q

                because these distances involve only the coordinates of the point

                Digital Image Processing

                Week 1

                Array versus Matrix Operations

                An array operation involving one or more images is carried out on a pixel-by-pixel basis

                11 12 11 12

                21 22 21 22

                a a b ba a b b

                Array product

                11 12 11 12 11 11 12 12

                21 22 21 22 21 21 22 21

                a a b b a b a ba a b b a b a b

                Matrix product

                11 12 11 12 11 11 12 21 11 12 12 21

                21 22 21 22 21 11 22 21 21 12 22 22

                a a b b a b a b a b a ba a b b a b a b a b a b

                We assume array operations unless stated otherwise

                Digital Image Processing

                Week 1

                Linear versus Nonlinear Operations

                One of the most important classifications of image-processing methods is whether it is

                linear or nonlinear

                ( ) ( )H f x y g x y

                H is said to be a linear operator if

                images1 2 1 2

                1 2

                ( ) ( ) ( ) ( )

                H a f x y b f x y a H f x y b H f x y

                a b f f

                Example of nonlinear operator

                the maximum value of the pixels of image max ( )H f f x y f

                1 2

                0 2 6 5 1 1

                2 3 4 7f f a b

                Digital Image Processing

                Week 1

                1 2

                0 2 6 5 6 3max max 1 ( 1) max 2

                2 3 4 7 2 4a f b f

                0 2 6 51 max ( 1) max 3 ( 1)7 4

                2 3 4 7

                Arithmetic Operations in Image Processing

                Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                The two random variables are uncorrelated when their covariance is 0

                Digital Image Processing

                Week 1

                Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                used in image enhancement)

                1

                1( ) ( )K

                ii

                g x y g x yK

                If the noise satisfies the properties stated above we have

                2 2( ) ( )

                1( ) ( ) g x y x yE g x y f x yK

                ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                and g respectively The standard deviation (square root of the variance) at any point in

                the average image is

                ( ) ( )1

                g x y x yK

                Digital Image Processing

                Week 1

                As K increases the variability (as measured by the variance or the standard deviation) of

                the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                means that ( )g x y approaches f(x y) as the number of noisy images used in the

                averaging process increases

                An important application of image averaging is in the field of astronomy where imaging

                under very low light levels frequently causes sensor noise to render single images

                virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                images respectively

                Digital Image Processing

                Week 1

                Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                100 noisy images

                a b c d e f

                Digital Image Processing

                Week 1

                A frequent application of image subtraction is in the enhancement of differences between

                images

                (a) (b) (c)

                Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                significant bit of each pixel (c) the difference between the two images

                Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                difference between images (a) and (b)

                Digital Image Processing

                Week 1

                Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                bloodstream taking a series of images called live images (denoted f(x y)) of the same

                anatomical region as h(x y) and subtracting the mask from the series of incoming live

                images after injection of the contrast medium

                In g(x y) we can find the differences between h and f as enhanced detail

                Images being captured at TV rates we obtain a movie showing how the contrast medium

                propagates through the various arteries in the area being observed

                Digital Image Processing

                Week 1

                a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                Digital Image Processing

                Week 1

                An important application of image multiplication (and division) is shading correction

                Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                When the shading function is known

                ( )( )( )

                g x yf x yh x y

                h(x y) is unknown but we have access to the imaging system we can obtain an

                approximation to the shading function by imaging a target of constant intensity When the

                sensor is not available often the shading pattern can be estimated from the image

                Digital Image Processing

                Week 1

                (a) (b) (c)

                Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                Digital Image Processing

                Week 1

                Another use of image multiplication is in masking also called region of interest (ROI)

                operations The process consists of multiplying a given image by a mask image that has

                1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                (a) (b) (c)

                Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                Digital Image Processing

                Week 1

                In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                min( )mf f f

                0 ( 255)max( )

                ms

                m

                ff K K K

                f

                Digital Image Processing

                Week 1

                Spatial Operations

                - are performed directly on the pixels of a given image

                There are three categories of spatial operations

                single-pixel operations

                neighborhood operations

                geometric spatial transformations

                Single-pixel operations

                - change the values of intensity for the individual pixels ( )s T z

                where z is the intensity of a pixel in the original image and s is the intensity of the

                corresponding pixel in the processed image

                Digital Image Processing

                Week 1

                Neighborhood operations

                Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                in an image f Neighborhood processing generates new intensity level at point (x y)

                based on the values of the intensities of the points in Sxy For example if Sxy is a

                rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                intensity by computing the average value of the pixels in Sxy

                ( )

                1( ) ( )xyr c S

                g x y f r cm n

                The net effect is to perform local blurring in the original image This type of process is

                used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                largest region of an image

                Digital Image Processing

                Week 1

                Geometric spatial transformations and image registration

                - modify the spatial relationship between pixels in an image

                - these transformations are often called rubber-sheet transformations (analogous to

                printing an image on a sheet of rubber and then stretching the sheet according to a

                predefined set of rules

                A geometric transformation consists of 2 basic operations

                1 a spatial transformation of coordinates

                2 intensity interpolation that assign intensity values to the spatial transformed

                pixels

                The coordinate system transformation ( ) [( )]x y T v w

                (v w) ndash pixel coordinates in the original image

                (x y) ndash pixel coordinates in the transformed image

                Digital Image Processing

                Week 1

                [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                Affine transform

                11 1211 21 31

                21 2212 22 33

                31 32

                0[ 1] [ 1] [ 1] 0

                1

                t tx t v t w t

                x y v w T v w t ty t v t w t

                t t

                (AT)

                This transform can scale rotate translate or shear a set of coordinate points depending

                on the elements of the matrix T If we want to resize an image rotate it and move the

                result to some location we simply form a 3x3 matrix equal to the matrix product of the

                scaling rotation and translation matrices from Table 1

                Digital Image Processing

                Week 1

                Affine transformations

                Digital Image Processing

                Week 1

                The preceding transformations relocate pixels on an image to new locations To complete

                the process we have to assign intensity values to those locations This task is done by

                using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                In practice we can use equation (AT) in two basic ways

                forward mapping scan the pixels of the input image (v w) compute the new spatial

                location (x y) of the corresponding pixel in the new image using (AT) directly

                Problems

                - intensity assignment when 2 or more pixels in the original image are transformed to

                the same location in the output image

                - some output locations have no correspondent in the original image (no intensity

                assignment)

                Digital Image Processing

                Week 1

                inverse mapping scans the output pixel locations and at each location (x y)

                computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                It then interpolates among the nearest input pixels to determine the intensity of the output

                pixel value

                Inverse mappings are more efficient to implement than forward mappings and are used in

                numerous commercial implementations of spatial transformations (MATLAB for ex)

                Digital Image Processing

                Week 1

                Digital Image Processing

                Week 1

                Image registration ndash align two or more images of the same scene

                In image registration we have available the input and output images but the specific

                transformation that produced the output image from the input is generally unknown

                The problem is to estimate the transformation function and then use it to register the two

                images

                - it may be of interest to align (register) two or more image taken at approximately the

                same time but using different imaging systems (MRI scanner and a PET scanner)

                - align images of a given location taken by the same instrument at different moments

                of time (satellite images)

                Solving the problem using tie points (also called control points) which are

                corresponding points whose locations are known precisely in the input and reference

                image

                Digital Image Processing

                Week 1

                How to select tie points

                - interactively selecting them

                - use of algorithms that try to detect these points

                - some imaging systems have physical artifacts (small metallic objects) embedded in

                the imaging sensors These objects produce a set of known points (called reseau

                marks) directly on all images captured by the system which can be used as guides

                for establishing tie points

                The problem of estimating the transformation is one of modeling Suppose we have a set

                of 4 tie points both on the input image and the reference image A simple model based on

                a bilinear approximation is given by

                1 2 3 4

                5 6 7 8

                x c v c w c v w cy c v c w c v w c

                (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                Digital Image Processing

                Week 1

                When 4 tie points are insufficient to obtain satisfactory registration an approach used

                frequently is to select a larger number of tie points and using this new set of tie points

                subdivide the image in rectangular regions marked by groups of 4 tie points On the

                subregions marked by 4 tie points we applied the transformation model described above

                The number of tie points and the sophistication of the model required to solve the register

                problem depend on the severity of the geometrical distortion

                Digital Image Processing

                Week 1

                a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                Digital Image Processing

                Week 1

                Probabilistic Methods

                zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                p(zk) = the probability that the intensity level zk occurs in the given image

                ( ) kk

                np zM N

                nk = the number of times that intensity zk occurs in the image (MN is the total number of

                pixels in the image) 1

                0( ) 1

                L

                kk

                p z

                The mean (average) intensity of an image is given by 1

                0( )

                L

                k kk

                m z p z

                Digital Image Processing

                Week 1

                The variance of the intensities is 1

                2 2

                0( ) ( )

                L

                k kk

                z m p z

                The variance is a measure of the spread of the values of z about the mean so it is a

                measure of image contrast Usually for measuring image contrast the standard deviation

                ( ) is used

                The n-th moment of a random variable z about the mean is defined as 1

                0( ) ( ) ( )

                Ln

                n k kk

                z z m p z

                ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                3( ) 0z the intensities are biased to values higher than the mean

                ( 3( ) 0z the intensities are biased to values lower than the mean

                Digital Image Processing

                Week 1

                3( ) 0z the intensities are distributed approximately equally on both side of the

                mean

                Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                Figure 1(a) ndash standard deviation 143 (variance = 2045)

                Figure 1(b) ndash standard deviation 316 (variance = 9986)

                Figure 1(c) ndash standard deviation 492 (variance = 24206)

                Digital Image Processing

                Week 1

                Intensity Transformations and Spatial Filtering

                ( ) ( )g x y T f x y

                f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                neighborhood of (x y)

                - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                and much smaller in size than the image

                Digital Image Processing

                Week 1

                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                called spatial filter (spatial mask kernel template or window)

                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                ( )s T r

                s and r are denoting respectively the intensity of g and f at (x y)

                Figure 2 left - T produces an output image of higher contrast than the original by

                darkening the intensity levels below k and brightening the levels above k ndash this technique

                is called contrast stretching

                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                Digital Image Processing

                Week 1

                Figure 2 right - T produces a binary output image A mapping of this form is called

                thresholding function

                Some Basic Intensity Transformation Functions

                Image Negatives

                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                - equivalent of a photographic negative

                - technique suited for enhancing white or gray detail embedded in dark regions of an

                image

                Digital Image Processing

                Week 1

                Original Negative image

                Digital Image Processing

                Week 1

                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                Some basic intensity transformation functions

                Digital Image Processing

                Week 1

                This transformation maps a narrow range of low intensity values in the input into a wider

                range An operator of this type is used to expand the values of dark pixels in an image

                while compressing the higher-level values The opposite is true for the inverse log

                transformation The log functions compress the dynamic range of images with large

                variations in pixel values

                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                Digital Image Processing

                Week 1

                Power-Law (Gamma) Transformations

                - positive constants( ) ( ( ) )s T r c r c s c r

                Plots of gamma transformation for different values of γ (c=1)

                Digital Image Processing

                Week 1

                Power-law curves with 1 map a narrow range of dark input values into a wider range

                of output values with the opposite being true for higher values of input values The

                curves with 1 have the opposite effect of those generated with values of 1

                1c - identity transformation

                A variety of devices used for image capture printing and display respond according to a

                power law The process used to correct these power-law response phenomena is called

                gamma correction

                Digital Image Processing

                Week 1

                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                Digital Image Processing

                Week 1

                Piecewise-Linear Transformations Functions

                Contrast stretching

                - a process that expands the range of intensity levels in an image so it spans the full

                intensity range of the recording tool or display device

                a b c d Fig5

                Digital Image Processing

                Week 1

                11

                1

                2 1 1 21 2

                2 1 2 1

                22

                2

                [0 ]

                ( ) ( )( ) [ ]( ) ( )

                ( 1 ) [ 1]( 1 )

                s r r rrs r r s r rT r r r r

                r r r rs L r r r L

                L r

                Digital Image Processing

                Week 1

                1 1 2 2r s r s identity transformation (no change)

                1 2 1 2 0 1r r s s L thresholding function

                Figure 5(b) shows an 8-bit image with low contrast

                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                in the image respectively Thus the transformation function stretched the levels linearly

                from their original range to the full range [0 L-1]

                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                2 2 1r s m L where m is the mean gray level in the image

                The original image on which these results are based is a scanning electron microscope

                image of pollen magnified approximately 700 times

                Digital Image Processing

                Week 1

                Intensity-level slicing

                - highlighting a specific range of intensities in an image

                There are two approaches for intensity-level slicing

                1 display in one value (white for example) all the values in the range of interest and in

                another (say black) all other intensities (Figure 311 (a))

                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                intensities in the image (Figure 311 (b))

                Digital Image Processing

                Week 1

                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                medium Figure 6(middle) shows the result of applying the first technique for a band near

                the top of the scale of intensities This type of enhancement produces a binary image

                Highlights intensity range [A B] and reduces all other intensities to a lower level

                Highlights range [A B] and preserves all other intensities

                Digital Image Processing

                Week 1

                which is useful for studying the shape of the flow of the contrast substance (to detect

                blockageshellip)

                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                image around the mean intensity was set to black the other intensities remain unchanged

                Fig 6 - Aortic angiogram and intensity sliced versions

                Digital Image Processing

                Week 1

                Bit-plane slicing

                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                This technique highlights the contribution made to the whole image appearances by each

                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                Digital Image Processing

                Week 1

                Digital Image Processing

                Week 1

                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                • DIP 1 2017
                • DIP 02 (2017)

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Image processing is not limited to the visual band of the electromagnetic (EM) spectrum

                  Image processing gamma to radio waves ultrasound electron microscopy computer-generated images

                  image processing image analysis computer vision

                  Image processing = discipline in which both the input and the output of a process are images

                  Computer Vision = use computer to emulate human vision (AI) ndashlearning making inferences and take actions based on visual inputs

                  Image analysis (image understanding) = segmentation partitioning images into regions or objects

                  (link between image processing and image analysis)

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Distinction between image processing image analysis computer vision

                  low-level mid-level high-level processes

                  Low-level processes image preprocessing to reduce noise contrast enhancement image sharpening both input and output are images

                  Mid-level processes segmentation partitioning images into regions or objects description of the objects for computer processing classificationrecognition of individual objects inputs are generally images outputs are attributes extracted from the input image (eg edges contours identity of individual objects)

                  High-level processes ldquomaking senserdquo of a set of recognized objects performing the cognitive functions associated with vision

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Digital Image Processing (Gonzalez + Woods) =

                  processes whose inputs and outputs are images +

                  processes that extract attributes from images recognition of individual objects

                  (low- and mid-level processes)

                  Example

                  automated analysis of text = acquiring an image containing text preprocessing the image (enhancement sharpening) extracting (segmenting) the individual characaters describing the characters in a form suitable for computer processing recognition of individual characters

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  The Origins of DIP

                  Newspaper industry pictures were sent by submarine cable between London and New York

                  Before Bartlane cable picture transmission system (early 1920s) ndash 1 week

                  With Bartlane system less than 3 hours

                  Specialized printing equipment coded pictures for cable transmission and reconstructed them at the receiving end (1920s -5 distict levels of gray 1929 ndash 15 levels)

                  This example is not DIP the computer is not involved

                  DIP is linked and devolps in the same rhythm as digital computers (data storage display and transmission)

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  A digital pictureproduced in 1921from a coded tapeby a telegraph printer withspecial type faces(McFarlane)

                  A digital picturemade in 1922from a tapepunched after the signals hadcrossed the Atlantic twice(McFarlane)

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  1964 Jet Propulsion Laboratory (Pasadena California) processed pictures of the moon transmitted by Ranger 7 (corrected image distortions)

                  The first picture of the moon by a US spacecraft Ranger 7 took this image July 31 1964 about 17 minutes before impacting the lunar surface(Courtesy of NASA)

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

                  1970s ndash invention of CAT (computerized axial tomography)

                  CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  loz geographers use DIP to study pollution patterns from aerial and satellite imagery

                  loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

                  loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

                  loz astronomy biology nuclear medicine law enforcement industry

                  DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

                  loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Examples of Fields that Use DIP

                  Images can be classified according to their sources (visual X-ray hellip)

                  Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Electromagnetic waves can be thought as propagating sinusoidal

                  waves of different wavelength or as a stream of massless particles

                  each moving in a wavelike pattern with the speed of light Each

                  massless particle contains a certain amount (bundle) of energy Each

                  bundle of energy is called a photon If spectral bands are grouped

                  according to energy per photon we obtain the spectrum shown in the

                  image above ranging from gamma-rays (highest energy) to radio

                  waves (lowest energy)

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Gamma-Ray Imaging

                  Nuclear medicine astronomical observations

                  Nuclear medicine

                  the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

                  Images are produced from the emissions collected by gamma-ray detectors

                  Images of this sort are used to locate sites of bone pathology (infections tumors)

                  PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Examples of gamma-ray imaging

                  Bone scan PET image

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  X-ray imaging

                  Medical diagnosticindustry astronomy

                  A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                  The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Angiography = contrast-enhancement radiography

                  Angiograms = images of blood vessels

                  A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                  X-rays are used in CAT (computerized axial tomography)

                  X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                  Industrial CAT scans are useful when the parts can be penetreted by X-rays

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Examples of X-ray imaging

                  Chest X-rayAortic angiogram

                  Head CT Cygnus LoopCircuit boards

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Imaging in the Ultraviolet Band

                  Litography industrial inspection microscopy biological imaging astronomical observations

                  Ultraviolet light is used in fluorescence microscopy

                  Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                  other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                  and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Imaging in the Visible and Infrared Bands

                  Light microscopy astronomy remote sensing industry law enforcement

                  LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                  Weather observations and prediction produce major applications of multispectral image from satellites

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Satellite images of Washington DC area in spectral bands of the Table 1

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Examples of light microscopy

                  Taxol (anticancer agent)magnified 250X

                  Cholesterol(40X)

                  Microprocessor(60X)

                  Nickel oxidethin film(600X)

                  Surface of audio CD(1750X)

                  Organicsuperconductor(450X)

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Automated visual inspection of manufactured goods

                  a bc de f

                  a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Imaging in the Microwave Band

                  The dominant aplication of imaging in the microwave band ndash radar

                  bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                  bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                  bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                  An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Spaceborne radar image of mountains in southeast Tibet

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Imaging in the Radio Band

                  medicine astronomy

                  MRI = Magnetic Resonance Imaging

                  This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                  Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                  The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  MRI images of a human knee (left) and spine (right)

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Images of the Crab Pulsar covering the electromagnetic spectrum

                  Gamma X-ray Optical Infrared Radio

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Other Imaging Modalities

                  acoustic imaging electron microscopy synthetic (computer-generated) imaging

                  Imaging using sound geological explorations industry medicine

                  Mineral and oil exploration

                  For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Biometry - iris

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Biometry - fingerprint

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Face detection and recognition

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Gender identification

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Image morphing

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Fundamental Steps in DIP

                  methods whose input and output are images

                  methods whose inputs are images but whose outputs are attributes extracted from those images

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Outputs are images

                  bull image acquisition

                  bull image filtering and enhancement

                  bull image restoration

                  bull color image processing

                  bull wavelets and multiresolution processing

                  bull compression

                  bull morphological processing

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Outputs are attributes

                  bull morphological processing

                  bull segmentation

                  bull representation and description

                  bull object recognition

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Image acquisition - may involve preprocessing such as scaling

                  Image enhancement

                  bull manipulating an image so that the result is more suitable than the original for a specific operation

                  bull enhancement is problem oriented

                  bull there is no general sbquotheoryrsquo of image enhancement

                  bull enhancement use subjective methods for image emprovement

                  bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Image restoration

                  bull improving the appearance of an image

                  bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                  Color image processing

                  bull fundamental concept in color models

                  bull basic color processing in a digital domain

                  Wavelets and multiresolution processing

                  representing images in various degree of resolution

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Compression

                  reducing the storage required to save an image or the bandwidth required to transmit it

                  Morphological processing

                  bull tools for extracting image components that are useful in the representation and description of shape

                  bull a transition from processes that output images to processes that outputimage attributes

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Segmentation

                  bull partitioning an image into its constituents parts or objects

                  bull autonomous segmentation is one of the most difficult tasks of DIP

                  bull the more accurate the segmentation the more likley recognition is to succeed

                  Representation and description (almost always follows segmentation)

                  bull segmentation produces either the boundary of a region or all the poits in the region itself

                  bull converting the data produced by segmentation to a form suitable for computer processing

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  bull boundary representation the focus is on external shape characteristics such as corners or inflections

                  bull complete region the focus is on internal properties such as texture or skeletal shape

                  bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                  Object recognition

                  the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                  Knowledge database

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Simplified diagramof a cross sectionof the human eye

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                  The cornea is a tough transparent tissue that covers the anterior surface of the eye

                  Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                  The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                  The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                  Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                  Fovea = the place where the image of the object of interest falls on

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                  Blind spot region without receptors

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Image formation in the eye

                  Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                  Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                  distance between lens and retina along visual axix = 17 mm

                  range of focal length = 14 mm to 17 mm

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Optical illusions

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                  gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                  quantities that describe the quality of a chromatic light source radiance

                  the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                  measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                  brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  the physical meaning is determined by the source of the image

                  ( )f D f x y

                  Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                  f(xy) ndash characterized by two components

                  i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                  r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                  ( ) ( ) ( )

                  0 ( ) 0 ( ) 1

                  f x y i x y r x y

                  i x y r x y

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  r(xy)=0 - total absorption r(xy)=1 - total reflectance

                  i(xy) ndash determined by the illumination source

                  r(xy) ndash determined by the characteristics of the imaged objects

                  is called gray (or intensity) scale

                  In practice

                  min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                  indoor values without additional illuminationmin max10 1000L L

                  black whitemin max0 1 0 1 0 1L L L L l l L

                  min maxL L

                  Digital Image ProcessingDigital Image Processing

                  Week 1Week 1

                  Digital Image Processing

                  Week 1

                  Image Sampling and Quantization

                  - the output of the sensors is a continuous voltage waveform related to the sensed

                  scene

                  converting a continuous image f to digital form

                  - digitizing (x y) is called sampling

                  - digitizing f(x y) is called quantization

                  Digital Image Processing

                  Week 1

                  Digital Image Processing

                  Week 1

                  Continuous image projected onto a sensor array Result of image sampling and quantization

                  Digital Image Processing

                  Week 1

                  Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                  (00) (01) (0 1)(10) (11) (1 1)

                  ( )

                  ( 10) ( 11) ( 1 1)

                  f f f Nf f f N

                  f x y

                  f M f M f M N

                  image element pixel

                  00 01 0 1

                  10 11 1 1

                  10 11 1 1

                  ( ) ( )

                  N

                  i jN M N

                  i j

                  M M M N

                  a a aa f x i y j f i ja a a

                  Aa

                  a a a

                  f(00) ndash the upper left corner of the image

                  Digital Image Processing

                  Week 1

                  M N ge 0 L=2k

                  [0 1]i j i ja a L

                  Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                  Digital Image Processing

                  Week 1

                  Digital Image Processing

                  Week 1

                  Number of bits required to store a digitized image

                  for 2 b M N k M N b N k

                  When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                  Digital Image Processing

                  Week 1

                  Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                  Measures line pairs per unit distance dots (pixels) per unit distance

                  Image resolution = the largest number of discernible line pairs per unit distance

                  (eg 100 line pairs per mm)

                  Dots per unit distance are commonly used in printing and publishing

                  In US the measure is expressed in dots per inch (dpi)

                  (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                  Intensity resolution ndash the smallest discernible change in intensity level

                  The number of intensity levels (L) is determined by hardware considerations

                  L=2k ndash most common k = 8

                  Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                  Digital Image Processing

                  Week 1

                  Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                  150 dpi (lower left) 72 dpi (lower right)

                  Digital Image Processing

                  Week 1

                  Reducing the number of gray levels 256 128 64 32

                  Digital Image Processing

                  Week 1

                  Reducing the number of gray levels 16 8 4 2

                  Digital Image Processing

                  Week 1

                  Image Interpolation - used in zooming shrinking rotating and geometric corrections

                  Shrinking zooming ndash image resizing ndash image resampling methods

                  Interpolation is the process of using known data to estimate values at unknown locations

                  Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                  750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                  same spacing as the original and then shrink it so that it fits exactly over the original

                  image The pixel spacing in the 750 times 750 grid will be less than in the original image

                  Problem assignment of intensity-level in the new 750 times 750 grid

                  Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                  intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                  This technique has the tendency to produce undesirable effects like severe distortion of

                  straight edges

                  Digital Image Processing

                  Week 1

                  Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                  where the four coefficients are determined from the 4 equations in 4 unknowns that can

                  be written using the 4 nearest neighbors of point (x y)

                  Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                  modest increase in computational effort

                  Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                  nearest neighbors of the point 3 3

                  0 0

                  ( ) i ji j

                  i jv x y c x y

                  The coefficients cij are obtained solving a 16x16 linear system

                  intensity levels of the 16 nearest neighbors of 3 3

                  0 0

                  ( )i ji j

                  i jc x y x y

                  Digital Image Processing

                  Week 1

                  Generally bicubic interpolation does a better job of preserving fine detail than the

                  bilinear technique Bicubic interpolation is the standard used in commercial image editing

                  programs such as Adobe Photoshop and Corel Photopaint

                  Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                  the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                  then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                  neighbor interpolation was used (both for shrinking and zooming)

                  Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                  interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                  from 1250 dpi to 150 dpi (instead of 72 dpi)

                  Digital Image Processing

                  Week 1

                  Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                  Digital Image Processing

                  Week 1

                  Neighbors of a Pixel

                  A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                  This set of pixels called the 4-neighbors of p denoted by N4 (p)

                  The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                  and are denoted ND(p)

                  The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                  N8 (p)

                  If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                  fall outside the image

                  Digital Image Processing

                  Week 1

                  Adjacency Connectivity Regions Boundaries

                  Denote by V the set of intensity levels used to define adjacency

                  - in a binary image V 01 (V=0 V=1)

                  - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                  We consider 3 types of adjacency

                  (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                  (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                  (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                  m-adjacent if

                  4( )q N p or

                  ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                  Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                  ambiguities that often arise when 8-adjacency is used Consider the example

                  Digital Image Processing

                  Week 1

                  binary image

                  0 1 1 0 1 1 0 1 1

                  1 0 1 0 0 1 0 0 1 0

                  0 0 1 0 0 1 0 0 1

                  V

                  The three pixels at the top (first line) in the above example show multiple (ambiguous)

                  8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                  m-adjacency

                  Digital Image Processing

                  Week 1

                  A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                  is a sequence of distinct pixels with coordinates

                  and are adjacent 0 0 1 1

                  1 1

                  ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                  n n

                  i i i i

                  x y x y x y x y s tx y x y i n

                  The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                  Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                  Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                  in S if there exists a path between them consisting only of pixels from S

                  S is a connected set if there is a path in S between any 2 pixels in S

                  Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                  Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                  that are not adjacent are said to be disjoint When referring to regions only 4- and

                  8-adjacency are considered

                  Digital Image Processing

                  Week 1

                  Suppose that an image contains K disjoint regions 1 kR k K none of which

                  touches the image border

                  the complement of 1

                  ( )K

                  cu k u u

                  k

                  R R R R

                  We call all the points in Ru the foreground of the image and the points in ( )cuR the

                  background of the image

                  The boundary (border or contour) of a region R is the set of points that are adjacent to

                  points in the complement of R (R)c The border of an image is the set of pixels in the

                  region that have at least one background neighbor This definition is referred to as the

                  inner border to distinguish it from the notion of outer border which is the corresponding

                  border in the background

                  Digital Image Processing

                  Week 1

                  Distance measures

                  For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                  function or metric if

                  (a) D(p q) ge 0 D(p q) = 0 iff p=q

                  (b) D(p q) = D(q p)

                  (c) D(p z) le D(p q) + D(q z)

                  The Euclidean distance between p and q is defined as 1

                  2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                  The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                  centered at (x y)

                  Digital Image Processing

                  Week 1

                  The D4 distance (also called city-block distance) between p and q is defined as

                  4( ) | | | |D p q x s y t

                  The pixels q for which 4( )D p q r form a diamond centered at (xy)

                  4

                  22 1 2

                  2 2 1 0 1 22 1 2

                  2

                  D

                  The pixels with D4 = 1 are the 4-neighbors of (x y)

                  The D8 distance (called the chessboard distance) between p and q is defined as

                  8( ) max| | | |D p q x s y t

                  The pixels q for which 8( )D p q r form a square centered at (x y)

                  Digital Image Processing

                  Week 1

                  8

                  2 2 2 2 22 1 1 1 2

                  2 2 1 0 1 22 1 1 1 22 2 2 2 2

                  D

                  The pixels with D8 = 1 are the 8-neighbors of (x y)

                  D4 and D8 distances are independent of any paths that might exist between p and q

                  because these distances involve only the coordinates of the point

                  Digital Image Processing

                  Week 1

                  Array versus Matrix Operations

                  An array operation involving one or more images is carried out on a pixel-by-pixel basis

                  11 12 11 12

                  21 22 21 22

                  a a b ba a b b

                  Array product

                  11 12 11 12 11 11 12 12

                  21 22 21 22 21 21 22 21

                  a a b b a b a ba a b b a b a b

                  Matrix product

                  11 12 11 12 11 11 12 21 11 12 12 21

                  21 22 21 22 21 11 22 21 21 12 22 22

                  a a b b a b a b a b a ba a b b a b a b a b a b

                  We assume array operations unless stated otherwise

                  Digital Image Processing

                  Week 1

                  Linear versus Nonlinear Operations

                  One of the most important classifications of image-processing methods is whether it is

                  linear or nonlinear

                  ( ) ( )H f x y g x y

                  H is said to be a linear operator if

                  images1 2 1 2

                  1 2

                  ( ) ( ) ( ) ( )

                  H a f x y b f x y a H f x y b H f x y

                  a b f f

                  Example of nonlinear operator

                  the maximum value of the pixels of image max ( )H f f x y f

                  1 2

                  0 2 6 5 1 1

                  2 3 4 7f f a b

                  Digital Image Processing

                  Week 1

                  1 2

                  0 2 6 5 6 3max max 1 ( 1) max 2

                  2 3 4 7 2 4a f b f

                  0 2 6 51 max ( 1) max 3 ( 1)7 4

                  2 3 4 7

                  Arithmetic Operations in Image Processing

                  Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                  f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                  For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                  value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                  The two random variables are uncorrelated when their covariance is 0

                  Digital Image Processing

                  Week 1

                  Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                  used in image enhancement)

                  1

                  1( ) ( )K

                  ii

                  g x y g x yK

                  If the noise satisfies the properties stated above we have

                  2 2( ) ( )

                  1( ) ( ) g x y x yE g x y f x yK

                  ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                  and g respectively The standard deviation (square root of the variance) at any point in

                  the average image is

                  ( ) ( )1

                  g x y x yK

                  Digital Image Processing

                  Week 1

                  As K increases the variability (as measured by the variance or the standard deviation) of

                  the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                  means that ( )g x y approaches f(x y) as the number of noisy images used in the

                  averaging process increases

                  An important application of image averaging is in the field of astronomy where imaging

                  under very low light levels frequently causes sensor noise to render single images

                  virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                  was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                  64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                  images respectively

                  Digital Image Processing

                  Week 1

                  Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                  100 noisy images

                  a b c d e f

                  Digital Image Processing

                  Week 1

                  A frequent application of image subtraction is in the enhancement of differences between

                  images

                  (a) (b) (c)

                  Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                  significant bit of each pixel (c) the difference between the two images

                  Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                  Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                  images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                  difference between images (a) and (b)

                  Digital Image Processing

                  Week 1

                  Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                  h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                  intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                  source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                  bloodstream taking a series of images called live images (denoted f(x y)) of the same

                  anatomical region as h(x y) and subtracting the mask from the series of incoming live

                  images after injection of the contrast medium

                  In g(x y) we can find the differences between h and f as enhanced detail

                  Images being captured at TV rates we obtain a movie showing how the contrast medium

                  propagates through the various arteries in the area being observed

                  Digital Image Processing

                  Week 1

                  a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                  Digital Image Processing

                  Week 1

                  An important application of image multiplication (and division) is shading correction

                  Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                  f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                  When the shading function is known

                  ( )( )( )

                  g x yf x yh x y

                  h(x y) is unknown but we have access to the imaging system we can obtain an

                  approximation to the shading function by imaging a target of constant intensity When the

                  sensor is not available often the shading pattern can be estimated from the image

                  Digital Image Processing

                  Week 1

                  (a) (b) (c)

                  Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                  Digital Image Processing

                  Week 1

                  Another use of image multiplication is in masking also called region of interest (ROI)

                  operations The process consists of multiplying a given image by a mask image that has

                  1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                  image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                  (a) (b) (c)

                  Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                  Digital Image Processing

                  Week 1

                  In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                  min( )mf f f

                  0 ( 255)max( )

                  ms

                  m

                  ff K K K

                  f

                  Digital Image Processing

                  Week 1

                  Spatial Operations

                  - are performed directly on the pixels of a given image

                  There are three categories of spatial operations

                  single-pixel operations

                  neighborhood operations

                  geometric spatial transformations

                  Single-pixel operations

                  - change the values of intensity for the individual pixels ( )s T z

                  where z is the intensity of a pixel in the original image and s is the intensity of the

                  corresponding pixel in the processed image

                  Digital Image Processing

                  Week 1

                  Neighborhood operations

                  Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                  in an image f Neighborhood processing generates new intensity level at point (x y)

                  based on the values of the intensities of the points in Sxy For example if Sxy is a

                  rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                  intensity by computing the average value of the pixels in Sxy

                  ( )

                  1( ) ( )xyr c S

                  g x y f r cm n

                  The net effect is to perform local blurring in the original image This type of process is

                  used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                  largest region of an image

                  Digital Image Processing

                  Week 1

                  Geometric spatial transformations and image registration

                  - modify the spatial relationship between pixels in an image

                  - these transformations are often called rubber-sheet transformations (analogous to

                  printing an image on a sheet of rubber and then stretching the sheet according to a

                  predefined set of rules

                  A geometric transformation consists of 2 basic operations

                  1 a spatial transformation of coordinates

                  2 intensity interpolation that assign intensity values to the spatial transformed

                  pixels

                  The coordinate system transformation ( ) [( )]x y T v w

                  (v w) ndash pixel coordinates in the original image

                  (x y) ndash pixel coordinates in the transformed image

                  Digital Image Processing

                  Week 1

                  [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                  Affine transform

                  11 1211 21 31

                  21 2212 22 33

                  31 32

                  0[ 1] [ 1] [ 1] 0

                  1

                  t tx t v t w t

                  x y v w T v w t ty t v t w t

                  t t

                  (AT)

                  This transform can scale rotate translate or shear a set of coordinate points depending

                  on the elements of the matrix T If we want to resize an image rotate it and move the

                  result to some location we simply form a 3x3 matrix equal to the matrix product of the

                  scaling rotation and translation matrices from Table 1

                  Digital Image Processing

                  Week 1

                  Affine transformations

                  Digital Image Processing

                  Week 1

                  The preceding transformations relocate pixels on an image to new locations To complete

                  the process we have to assign intensity values to those locations This task is done by

                  using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                  In practice we can use equation (AT) in two basic ways

                  forward mapping scan the pixels of the input image (v w) compute the new spatial

                  location (x y) of the corresponding pixel in the new image using (AT) directly

                  Problems

                  - intensity assignment when 2 or more pixels in the original image are transformed to

                  the same location in the output image

                  - some output locations have no correspondent in the original image (no intensity

                  assignment)

                  Digital Image Processing

                  Week 1

                  inverse mapping scans the output pixel locations and at each location (x y)

                  computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                  It then interpolates among the nearest input pixels to determine the intensity of the output

                  pixel value

                  Inverse mappings are more efficient to implement than forward mappings and are used in

                  numerous commercial implementations of spatial transformations (MATLAB for ex)

                  Digital Image Processing

                  Week 1

                  Digital Image Processing

                  Week 1

                  Image registration ndash align two or more images of the same scene

                  In image registration we have available the input and output images but the specific

                  transformation that produced the output image from the input is generally unknown

                  The problem is to estimate the transformation function and then use it to register the two

                  images

                  - it may be of interest to align (register) two or more image taken at approximately the

                  same time but using different imaging systems (MRI scanner and a PET scanner)

                  - align images of a given location taken by the same instrument at different moments

                  of time (satellite images)

                  Solving the problem using tie points (also called control points) which are

                  corresponding points whose locations are known precisely in the input and reference

                  image

                  Digital Image Processing

                  Week 1

                  How to select tie points

                  - interactively selecting them

                  - use of algorithms that try to detect these points

                  - some imaging systems have physical artifacts (small metallic objects) embedded in

                  the imaging sensors These objects produce a set of known points (called reseau

                  marks) directly on all images captured by the system which can be used as guides

                  for establishing tie points

                  The problem of estimating the transformation is one of modeling Suppose we have a set

                  of 4 tie points both on the input image and the reference image A simple model based on

                  a bilinear approximation is given by

                  1 2 3 4

                  5 6 7 8

                  x c v c w c v w cy c v c w c v w c

                  (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                  Digital Image Processing

                  Week 1

                  When 4 tie points are insufficient to obtain satisfactory registration an approach used

                  frequently is to select a larger number of tie points and using this new set of tie points

                  subdivide the image in rectangular regions marked by groups of 4 tie points On the

                  subregions marked by 4 tie points we applied the transformation model described above

                  The number of tie points and the sophistication of the model required to solve the register

                  problem depend on the severity of the geometrical distortion

                  Digital Image Processing

                  Week 1

                  a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                  Digital Image Processing

                  Week 1

                  Probabilistic Methods

                  zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                  p(zk) = the probability that the intensity level zk occurs in the given image

                  ( ) kk

                  np zM N

                  nk = the number of times that intensity zk occurs in the image (MN is the total number of

                  pixels in the image) 1

                  0( ) 1

                  L

                  kk

                  p z

                  The mean (average) intensity of an image is given by 1

                  0( )

                  L

                  k kk

                  m z p z

                  Digital Image Processing

                  Week 1

                  The variance of the intensities is 1

                  2 2

                  0( ) ( )

                  L

                  k kk

                  z m p z

                  The variance is a measure of the spread of the values of z about the mean so it is a

                  measure of image contrast Usually for measuring image contrast the standard deviation

                  ( ) is used

                  The n-th moment of a random variable z about the mean is defined as 1

                  0( ) ( ) ( )

                  Ln

                  n k kk

                  z z m p z

                  ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                  3( ) 0z the intensities are biased to values higher than the mean

                  ( 3( ) 0z the intensities are biased to values lower than the mean

                  Digital Image Processing

                  Week 1

                  3( ) 0z the intensities are distributed approximately equally on both side of the

                  mean

                  Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                  Figure 1(a) ndash standard deviation 143 (variance = 2045)

                  Figure 1(b) ndash standard deviation 316 (variance = 9986)

                  Figure 1(c) ndash standard deviation 492 (variance = 24206)

                  Digital Image Processing

                  Week 1

                  Intensity Transformations and Spatial Filtering

                  ( ) ( )g x y T f x y

                  f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                  neighborhood of (x y)

                  - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                  and much smaller in size than the image

                  Digital Image Processing

                  Week 1

                  - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                  called spatial filter (spatial mask kernel template or window)

                  ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                  ( )s T r

                  s and r are denoting respectively the intensity of g and f at (x y)

                  Figure 2 left - T produces an output image of higher contrast than the original by

                  darkening the intensity levels below k and brightening the levels above k ndash this technique

                  is called contrast stretching

                  Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                  Digital Image Processing

                  Week 1

                  Figure 2 right - T produces a binary output image A mapping of this form is called

                  thresholding function

                  Some Basic Intensity Transformation Functions

                  Image Negatives

                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                  - equivalent of a photographic negative

                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                  image

                  Digital Image Processing

                  Week 1

                  Original Negative image

                  Digital Image Processing

                  Week 1

                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                  Some basic intensity transformation functions

                  Digital Image Processing

                  Week 1

                  This transformation maps a narrow range of low intensity values in the input into a wider

                  range An operator of this type is used to expand the values of dark pixels in an image

                  while compressing the higher-level values The opposite is true for the inverse log

                  transformation The log functions compress the dynamic range of images with large

                  variations in pixel values

                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                  Digital Image Processing

                  Week 1

                  Power-Law (Gamma) Transformations

                  - positive constants( ) ( ( ) )s T r c r c s c r

                  Plots of gamma transformation for different values of γ (c=1)

                  Digital Image Processing

                  Week 1

                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                  of output values with the opposite being true for higher values of input values The

                  curves with 1 have the opposite effect of those generated with values of 1

                  1c - identity transformation

                  A variety of devices used for image capture printing and display respond according to a

                  power law The process used to correct these power-law response phenomena is called

                  gamma correction

                  Digital Image Processing

                  Week 1

                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                  Digital Image Processing

                  Week 1

                  Piecewise-Linear Transformations Functions

                  Contrast stretching

                  - a process that expands the range of intensity levels in an image so it spans the full

                  intensity range of the recording tool or display device

                  a b c d Fig5

                  Digital Image Processing

                  Week 1

                  11

                  1

                  2 1 1 21 2

                  2 1 2 1

                  22

                  2

                  [0 ]

                  ( ) ( )( ) [ ]( ) ( )

                  ( 1 ) [ 1]( 1 )

                  s r r rrs r r s r rT r r r r

                  r r r rs L r r r L

                  L r

                  Digital Image Processing

                  Week 1

                  1 1 2 2r s r s identity transformation (no change)

                  1 2 1 2 0 1r r s s L thresholding function

                  Figure 5(b) shows an 8-bit image with low contrast

                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                  in the image respectively Thus the transformation function stretched the levels linearly

                  from their original range to the full range [0 L-1]

                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                  2 2 1r s m L where m is the mean gray level in the image

                  The original image on which these results are based is a scanning electron microscope

                  image of pollen magnified approximately 700 times

                  Digital Image Processing

                  Week 1

                  Intensity-level slicing

                  - highlighting a specific range of intensities in an image

                  There are two approaches for intensity-level slicing

                  1 display in one value (white for example) all the values in the range of interest and in

                  another (say black) all other intensities (Figure 311 (a))

                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                  intensities in the image (Figure 311 (b))

                  Digital Image Processing

                  Week 1

                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                  the top of the scale of intensities This type of enhancement produces a binary image

                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                  Highlights range [A B] and preserves all other intensities

                  Digital Image Processing

                  Week 1

                  which is useful for studying the shape of the flow of the contrast substance (to detect

                  blockageshellip)

                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                  image around the mean intensity was set to black the other intensities remain unchanged

                  Fig 6 - Aortic angiogram and intensity sliced versions

                  Digital Image Processing

                  Week 1

                  Bit-plane slicing

                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                  This technique highlights the contribution made to the whole image appearances by each

                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                  Digital Image Processing

                  Week 1

                  Digital Image Processing

                  Week 1

                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                  • DIP 1 2017
                  • DIP 02 (2017)

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Distinction between image processing image analysis computer vision

                    low-level mid-level high-level processes

                    Low-level processes image preprocessing to reduce noise contrast enhancement image sharpening both input and output are images

                    Mid-level processes segmentation partitioning images into regions or objects description of the objects for computer processing classificationrecognition of individual objects inputs are generally images outputs are attributes extracted from the input image (eg edges contours identity of individual objects)

                    High-level processes ldquomaking senserdquo of a set of recognized objects performing the cognitive functions associated with vision

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Digital Image Processing (Gonzalez + Woods) =

                    processes whose inputs and outputs are images +

                    processes that extract attributes from images recognition of individual objects

                    (low- and mid-level processes)

                    Example

                    automated analysis of text = acquiring an image containing text preprocessing the image (enhancement sharpening) extracting (segmenting) the individual characaters describing the characters in a form suitable for computer processing recognition of individual characters

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    The Origins of DIP

                    Newspaper industry pictures were sent by submarine cable between London and New York

                    Before Bartlane cable picture transmission system (early 1920s) ndash 1 week

                    With Bartlane system less than 3 hours

                    Specialized printing equipment coded pictures for cable transmission and reconstructed them at the receiving end (1920s -5 distict levels of gray 1929 ndash 15 levels)

                    This example is not DIP the computer is not involved

                    DIP is linked and devolps in the same rhythm as digital computers (data storage display and transmission)

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    A digital pictureproduced in 1921from a coded tapeby a telegraph printer withspecial type faces(McFarlane)

                    A digital picturemade in 1922from a tapepunched after the signals hadcrossed the Atlantic twice(McFarlane)

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    1964 Jet Propulsion Laboratory (Pasadena California) processed pictures of the moon transmitted by Ranger 7 (corrected image distortions)

                    The first picture of the moon by a US spacecraft Ranger 7 took this image July 31 1964 about 17 minutes before impacting the lunar surface(Courtesy of NASA)

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

                    1970s ndash invention of CAT (computerized axial tomography)

                    CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    loz geographers use DIP to study pollution patterns from aerial and satellite imagery

                    loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

                    loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

                    loz astronomy biology nuclear medicine law enforcement industry

                    DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

                    loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Examples of Fields that Use DIP

                    Images can be classified according to their sources (visual X-ray hellip)

                    Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Electromagnetic waves can be thought as propagating sinusoidal

                    waves of different wavelength or as a stream of massless particles

                    each moving in a wavelike pattern with the speed of light Each

                    massless particle contains a certain amount (bundle) of energy Each

                    bundle of energy is called a photon If spectral bands are grouped

                    according to energy per photon we obtain the spectrum shown in the

                    image above ranging from gamma-rays (highest energy) to radio

                    waves (lowest energy)

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Gamma-Ray Imaging

                    Nuclear medicine astronomical observations

                    Nuclear medicine

                    the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

                    Images are produced from the emissions collected by gamma-ray detectors

                    Images of this sort are used to locate sites of bone pathology (infections tumors)

                    PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Examples of gamma-ray imaging

                    Bone scan PET image

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    X-ray imaging

                    Medical diagnosticindustry astronomy

                    A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                    The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Angiography = contrast-enhancement radiography

                    Angiograms = images of blood vessels

                    A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                    X-rays are used in CAT (computerized axial tomography)

                    X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                    Industrial CAT scans are useful when the parts can be penetreted by X-rays

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Examples of X-ray imaging

                    Chest X-rayAortic angiogram

                    Head CT Cygnus LoopCircuit boards

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Imaging in the Ultraviolet Band

                    Litography industrial inspection microscopy biological imaging astronomical observations

                    Ultraviolet light is used in fluorescence microscopy

                    Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                    other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                    and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Imaging in the Visible and Infrared Bands

                    Light microscopy astronomy remote sensing industry law enforcement

                    LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                    Weather observations and prediction produce major applications of multispectral image from satellites

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Satellite images of Washington DC area in spectral bands of the Table 1

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Examples of light microscopy

                    Taxol (anticancer agent)magnified 250X

                    Cholesterol(40X)

                    Microprocessor(60X)

                    Nickel oxidethin film(600X)

                    Surface of audio CD(1750X)

                    Organicsuperconductor(450X)

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Automated visual inspection of manufactured goods

                    a bc de f

                    a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Imaging in the Microwave Band

                    The dominant aplication of imaging in the microwave band ndash radar

                    bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                    bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                    bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                    An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Spaceborne radar image of mountains in southeast Tibet

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Imaging in the Radio Band

                    medicine astronomy

                    MRI = Magnetic Resonance Imaging

                    This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                    Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                    The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    MRI images of a human knee (left) and spine (right)

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Images of the Crab Pulsar covering the electromagnetic spectrum

                    Gamma X-ray Optical Infrared Radio

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Other Imaging Modalities

                    acoustic imaging electron microscopy synthetic (computer-generated) imaging

                    Imaging using sound geological explorations industry medicine

                    Mineral and oil exploration

                    For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Digital Image ProcessingDigital Image Processing

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                    Biometry - iris

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Biometry - fingerprint

                    Digital Image ProcessingDigital Image Processing

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                    Face detection and recognition

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Gender identification

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Image morphing

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Fundamental Steps in DIP

                    methods whose input and output are images

                    methods whose inputs are images but whose outputs are attributes extracted from those images

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Outputs are images

                    bull image acquisition

                    bull image filtering and enhancement

                    bull image restoration

                    bull color image processing

                    bull wavelets and multiresolution processing

                    bull compression

                    bull morphological processing

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Outputs are attributes

                    bull morphological processing

                    bull segmentation

                    bull representation and description

                    bull object recognition

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Image acquisition - may involve preprocessing such as scaling

                    Image enhancement

                    bull manipulating an image so that the result is more suitable than the original for a specific operation

                    bull enhancement is problem oriented

                    bull there is no general sbquotheoryrsquo of image enhancement

                    bull enhancement use subjective methods for image emprovement

                    bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Image restoration

                    bull improving the appearance of an image

                    bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                    Color image processing

                    bull fundamental concept in color models

                    bull basic color processing in a digital domain

                    Wavelets and multiresolution processing

                    representing images in various degree of resolution

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Compression

                    reducing the storage required to save an image or the bandwidth required to transmit it

                    Morphological processing

                    bull tools for extracting image components that are useful in the representation and description of shape

                    bull a transition from processes that output images to processes that outputimage attributes

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Segmentation

                    bull partitioning an image into its constituents parts or objects

                    bull autonomous segmentation is one of the most difficult tasks of DIP

                    bull the more accurate the segmentation the more likley recognition is to succeed

                    Representation and description (almost always follows segmentation)

                    bull segmentation produces either the boundary of a region or all the poits in the region itself

                    bull converting the data produced by segmentation to a form suitable for computer processing

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    bull boundary representation the focus is on external shape characteristics such as corners or inflections

                    bull complete region the focus is on internal properties such as texture or skeletal shape

                    bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                    Object recognition

                    the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                    Knowledge database

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Simplified diagramof a cross sectionof the human eye

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                    The cornea is a tough transparent tissue that covers the anterior surface of the eye

                    Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                    The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                    The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                    Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                    Fovea = the place where the image of the object of interest falls on

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                    Blind spot region without receptors

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Image formation in the eye

                    Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                    Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                    distance between lens and retina along visual axix = 17 mm

                    range of focal length = 14 mm to 17 mm

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Optical illusions

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                    gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                    quantities that describe the quality of a chromatic light source radiance

                    the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                    measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                    brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    the physical meaning is determined by the source of the image

                    ( )f D f x y

                    Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                    f(xy) ndash characterized by two components

                    i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                    r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                    ( ) ( ) ( )

                    0 ( ) 0 ( ) 1

                    f x y i x y r x y

                    i x y r x y

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    r(xy)=0 - total absorption r(xy)=1 - total reflectance

                    i(xy) ndash determined by the illumination source

                    r(xy) ndash determined by the characteristics of the imaged objects

                    is called gray (or intensity) scale

                    In practice

                    min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                    indoor values without additional illuminationmin max10 1000L L

                    black whitemin max0 1 0 1 0 1L L L L l l L

                    min maxL L

                    Digital Image ProcessingDigital Image Processing

                    Week 1Week 1

                    Digital Image Processing

                    Week 1

                    Image Sampling and Quantization

                    - the output of the sensors is a continuous voltage waveform related to the sensed

                    scene

                    converting a continuous image f to digital form

                    - digitizing (x y) is called sampling

                    - digitizing f(x y) is called quantization

                    Digital Image Processing

                    Week 1

                    Digital Image Processing

                    Week 1

                    Continuous image projected onto a sensor array Result of image sampling and quantization

                    Digital Image Processing

                    Week 1

                    Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                    (00) (01) (0 1)(10) (11) (1 1)

                    ( )

                    ( 10) ( 11) ( 1 1)

                    f f f Nf f f N

                    f x y

                    f M f M f M N

                    image element pixel

                    00 01 0 1

                    10 11 1 1

                    10 11 1 1

                    ( ) ( )

                    N

                    i jN M N

                    i j

                    M M M N

                    a a aa f x i y j f i ja a a

                    Aa

                    a a a

                    f(00) ndash the upper left corner of the image

                    Digital Image Processing

                    Week 1

                    M N ge 0 L=2k

                    [0 1]i j i ja a L

                    Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                    Digital Image Processing

                    Week 1

                    Digital Image Processing

                    Week 1

                    Number of bits required to store a digitized image

                    for 2 b M N k M N b N k

                    When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                    Digital Image Processing

                    Week 1

                    Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                    Measures line pairs per unit distance dots (pixels) per unit distance

                    Image resolution = the largest number of discernible line pairs per unit distance

                    (eg 100 line pairs per mm)

                    Dots per unit distance are commonly used in printing and publishing

                    In US the measure is expressed in dots per inch (dpi)

                    (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                    Intensity resolution ndash the smallest discernible change in intensity level

                    The number of intensity levels (L) is determined by hardware considerations

                    L=2k ndash most common k = 8

                    Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                    Digital Image Processing

                    Week 1

                    Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                    150 dpi (lower left) 72 dpi (lower right)

                    Digital Image Processing

                    Week 1

                    Reducing the number of gray levels 256 128 64 32

                    Digital Image Processing

                    Week 1

                    Reducing the number of gray levels 16 8 4 2

                    Digital Image Processing

                    Week 1

                    Image Interpolation - used in zooming shrinking rotating and geometric corrections

                    Shrinking zooming ndash image resizing ndash image resampling methods

                    Interpolation is the process of using known data to estimate values at unknown locations

                    Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                    750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                    same spacing as the original and then shrink it so that it fits exactly over the original

                    image The pixel spacing in the 750 times 750 grid will be less than in the original image

                    Problem assignment of intensity-level in the new 750 times 750 grid

                    Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                    intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                    This technique has the tendency to produce undesirable effects like severe distortion of

                    straight edges

                    Digital Image Processing

                    Week 1

                    Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                    where the four coefficients are determined from the 4 equations in 4 unknowns that can

                    be written using the 4 nearest neighbors of point (x y)

                    Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                    modest increase in computational effort

                    Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                    nearest neighbors of the point 3 3

                    0 0

                    ( ) i ji j

                    i jv x y c x y

                    The coefficients cij are obtained solving a 16x16 linear system

                    intensity levels of the 16 nearest neighbors of 3 3

                    0 0

                    ( )i ji j

                    i jc x y x y

                    Digital Image Processing

                    Week 1

                    Generally bicubic interpolation does a better job of preserving fine detail than the

                    bilinear technique Bicubic interpolation is the standard used in commercial image editing

                    programs such as Adobe Photoshop and Corel Photopaint

                    Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                    the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                    then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                    neighbor interpolation was used (both for shrinking and zooming)

                    Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                    interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                    from 1250 dpi to 150 dpi (instead of 72 dpi)

                    Digital Image Processing

                    Week 1

                    Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                    Digital Image Processing

                    Week 1

                    Neighbors of a Pixel

                    A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                    This set of pixels called the 4-neighbors of p denoted by N4 (p)

                    The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                    and are denoted ND(p)

                    The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                    N8 (p)

                    If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                    fall outside the image

                    Digital Image Processing

                    Week 1

                    Adjacency Connectivity Regions Boundaries

                    Denote by V the set of intensity levels used to define adjacency

                    - in a binary image V 01 (V=0 V=1)

                    - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                    We consider 3 types of adjacency

                    (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                    (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                    (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                    m-adjacent if

                    4( )q N p or

                    ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                    Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                    ambiguities that often arise when 8-adjacency is used Consider the example

                    Digital Image Processing

                    Week 1

                    binary image

                    0 1 1 0 1 1 0 1 1

                    1 0 1 0 0 1 0 0 1 0

                    0 0 1 0 0 1 0 0 1

                    V

                    The three pixels at the top (first line) in the above example show multiple (ambiguous)

                    8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                    m-adjacency

                    Digital Image Processing

                    Week 1

                    A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                    is a sequence of distinct pixels with coordinates

                    and are adjacent 0 0 1 1

                    1 1

                    ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                    n n

                    i i i i

                    x y x y x y x y s tx y x y i n

                    The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                    Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                    Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                    in S if there exists a path between them consisting only of pixels from S

                    S is a connected set if there is a path in S between any 2 pixels in S

                    Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                    Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                    that are not adjacent are said to be disjoint When referring to regions only 4- and

                    8-adjacency are considered

                    Digital Image Processing

                    Week 1

                    Suppose that an image contains K disjoint regions 1 kR k K none of which

                    touches the image border

                    the complement of 1

                    ( )K

                    cu k u u

                    k

                    R R R R

                    We call all the points in Ru the foreground of the image and the points in ( )cuR the

                    background of the image

                    The boundary (border or contour) of a region R is the set of points that are adjacent to

                    points in the complement of R (R)c The border of an image is the set of pixels in the

                    region that have at least one background neighbor This definition is referred to as the

                    inner border to distinguish it from the notion of outer border which is the corresponding

                    border in the background

                    Digital Image Processing

                    Week 1

                    Distance measures

                    For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                    function or metric if

                    (a) D(p q) ge 0 D(p q) = 0 iff p=q

                    (b) D(p q) = D(q p)

                    (c) D(p z) le D(p q) + D(q z)

                    The Euclidean distance between p and q is defined as 1

                    2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                    The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                    centered at (x y)

                    Digital Image Processing

                    Week 1

                    The D4 distance (also called city-block distance) between p and q is defined as

                    4( ) | | | |D p q x s y t

                    The pixels q for which 4( )D p q r form a diamond centered at (xy)

                    4

                    22 1 2

                    2 2 1 0 1 22 1 2

                    2

                    D

                    The pixels with D4 = 1 are the 4-neighbors of (x y)

                    The D8 distance (called the chessboard distance) between p and q is defined as

                    8( ) max| | | |D p q x s y t

                    The pixels q for which 8( )D p q r form a square centered at (x y)

                    Digital Image Processing

                    Week 1

                    8

                    2 2 2 2 22 1 1 1 2

                    2 2 1 0 1 22 1 1 1 22 2 2 2 2

                    D

                    The pixels with D8 = 1 are the 8-neighbors of (x y)

                    D4 and D8 distances are independent of any paths that might exist between p and q

                    because these distances involve only the coordinates of the point

                    Digital Image Processing

                    Week 1

                    Array versus Matrix Operations

                    An array operation involving one or more images is carried out on a pixel-by-pixel basis

                    11 12 11 12

                    21 22 21 22

                    a a b ba a b b

                    Array product

                    11 12 11 12 11 11 12 12

                    21 22 21 22 21 21 22 21

                    a a b b a b a ba a b b a b a b

                    Matrix product

                    11 12 11 12 11 11 12 21 11 12 12 21

                    21 22 21 22 21 11 22 21 21 12 22 22

                    a a b b a b a b a b a ba a b b a b a b a b a b

                    We assume array operations unless stated otherwise

                    Digital Image Processing

                    Week 1

                    Linear versus Nonlinear Operations

                    One of the most important classifications of image-processing methods is whether it is

                    linear or nonlinear

                    ( ) ( )H f x y g x y

                    H is said to be a linear operator if

                    images1 2 1 2

                    1 2

                    ( ) ( ) ( ) ( )

                    H a f x y b f x y a H f x y b H f x y

                    a b f f

                    Example of nonlinear operator

                    the maximum value of the pixels of image max ( )H f f x y f

                    1 2

                    0 2 6 5 1 1

                    2 3 4 7f f a b

                    Digital Image Processing

                    Week 1

                    1 2

                    0 2 6 5 6 3max max 1 ( 1) max 2

                    2 3 4 7 2 4a f b f

                    0 2 6 51 max ( 1) max 3 ( 1)7 4

                    2 3 4 7

                    Arithmetic Operations in Image Processing

                    Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                    f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                    For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                    value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                    The two random variables are uncorrelated when their covariance is 0

                    Digital Image Processing

                    Week 1

                    Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                    used in image enhancement)

                    1

                    1( ) ( )K

                    ii

                    g x y g x yK

                    If the noise satisfies the properties stated above we have

                    2 2( ) ( )

                    1( ) ( ) g x y x yE g x y f x yK

                    ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                    and g respectively The standard deviation (square root of the variance) at any point in

                    the average image is

                    ( ) ( )1

                    g x y x yK

                    Digital Image Processing

                    Week 1

                    As K increases the variability (as measured by the variance or the standard deviation) of

                    the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                    means that ( )g x y approaches f(x y) as the number of noisy images used in the

                    averaging process increases

                    An important application of image averaging is in the field of astronomy where imaging

                    under very low light levels frequently causes sensor noise to render single images

                    virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                    was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                    64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                    images respectively

                    Digital Image Processing

                    Week 1

                    Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                    100 noisy images

                    a b c d e f

                    Digital Image Processing

                    Week 1

                    A frequent application of image subtraction is in the enhancement of differences between

                    images

                    (a) (b) (c)

                    Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                    significant bit of each pixel (c) the difference between the two images

                    Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                    Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                    images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                    difference between images (a) and (b)

                    Digital Image Processing

                    Week 1

                    Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                    h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                    intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                    source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                    bloodstream taking a series of images called live images (denoted f(x y)) of the same

                    anatomical region as h(x y) and subtracting the mask from the series of incoming live

                    images after injection of the contrast medium

                    In g(x y) we can find the differences between h and f as enhanced detail

                    Images being captured at TV rates we obtain a movie showing how the contrast medium

                    propagates through the various arteries in the area being observed

                    Digital Image Processing

                    Week 1

                    a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                    Digital Image Processing

                    Week 1

                    An important application of image multiplication (and division) is shading correction

                    Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                    f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                    When the shading function is known

                    ( )( )( )

                    g x yf x yh x y

                    h(x y) is unknown but we have access to the imaging system we can obtain an

                    approximation to the shading function by imaging a target of constant intensity When the

                    sensor is not available often the shading pattern can be estimated from the image

                    Digital Image Processing

                    Week 1

                    (a) (b) (c)

                    Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                    Digital Image Processing

                    Week 1

                    Another use of image multiplication is in masking also called region of interest (ROI)

                    operations The process consists of multiplying a given image by a mask image that has

                    1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                    image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                    (a) (b) (c)

                    Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                    Digital Image Processing

                    Week 1

                    In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                    min( )mf f f

                    0 ( 255)max( )

                    ms

                    m

                    ff K K K

                    f

                    Digital Image Processing

                    Week 1

                    Spatial Operations

                    - are performed directly on the pixels of a given image

                    There are three categories of spatial operations

                    single-pixel operations

                    neighborhood operations

                    geometric spatial transformations

                    Single-pixel operations

                    - change the values of intensity for the individual pixels ( )s T z

                    where z is the intensity of a pixel in the original image and s is the intensity of the

                    corresponding pixel in the processed image

                    Digital Image Processing

                    Week 1

                    Neighborhood operations

                    Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                    in an image f Neighborhood processing generates new intensity level at point (x y)

                    based on the values of the intensities of the points in Sxy For example if Sxy is a

                    rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                    intensity by computing the average value of the pixels in Sxy

                    ( )

                    1( ) ( )xyr c S

                    g x y f r cm n

                    The net effect is to perform local blurring in the original image This type of process is

                    used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                    largest region of an image

                    Digital Image Processing

                    Week 1

                    Geometric spatial transformations and image registration

                    - modify the spatial relationship between pixels in an image

                    - these transformations are often called rubber-sheet transformations (analogous to

                    printing an image on a sheet of rubber and then stretching the sheet according to a

                    predefined set of rules

                    A geometric transformation consists of 2 basic operations

                    1 a spatial transformation of coordinates

                    2 intensity interpolation that assign intensity values to the spatial transformed

                    pixels

                    The coordinate system transformation ( ) [( )]x y T v w

                    (v w) ndash pixel coordinates in the original image

                    (x y) ndash pixel coordinates in the transformed image

                    Digital Image Processing

                    Week 1

                    [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                    Affine transform

                    11 1211 21 31

                    21 2212 22 33

                    31 32

                    0[ 1] [ 1] [ 1] 0

                    1

                    t tx t v t w t

                    x y v w T v w t ty t v t w t

                    t t

                    (AT)

                    This transform can scale rotate translate or shear a set of coordinate points depending

                    on the elements of the matrix T If we want to resize an image rotate it and move the

                    result to some location we simply form a 3x3 matrix equal to the matrix product of the

                    scaling rotation and translation matrices from Table 1

                    Digital Image Processing

                    Week 1

                    Affine transformations

                    Digital Image Processing

                    Week 1

                    The preceding transformations relocate pixels on an image to new locations To complete

                    the process we have to assign intensity values to those locations This task is done by

                    using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                    In practice we can use equation (AT) in two basic ways

                    forward mapping scan the pixels of the input image (v w) compute the new spatial

                    location (x y) of the corresponding pixel in the new image using (AT) directly

                    Problems

                    - intensity assignment when 2 or more pixels in the original image are transformed to

                    the same location in the output image

                    - some output locations have no correspondent in the original image (no intensity

                    assignment)

                    Digital Image Processing

                    Week 1

                    inverse mapping scans the output pixel locations and at each location (x y)

                    computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                    It then interpolates among the nearest input pixels to determine the intensity of the output

                    pixel value

                    Inverse mappings are more efficient to implement than forward mappings and are used in

                    numerous commercial implementations of spatial transformations (MATLAB for ex)

                    Digital Image Processing

                    Week 1

                    Digital Image Processing

                    Week 1

                    Image registration ndash align two or more images of the same scene

                    In image registration we have available the input and output images but the specific

                    transformation that produced the output image from the input is generally unknown

                    The problem is to estimate the transformation function and then use it to register the two

                    images

                    - it may be of interest to align (register) two or more image taken at approximately the

                    same time but using different imaging systems (MRI scanner and a PET scanner)

                    - align images of a given location taken by the same instrument at different moments

                    of time (satellite images)

                    Solving the problem using tie points (also called control points) which are

                    corresponding points whose locations are known precisely in the input and reference

                    image

                    Digital Image Processing

                    Week 1

                    How to select tie points

                    - interactively selecting them

                    - use of algorithms that try to detect these points

                    - some imaging systems have physical artifacts (small metallic objects) embedded in

                    the imaging sensors These objects produce a set of known points (called reseau

                    marks) directly on all images captured by the system which can be used as guides

                    for establishing tie points

                    The problem of estimating the transformation is one of modeling Suppose we have a set

                    of 4 tie points both on the input image and the reference image A simple model based on

                    a bilinear approximation is given by

                    1 2 3 4

                    5 6 7 8

                    x c v c w c v w cy c v c w c v w c

                    (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                    Digital Image Processing

                    Week 1

                    When 4 tie points are insufficient to obtain satisfactory registration an approach used

                    frequently is to select a larger number of tie points and using this new set of tie points

                    subdivide the image in rectangular regions marked by groups of 4 tie points On the

                    subregions marked by 4 tie points we applied the transformation model described above

                    The number of tie points and the sophistication of the model required to solve the register

                    problem depend on the severity of the geometrical distortion

                    Digital Image Processing

                    Week 1

                    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                    Digital Image Processing

                    Week 1

                    Probabilistic Methods

                    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                    p(zk) = the probability that the intensity level zk occurs in the given image

                    ( ) kk

                    np zM N

                    nk = the number of times that intensity zk occurs in the image (MN is the total number of

                    pixels in the image) 1

                    0( ) 1

                    L

                    kk

                    p z

                    The mean (average) intensity of an image is given by 1

                    0( )

                    L

                    k kk

                    m z p z

                    Digital Image Processing

                    Week 1

                    The variance of the intensities is 1

                    2 2

                    0( ) ( )

                    L

                    k kk

                    z m p z

                    The variance is a measure of the spread of the values of z about the mean so it is a

                    measure of image contrast Usually for measuring image contrast the standard deviation

                    ( ) is used

                    The n-th moment of a random variable z about the mean is defined as 1

                    0( ) ( ) ( )

                    Ln

                    n k kk

                    z z m p z

                    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                    3( ) 0z the intensities are biased to values higher than the mean

                    ( 3( ) 0z the intensities are biased to values lower than the mean

                    Digital Image Processing

                    Week 1

                    3( ) 0z the intensities are distributed approximately equally on both side of the

                    mean

                    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                    Figure 1(a) ndash standard deviation 143 (variance = 2045)

                    Figure 1(b) ndash standard deviation 316 (variance = 9986)

                    Figure 1(c) ndash standard deviation 492 (variance = 24206)

                    Digital Image Processing

                    Week 1

                    Intensity Transformations and Spatial Filtering

                    ( ) ( )g x y T f x y

                    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                    neighborhood of (x y)

                    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                    and much smaller in size than the image

                    Digital Image Processing

                    Week 1

                    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                    called spatial filter (spatial mask kernel template or window)

                    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                    ( )s T r

                    s and r are denoting respectively the intensity of g and f at (x y)

                    Figure 2 left - T produces an output image of higher contrast than the original by

                    darkening the intensity levels below k and brightening the levels above k ndash this technique

                    is called contrast stretching

                    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                    Digital Image Processing

                    Week 1

                    Figure 2 right - T produces a binary output image A mapping of this form is called

                    thresholding function

                    Some Basic Intensity Transformation Functions

                    Image Negatives

                    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                    - equivalent of a photographic negative

                    - technique suited for enhancing white or gray detail embedded in dark regions of an

                    image

                    Digital Image Processing

                    Week 1

                    Original Negative image

                    Digital Image Processing

                    Week 1

                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                    Some basic intensity transformation functions

                    Digital Image Processing

                    Week 1

                    This transformation maps a narrow range of low intensity values in the input into a wider

                    range An operator of this type is used to expand the values of dark pixels in an image

                    while compressing the higher-level values The opposite is true for the inverse log

                    transformation The log functions compress the dynamic range of images with large

                    variations in pixel values

                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                    Digital Image Processing

                    Week 1

                    Power-Law (Gamma) Transformations

                    - positive constants( ) ( ( ) )s T r c r c s c r

                    Plots of gamma transformation for different values of γ (c=1)

                    Digital Image Processing

                    Week 1

                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                    of output values with the opposite being true for higher values of input values The

                    curves with 1 have the opposite effect of those generated with values of 1

                    1c - identity transformation

                    A variety of devices used for image capture printing and display respond according to a

                    power law The process used to correct these power-law response phenomena is called

                    gamma correction

                    Digital Image Processing

                    Week 1

                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                    Digital Image Processing

                    Week 1

                    Piecewise-Linear Transformations Functions

                    Contrast stretching

                    - a process that expands the range of intensity levels in an image so it spans the full

                    intensity range of the recording tool or display device

                    a b c d Fig5

                    Digital Image Processing

                    Week 1

                    11

                    1

                    2 1 1 21 2

                    2 1 2 1

                    22

                    2

                    [0 ]

                    ( ) ( )( ) [ ]( ) ( )

                    ( 1 ) [ 1]( 1 )

                    s r r rrs r r s r rT r r r r

                    r r r rs L r r r L

                    L r

                    Digital Image Processing

                    Week 1

                    1 1 2 2r s r s identity transformation (no change)

                    1 2 1 2 0 1r r s s L thresholding function

                    Figure 5(b) shows an 8-bit image with low contrast

                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                    in the image respectively Thus the transformation function stretched the levels linearly

                    from their original range to the full range [0 L-1]

                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                    2 2 1r s m L where m is the mean gray level in the image

                    The original image on which these results are based is a scanning electron microscope

                    image of pollen magnified approximately 700 times

                    Digital Image Processing

                    Week 1

                    Intensity-level slicing

                    - highlighting a specific range of intensities in an image

                    There are two approaches for intensity-level slicing

                    1 display in one value (white for example) all the values in the range of interest and in

                    another (say black) all other intensities (Figure 311 (a))

                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                    intensities in the image (Figure 311 (b))

                    Digital Image Processing

                    Week 1

                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                    the top of the scale of intensities This type of enhancement produces a binary image

                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                    Highlights range [A B] and preserves all other intensities

                    Digital Image Processing

                    Week 1

                    which is useful for studying the shape of the flow of the contrast substance (to detect

                    blockageshellip)

                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                    image around the mean intensity was set to black the other intensities remain unchanged

                    Fig 6 - Aortic angiogram and intensity sliced versions

                    Digital Image Processing

                    Week 1

                    Bit-plane slicing

                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                    This technique highlights the contribution made to the whole image appearances by each

                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                    Digital Image Processing

                    Week 1

                    Digital Image Processing

                    Week 1

                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                    • DIP 1 2017
                    • DIP 02 (2017)

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Digital Image Processing (Gonzalez + Woods) =

                      processes whose inputs and outputs are images +

                      processes that extract attributes from images recognition of individual objects

                      (low- and mid-level processes)

                      Example

                      automated analysis of text = acquiring an image containing text preprocessing the image (enhancement sharpening) extracting (segmenting) the individual characaters describing the characters in a form suitable for computer processing recognition of individual characters

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      The Origins of DIP

                      Newspaper industry pictures were sent by submarine cable between London and New York

                      Before Bartlane cable picture transmission system (early 1920s) ndash 1 week

                      With Bartlane system less than 3 hours

                      Specialized printing equipment coded pictures for cable transmission and reconstructed them at the receiving end (1920s -5 distict levels of gray 1929 ndash 15 levels)

                      This example is not DIP the computer is not involved

                      DIP is linked and devolps in the same rhythm as digital computers (data storage display and transmission)

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      A digital pictureproduced in 1921from a coded tapeby a telegraph printer withspecial type faces(McFarlane)

                      A digital picturemade in 1922from a tapepunched after the signals hadcrossed the Atlantic twice(McFarlane)

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      1964 Jet Propulsion Laboratory (Pasadena California) processed pictures of the moon transmitted by Ranger 7 (corrected image distortions)

                      The first picture of the moon by a US spacecraft Ranger 7 took this image July 31 1964 about 17 minutes before impacting the lunar surface(Courtesy of NASA)

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

                      1970s ndash invention of CAT (computerized axial tomography)

                      CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      loz geographers use DIP to study pollution patterns from aerial and satellite imagery

                      loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

                      loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

                      loz astronomy biology nuclear medicine law enforcement industry

                      DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

                      loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Examples of Fields that Use DIP

                      Images can be classified according to their sources (visual X-ray hellip)

                      Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Electromagnetic waves can be thought as propagating sinusoidal

                      waves of different wavelength or as a stream of massless particles

                      each moving in a wavelike pattern with the speed of light Each

                      massless particle contains a certain amount (bundle) of energy Each

                      bundle of energy is called a photon If spectral bands are grouped

                      according to energy per photon we obtain the spectrum shown in the

                      image above ranging from gamma-rays (highest energy) to radio

                      waves (lowest energy)

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Gamma-Ray Imaging

                      Nuclear medicine astronomical observations

                      Nuclear medicine

                      the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

                      Images are produced from the emissions collected by gamma-ray detectors

                      Images of this sort are used to locate sites of bone pathology (infections tumors)

                      PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Examples of gamma-ray imaging

                      Bone scan PET image

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      X-ray imaging

                      Medical diagnosticindustry astronomy

                      A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                      The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Angiography = contrast-enhancement radiography

                      Angiograms = images of blood vessels

                      A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                      X-rays are used in CAT (computerized axial tomography)

                      X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                      Industrial CAT scans are useful when the parts can be penetreted by X-rays

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Examples of X-ray imaging

                      Chest X-rayAortic angiogram

                      Head CT Cygnus LoopCircuit boards

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Imaging in the Ultraviolet Band

                      Litography industrial inspection microscopy biological imaging astronomical observations

                      Ultraviolet light is used in fluorescence microscopy

                      Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                      other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                      and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Imaging in the Visible and Infrared Bands

                      Light microscopy astronomy remote sensing industry law enforcement

                      LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                      Weather observations and prediction produce major applications of multispectral image from satellites

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Satellite images of Washington DC area in spectral bands of the Table 1

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Examples of light microscopy

                      Taxol (anticancer agent)magnified 250X

                      Cholesterol(40X)

                      Microprocessor(60X)

                      Nickel oxidethin film(600X)

                      Surface of audio CD(1750X)

                      Organicsuperconductor(450X)

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Automated visual inspection of manufactured goods

                      a bc de f

                      a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Imaging in the Microwave Band

                      The dominant aplication of imaging in the microwave band ndash radar

                      bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                      bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                      bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                      An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Spaceborne radar image of mountains in southeast Tibet

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Imaging in the Radio Band

                      medicine astronomy

                      MRI = Magnetic Resonance Imaging

                      This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                      Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                      The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      MRI images of a human knee (left) and spine (right)

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Images of the Crab Pulsar covering the electromagnetic spectrum

                      Gamma X-ray Optical Infrared Radio

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Other Imaging Modalities

                      acoustic imaging electron microscopy synthetic (computer-generated) imaging

                      Imaging using sound geological explorations industry medicine

                      Mineral and oil exploration

                      For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Biometry - iris

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Biometry - fingerprint

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Face detection and recognition

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Gender identification

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Image morphing

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Fundamental Steps in DIP

                      methods whose input and output are images

                      methods whose inputs are images but whose outputs are attributes extracted from those images

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Outputs are images

                      bull image acquisition

                      bull image filtering and enhancement

                      bull image restoration

                      bull color image processing

                      bull wavelets and multiresolution processing

                      bull compression

                      bull morphological processing

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Outputs are attributes

                      bull morphological processing

                      bull segmentation

                      bull representation and description

                      bull object recognition

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Image acquisition - may involve preprocessing such as scaling

                      Image enhancement

                      bull manipulating an image so that the result is more suitable than the original for a specific operation

                      bull enhancement is problem oriented

                      bull there is no general sbquotheoryrsquo of image enhancement

                      bull enhancement use subjective methods for image emprovement

                      bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Image restoration

                      bull improving the appearance of an image

                      bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                      Color image processing

                      bull fundamental concept in color models

                      bull basic color processing in a digital domain

                      Wavelets and multiresolution processing

                      representing images in various degree of resolution

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Compression

                      reducing the storage required to save an image or the bandwidth required to transmit it

                      Morphological processing

                      bull tools for extracting image components that are useful in the representation and description of shape

                      bull a transition from processes that output images to processes that outputimage attributes

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Segmentation

                      bull partitioning an image into its constituents parts or objects

                      bull autonomous segmentation is one of the most difficult tasks of DIP

                      bull the more accurate the segmentation the more likley recognition is to succeed

                      Representation and description (almost always follows segmentation)

                      bull segmentation produces either the boundary of a region or all the poits in the region itself

                      bull converting the data produced by segmentation to a form suitable for computer processing

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      bull boundary representation the focus is on external shape characteristics such as corners or inflections

                      bull complete region the focus is on internal properties such as texture or skeletal shape

                      bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                      Object recognition

                      the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                      Knowledge database

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Simplified diagramof a cross sectionof the human eye

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                      The cornea is a tough transparent tissue that covers the anterior surface of the eye

                      Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                      The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                      The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                      Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                      Fovea = the place where the image of the object of interest falls on

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                      Blind spot region without receptors

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Image formation in the eye

                      Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                      Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                      distance between lens and retina along visual axix = 17 mm

                      range of focal length = 14 mm to 17 mm

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Optical illusions

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                      gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                      quantities that describe the quality of a chromatic light source radiance

                      the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                      measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                      brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      the physical meaning is determined by the source of the image

                      ( )f D f x y

                      Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                      f(xy) ndash characterized by two components

                      i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                      r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                      ( ) ( ) ( )

                      0 ( ) 0 ( ) 1

                      f x y i x y r x y

                      i x y r x y

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      r(xy)=0 - total absorption r(xy)=1 - total reflectance

                      i(xy) ndash determined by the illumination source

                      r(xy) ndash determined by the characteristics of the imaged objects

                      is called gray (or intensity) scale

                      In practice

                      min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                      indoor values without additional illuminationmin max10 1000L L

                      black whitemin max0 1 0 1 0 1L L L L l l L

                      min maxL L

                      Digital Image ProcessingDigital Image Processing

                      Week 1Week 1

                      Digital Image Processing

                      Week 1

                      Image Sampling and Quantization

                      - the output of the sensors is a continuous voltage waveform related to the sensed

                      scene

                      converting a continuous image f to digital form

                      - digitizing (x y) is called sampling

                      - digitizing f(x y) is called quantization

                      Digital Image Processing

                      Week 1

                      Digital Image Processing

                      Week 1

                      Continuous image projected onto a sensor array Result of image sampling and quantization

                      Digital Image Processing

                      Week 1

                      Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                      (00) (01) (0 1)(10) (11) (1 1)

                      ( )

                      ( 10) ( 11) ( 1 1)

                      f f f Nf f f N

                      f x y

                      f M f M f M N

                      image element pixel

                      00 01 0 1

                      10 11 1 1

                      10 11 1 1

                      ( ) ( )

                      N

                      i jN M N

                      i j

                      M M M N

                      a a aa f x i y j f i ja a a

                      Aa

                      a a a

                      f(00) ndash the upper left corner of the image

                      Digital Image Processing

                      Week 1

                      M N ge 0 L=2k

                      [0 1]i j i ja a L

                      Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                      Digital Image Processing

                      Week 1

                      Digital Image Processing

                      Week 1

                      Number of bits required to store a digitized image

                      for 2 b M N k M N b N k

                      When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                      Digital Image Processing

                      Week 1

                      Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                      Measures line pairs per unit distance dots (pixels) per unit distance

                      Image resolution = the largest number of discernible line pairs per unit distance

                      (eg 100 line pairs per mm)

                      Dots per unit distance are commonly used in printing and publishing

                      In US the measure is expressed in dots per inch (dpi)

                      (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                      Intensity resolution ndash the smallest discernible change in intensity level

                      The number of intensity levels (L) is determined by hardware considerations

                      L=2k ndash most common k = 8

                      Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                      Digital Image Processing

                      Week 1

                      Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                      150 dpi (lower left) 72 dpi (lower right)

                      Digital Image Processing

                      Week 1

                      Reducing the number of gray levels 256 128 64 32

                      Digital Image Processing

                      Week 1

                      Reducing the number of gray levels 16 8 4 2

                      Digital Image Processing

                      Week 1

                      Image Interpolation - used in zooming shrinking rotating and geometric corrections

                      Shrinking zooming ndash image resizing ndash image resampling methods

                      Interpolation is the process of using known data to estimate values at unknown locations

                      Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                      750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                      same spacing as the original and then shrink it so that it fits exactly over the original

                      image The pixel spacing in the 750 times 750 grid will be less than in the original image

                      Problem assignment of intensity-level in the new 750 times 750 grid

                      Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                      intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                      This technique has the tendency to produce undesirable effects like severe distortion of

                      straight edges

                      Digital Image Processing

                      Week 1

                      Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                      where the four coefficients are determined from the 4 equations in 4 unknowns that can

                      be written using the 4 nearest neighbors of point (x y)

                      Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                      modest increase in computational effort

                      Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                      nearest neighbors of the point 3 3

                      0 0

                      ( ) i ji j

                      i jv x y c x y

                      The coefficients cij are obtained solving a 16x16 linear system

                      intensity levels of the 16 nearest neighbors of 3 3

                      0 0

                      ( )i ji j

                      i jc x y x y

                      Digital Image Processing

                      Week 1

                      Generally bicubic interpolation does a better job of preserving fine detail than the

                      bilinear technique Bicubic interpolation is the standard used in commercial image editing

                      programs such as Adobe Photoshop and Corel Photopaint

                      Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                      the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                      then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                      neighbor interpolation was used (both for shrinking and zooming)

                      Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                      interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                      from 1250 dpi to 150 dpi (instead of 72 dpi)

                      Digital Image Processing

                      Week 1

                      Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                      Digital Image Processing

                      Week 1

                      Neighbors of a Pixel

                      A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                      This set of pixels called the 4-neighbors of p denoted by N4 (p)

                      The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                      and are denoted ND(p)

                      The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                      N8 (p)

                      If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                      fall outside the image

                      Digital Image Processing

                      Week 1

                      Adjacency Connectivity Regions Boundaries

                      Denote by V the set of intensity levels used to define adjacency

                      - in a binary image V 01 (V=0 V=1)

                      - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                      We consider 3 types of adjacency

                      (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                      (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                      (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                      m-adjacent if

                      4( )q N p or

                      ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                      Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                      ambiguities that often arise when 8-adjacency is used Consider the example

                      Digital Image Processing

                      Week 1

                      binary image

                      0 1 1 0 1 1 0 1 1

                      1 0 1 0 0 1 0 0 1 0

                      0 0 1 0 0 1 0 0 1

                      V

                      The three pixels at the top (first line) in the above example show multiple (ambiguous)

                      8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                      m-adjacency

                      Digital Image Processing

                      Week 1

                      A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                      is a sequence of distinct pixels with coordinates

                      and are adjacent 0 0 1 1

                      1 1

                      ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                      n n

                      i i i i

                      x y x y x y x y s tx y x y i n

                      The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                      Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                      Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                      in S if there exists a path between them consisting only of pixels from S

                      S is a connected set if there is a path in S between any 2 pixels in S

                      Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                      Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                      that are not adjacent are said to be disjoint When referring to regions only 4- and

                      8-adjacency are considered

                      Digital Image Processing

                      Week 1

                      Suppose that an image contains K disjoint regions 1 kR k K none of which

                      touches the image border

                      the complement of 1

                      ( )K

                      cu k u u

                      k

                      R R R R

                      We call all the points in Ru the foreground of the image and the points in ( )cuR the

                      background of the image

                      The boundary (border or contour) of a region R is the set of points that are adjacent to

                      points in the complement of R (R)c The border of an image is the set of pixels in the

                      region that have at least one background neighbor This definition is referred to as the

                      inner border to distinguish it from the notion of outer border which is the corresponding

                      border in the background

                      Digital Image Processing

                      Week 1

                      Distance measures

                      For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                      function or metric if

                      (a) D(p q) ge 0 D(p q) = 0 iff p=q

                      (b) D(p q) = D(q p)

                      (c) D(p z) le D(p q) + D(q z)

                      The Euclidean distance between p and q is defined as 1

                      2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                      The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                      centered at (x y)

                      Digital Image Processing

                      Week 1

                      The D4 distance (also called city-block distance) between p and q is defined as

                      4( ) | | | |D p q x s y t

                      The pixels q for which 4( )D p q r form a diamond centered at (xy)

                      4

                      22 1 2

                      2 2 1 0 1 22 1 2

                      2

                      D

                      The pixels with D4 = 1 are the 4-neighbors of (x y)

                      The D8 distance (called the chessboard distance) between p and q is defined as

                      8( ) max| | | |D p q x s y t

                      The pixels q for which 8( )D p q r form a square centered at (x y)

                      Digital Image Processing

                      Week 1

                      8

                      2 2 2 2 22 1 1 1 2

                      2 2 1 0 1 22 1 1 1 22 2 2 2 2

                      D

                      The pixels with D8 = 1 are the 8-neighbors of (x y)

                      D4 and D8 distances are independent of any paths that might exist between p and q

                      because these distances involve only the coordinates of the point

                      Digital Image Processing

                      Week 1

                      Array versus Matrix Operations

                      An array operation involving one or more images is carried out on a pixel-by-pixel basis

                      11 12 11 12

                      21 22 21 22

                      a a b ba a b b

                      Array product

                      11 12 11 12 11 11 12 12

                      21 22 21 22 21 21 22 21

                      a a b b a b a ba a b b a b a b

                      Matrix product

                      11 12 11 12 11 11 12 21 11 12 12 21

                      21 22 21 22 21 11 22 21 21 12 22 22

                      a a b b a b a b a b a ba a b b a b a b a b a b

                      We assume array operations unless stated otherwise

                      Digital Image Processing

                      Week 1

                      Linear versus Nonlinear Operations

                      One of the most important classifications of image-processing methods is whether it is

                      linear or nonlinear

                      ( ) ( )H f x y g x y

                      H is said to be a linear operator if

                      images1 2 1 2

                      1 2

                      ( ) ( ) ( ) ( )

                      H a f x y b f x y a H f x y b H f x y

                      a b f f

                      Example of nonlinear operator

                      the maximum value of the pixels of image max ( )H f f x y f

                      1 2

                      0 2 6 5 1 1

                      2 3 4 7f f a b

                      Digital Image Processing

                      Week 1

                      1 2

                      0 2 6 5 6 3max max 1 ( 1) max 2

                      2 3 4 7 2 4a f b f

                      0 2 6 51 max ( 1) max 3 ( 1)7 4

                      2 3 4 7

                      Arithmetic Operations in Image Processing

                      Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                      f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                      For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                      value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                      The two random variables are uncorrelated when their covariance is 0

                      Digital Image Processing

                      Week 1

                      Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                      used in image enhancement)

                      1

                      1( ) ( )K

                      ii

                      g x y g x yK

                      If the noise satisfies the properties stated above we have

                      2 2( ) ( )

                      1( ) ( ) g x y x yE g x y f x yK

                      ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                      and g respectively The standard deviation (square root of the variance) at any point in

                      the average image is

                      ( ) ( )1

                      g x y x yK

                      Digital Image Processing

                      Week 1

                      As K increases the variability (as measured by the variance or the standard deviation) of

                      the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                      means that ( )g x y approaches f(x y) as the number of noisy images used in the

                      averaging process increases

                      An important application of image averaging is in the field of astronomy where imaging

                      under very low light levels frequently causes sensor noise to render single images

                      virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                      was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                      64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                      images respectively

                      Digital Image Processing

                      Week 1

                      Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                      100 noisy images

                      a b c d e f

                      Digital Image Processing

                      Week 1

                      A frequent application of image subtraction is in the enhancement of differences between

                      images

                      (a) (b) (c)

                      Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                      significant bit of each pixel (c) the difference between the two images

                      Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                      Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                      images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                      difference between images (a) and (b)

                      Digital Image Processing

                      Week 1

                      Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                      h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                      intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                      source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                      bloodstream taking a series of images called live images (denoted f(x y)) of the same

                      anatomical region as h(x y) and subtracting the mask from the series of incoming live

                      images after injection of the contrast medium

                      In g(x y) we can find the differences between h and f as enhanced detail

                      Images being captured at TV rates we obtain a movie showing how the contrast medium

                      propagates through the various arteries in the area being observed

                      Digital Image Processing

                      Week 1

                      a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                      Digital Image Processing

                      Week 1

                      An important application of image multiplication (and division) is shading correction

                      Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                      f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                      When the shading function is known

                      ( )( )( )

                      g x yf x yh x y

                      h(x y) is unknown but we have access to the imaging system we can obtain an

                      approximation to the shading function by imaging a target of constant intensity When the

                      sensor is not available often the shading pattern can be estimated from the image

                      Digital Image Processing

                      Week 1

                      (a) (b) (c)

                      Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                      Digital Image Processing

                      Week 1

                      Another use of image multiplication is in masking also called region of interest (ROI)

                      operations The process consists of multiplying a given image by a mask image that has

                      1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                      image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                      (a) (b) (c)

                      Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                      Digital Image Processing

                      Week 1

                      In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                      min( )mf f f

                      0 ( 255)max( )

                      ms

                      m

                      ff K K K

                      f

                      Digital Image Processing

                      Week 1

                      Spatial Operations

                      - are performed directly on the pixels of a given image

                      There are three categories of spatial operations

                      single-pixel operations

                      neighborhood operations

                      geometric spatial transformations

                      Single-pixel operations

                      - change the values of intensity for the individual pixels ( )s T z

                      where z is the intensity of a pixel in the original image and s is the intensity of the

                      corresponding pixel in the processed image

                      Digital Image Processing

                      Week 1

                      Neighborhood operations

                      Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                      in an image f Neighborhood processing generates new intensity level at point (x y)

                      based on the values of the intensities of the points in Sxy For example if Sxy is a

                      rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                      intensity by computing the average value of the pixels in Sxy

                      ( )

                      1( ) ( )xyr c S

                      g x y f r cm n

                      The net effect is to perform local blurring in the original image This type of process is

                      used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                      largest region of an image

                      Digital Image Processing

                      Week 1

                      Geometric spatial transformations and image registration

                      - modify the spatial relationship between pixels in an image

                      - these transformations are often called rubber-sheet transformations (analogous to

                      printing an image on a sheet of rubber and then stretching the sheet according to a

                      predefined set of rules

                      A geometric transformation consists of 2 basic operations

                      1 a spatial transformation of coordinates

                      2 intensity interpolation that assign intensity values to the spatial transformed

                      pixels

                      The coordinate system transformation ( ) [( )]x y T v w

                      (v w) ndash pixel coordinates in the original image

                      (x y) ndash pixel coordinates in the transformed image

                      Digital Image Processing

                      Week 1

                      [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                      Affine transform

                      11 1211 21 31

                      21 2212 22 33

                      31 32

                      0[ 1] [ 1] [ 1] 0

                      1

                      t tx t v t w t

                      x y v w T v w t ty t v t w t

                      t t

                      (AT)

                      This transform can scale rotate translate or shear a set of coordinate points depending

                      on the elements of the matrix T If we want to resize an image rotate it and move the

                      result to some location we simply form a 3x3 matrix equal to the matrix product of the

                      scaling rotation and translation matrices from Table 1

                      Digital Image Processing

                      Week 1

                      Affine transformations

                      Digital Image Processing

                      Week 1

                      The preceding transformations relocate pixels on an image to new locations To complete

                      the process we have to assign intensity values to those locations This task is done by

                      using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                      In practice we can use equation (AT) in two basic ways

                      forward mapping scan the pixels of the input image (v w) compute the new spatial

                      location (x y) of the corresponding pixel in the new image using (AT) directly

                      Problems

                      - intensity assignment when 2 or more pixels in the original image are transformed to

                      the same location in the output image

                      - some output locations have no correspondent in the original image (no intensity

                      assignment)

                      Digital Image Processing

                      Week 1

                      inverse mapping scans the output pixel locations and at each location (x y)

                      computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                      It then interpolates among the nearest input pixels to determine the intensity of the output

                      pixel value

                      Inverse mappings are more efficient to implement than forward mappings and are used in

                      numerous commercial implementations of spatial transformations (MATLAB for ex)

                      Digital Image Processing

                      Week 1

                      Digital Image Processing

                      Week 1

                      Image registration ndash align two or more images of the same scene

                      In image registration we have available the input and output images but the specific

                      transformation that produced the output image from the input is generally unknown

                      The problem is to estimate the transformation function and then use it to register the two

                      images

                      - it may be of interest to align (register) two or more image taken at approximately the

                      same time but using different imaging systems (MRI scanner and a PET scanner)

                      - align images of a given location taken by the same instrument at different moments

                      of time (satellite images)

                      Solving the problem using tie points (also called control points) which are

                      corresponding points whose locations are known precisely in the input and reference

                      image

                      Digital Image Processing

                      Week 1

                      How to select tie points

                      - interactively selecting them

                      - use of algorithms that try to detect these points

                      - some imaging systems have physical artifacts (small metallic objects) embedded in

                      the imaging sensors These objects produce a set of known points (called reseau

                      marks) directly on all images captured by the system which can be used as guides

                      for establishing tie points

                      The problem of estimating the transformation is one of modeling Suppose we have a set

                      of 4 tie points both on the input image and the reference image A simple model based on

                      a bilinear approximation is given by

                      1 2 3 4

                      5 6 7 8

                      x c v c w c v w cy c v c w c v w c

                      (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                      Digital Image Processing

                      Week 1

                      When 4 tie points are insufficient to obtain satisfactory registration an approach used

                      frequently is to select a larger number of tie points and using this new set of tie points

                      subdivide the image in rectangular regions marked by groups of 4 tie points On the

                      subregions marked by 4 tie points we applied the transformation model described above

                      The number of tie points and the sophistication of the model required to solve the register

                      problem depend on the severity of the geometrical distortion

                      Digital Image Processing

                      Week 1

                      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                      Digital Image Processing

                      Week 1

                      Probabilistic Methods

                      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                      p(zk) = the probability that the intensity level zk occurs in the given image

                      ( ) kk

                      np zM N

                      nk = the number of times that intensity zk occurs in the image (MN is the total number of

                      pixels in the image) 1

                      0( ) 1

                      L

                      kk

                      p z

                      The mean (average) intensity of an image is given by 1

                      0( )

                      L

                      k kk

                      m z p z

                      Digital Image Processing

                      Week 1

                      The variance of the intensities is 1

                      2 2

                      0( ) ( )

                      L

                      k kk

                      z m p z

                      The variance is a measure of the spread of the values of z about the mean so it is a

                      measure of image contrast Usually for measuring image contrast the standard deviation

                      ( ) is used

                      The n-th moment of a random variable z about the mean is defined as 1

                      0( ) ( ) ( )

                      Ln

                      n k kk

                      z z m p z

                      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                      3( ) 0z the intensities are biased to values higher than the mean

                      ( 3( ) 0z the intensities are biased to values lower than the mean

                      Digital Image Processing

                      Week 1

                      3( ) 0z the intensities are distributed approximately equally on both side of the

                      mean

                      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                      Figure 1(a) ndash standard deviation 143 (variance = 2045)

                      Figure 1(b) ndash standard deviation 316 (variance = 9986)

                      Figure 1(c) ndash standard deviation 492 (variance = 24206)

                      Digital Image Processing

                      Week 1

                      Intensity Transformations and Spatial Filtering

                      ( ) ( )g x y T f x y

                      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                      neighborhood of (x y)

                      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                      and much smaller in size than the image

                      Digital Image Processing

                      Week 1

                      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                      called spatial filter (spatial mask kernel template or window)

                      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                      ( )s T r

                      s and r are denoting respectively the intensity of g and f at (x y)

                      Figure 2 left - T produces an output image of higher contrast than the original by

                      darkening the intensity levels below k and brightening the levels above k ndash this technique

                      is called contrast stretching

                      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                      Digital Image Processing

                      Week 1

                      Figure 2 right - T produces a binary output image A mapping of this form is called

                      thresholding function

                      Some Basic Intensity Transformation Functions

                      Image Negatives

                      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                      - equivalent of a photographic negative

                      - technique suited for enhancing white or gray detail embedded in dark regions of an

                      image

                      Digital Image Processing

                      Week 1

                      Original Negative image

                      Digital Image Processing

                      Week 1

                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                      Some basic intensity transformation functions

                      Digital Image Processing

                      Week 1

                      This transformation maps a narrow range of low intensity values in the input into a wider

                      range An operator of this type is used to expand the values of dark pixels in an image

                      while compressing the higher-level values The opposite is true for the inverse log

                      transformation The log functions compress the dynamic range of images with large

                      variations in pixel values

                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                      Digital Image Processing

                      Week 1

                      Power-Law (Gamma) Transformations

                      - positive constants( ) ( ( ) )s T r c r c s c r

                      Plots of gamma transformation for different values of γ (c=1)

                      Digital Image Processing

                      Week 1

                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                      of output values with the opposite being true for higher values of input values The

                      curves with 1 have the opposite effect of those generated with values of 1

                      1c - identity transformation

                      A variety of devices used for image capture printing and display respond according to a

                      power law The process used to correct these power-law response phenomena is called

                      gamma correction

                      Digital Image Processing

                      Week 1

                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                      Digital Image Processing

                      Week 1

                      Piecewise-Linear Transformations Functions

                      Contrast stretching

                      - a process that expands the range of intensity levels in an image so it spans the full

                      intensity range of the recording tool or display device

                      a b c d Fig5

                      Digital Image Processing

                      Week 1

                      11

                      1

                      2 1 1 21 2

                      2 1 2 1

                      22

                      2

                      [0 ]

                      ( ) ( )( ) [ ]( ) ( )

                      ( 1 ) [ 1]( 1 )

                      s r r rrs r r s r rT r r r r

                      r r r rs L r r r L

                      L r

                      Digital Image Processing

                      Week 1

                      1 1 2 2r s r s identity transformation (no change)

                      1 2 1 2 0 1r r s s L thresholding function

                      Figure 5(b) shows an 8-bit image with low contrast

                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                      in the image respectively Thus the transformation function stretched the levels linearly

                      from their original range to the full range [0 L-1]

                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                      2 2 1r s m L where m is the mean gray level in the image

                      The original image on which these results are based is a scanning electron microscope

                      image of pollen magnified approximately 700 times

                      Digital Image Processing

                      Week 1

                      Intensity-level slicing

                      - highlighting a specific range of intensities in an image

                      There are two approaches for intensity-level slicing

                      1 display in one value (white for example) all the values in the range of interest and in

                      another (say black) all other intensities (Figure 311 (a))

                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                      intensities in the image (Figure 311 (b))

                      Digital Image Processing

                      Week 1

                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                      the top of the scale of intensities This type of enhancement produces a binary image

                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                      Highlights range [A B] and preserves all other intensities

                      Digital Image Processing

                      Week 1

                      which is useful for studying the shape of the flow of the contrast substance (to detect

                      blockageshellip)

                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                      image around the mean intensity was set to black the other intensities remain unchanged

                      Fig 6 - Aortic angiogram and intensity sliced versions

                      Digital Image Processing

                      Week 1

                      Bit-plane slicing

                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                      This technique highlights the contribution made to the whole image appearances by each

                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                      Digital Image Processing

                      Week 1

                      Digital Image Processing

                      Week 1

                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                      • DIP 1 2017
                      • DIP 02 (2017)

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        The Origins of DIP

                        Newspaper industry pictures were sent by submarine cable between London and New York

                        Before Bartlane cable picture transmission system (early 1920s) ndash 1 week

                        With Bartlane system less than 3 hours

                        Specialized printing equipment coded pictures for cable transmission and reconstructed them at the receiving end (1920s -5 distict levels of gray 1929 ndash 15 levels)

                        This example is not DIP the computer is not involved

                        DIP is linked and devolps in the same rhythm as digital computers (data storage display and transmission)

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        A digital pictureproduced in 1921from a coded tapeby a telegraph printer withspecial type faces(McFarlane)

                        A digital picturemade in 1922from a tapepunched after the signals hadcrossed the Atlantic twice(McFarlane)

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        1964 Jet Propulsion Laboratory (Pasadena California) processed pictures of the moon transmitted by Ranger 7 (corrected image distortions)

                        The first picture of the moon by a US spacecraft Ranger 7 took this image July 31 1964 about 17 minutes before impacting the lunar surface(Courtesy of NASA)

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

                        1970s ndash invention of CAT (computerized axial tomography)

                        CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        loz geographers use DIP to study pollution patterns from aerial and satellite imagery

                        loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

                        loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

                        loz astronomy biology nuclear medicine law enforcement industry

                        DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

                        loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Examples of Fields that Use DIP

                        Images can be classified according to their sources (visual X-ray hellip)

                        Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Electromagnetic waves can be thought as propagating sinusoidal

                        waves of different wavelength or as a stream of massless particles

                        each moving in a wavelike pattern with the speed of light Each

                        massless particle contains a certain amount (bundle) of energy Each

                        bundle of energy is called a photon If spectral bands are grouped

                        according to energy per photon we obtain the spectrum shown in the

                        image above ranging from gamma-rays (highest energy) to radio

                        waves (lowest energy)

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                        Week 1Week 1

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                        Gamma-Ray Imaging

                        Nuclear medicine astronomical observations

                        Nuclear medicine

                        the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

                        Images are produced from the emissions collected by gamma-ray detectors

                        Images of this sort are used to locate sites of bone pathology (infections tumors)

                        PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Examples of gamma-ray imaging

                        Bone scan PET image

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        X-ray imaging

                        Medical diagnosticindustry astronomy

                        A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                        The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Angiography = contrast-enhancement radiography

                        Angiograms = images of blood vessels

                        A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                        X-rays are used in CAT (computerized axial tomography)

                        X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                        Industrial CAT scans are useful when the parts can be penetreted by X-rays

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Examples of X-ray imaging

                        Chest X-rayAortic angiogram

                        Head CT Cygnus LoopCircuit boards

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Imaging in the Ultraviolet Band

                        Litography industrial inspection microscopy biological imaging astronomical observations

                        Ultraviolet light is used in fluorescence microscopy

                        Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                        other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                        and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Imaging in the Visible and Infrared Bands

                        Light microscopy astronomy remote sensing industry law enforcement

                        LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                        Weather observations and prediction produce major applications of multispectral image from satellites

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Satellite images of Washington DC area in spectral bands of the Table 1

                        Digital Image ProcessingDigital Image Processing

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                        Examples of light microscopy

                        Taxol (anticancer agent)magnified 250X

                        Cholesterol(40X)

                        Microprocessor(60X)

                        Nickel oxidethin film(600X)

                        Surface of audio CD(1750X)

                        Organicsuperconductor(450X)

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Automated visual inspection of manufactured goods

                        a bc de f

                        a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

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                        Imaging in the Microwave Band

                        The dominant aplication of imaging in the microwave band ndash radar

                        bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                        bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                        bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                        An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Spaceborne radar image of mountains in southeast Tibet

                        Digital Image ProcessingDigital Image Processing

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                        Imaging in the Radio Band

                        medicine astronomy

                        MRI = Magnetic Resonance Imaging

                        This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                        Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                        The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        MRI images of a human knee (left) and spine (right)

                        Digital Image ProcessingDigital Image Processing

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                        Images of the Crab Pulsar covering the electromagnetic spectrum

                        Gamma X-ray Optical Infrared Radio

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Other Imaging Modalities

                        acoustic imaging electron microscopy synthetic (computer-generated) imaging

                        Imaging using sound geological explorations industry medicine

                        Mineral and oil exploration

                        For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                        Digital Image ProcessingDigital Image Processing

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                        Biometry - iris

                        Digital Image ProcessingDigital Image Processing

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                        Biometry - fingerprint

                        Digital Image ProcessingDigital Image Processing

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                        Face detection and recognition

                        Digital Image ProcessingDigital Image Processing

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                        Gender identification

                        Digital Image ProcessingDigital Image Processing

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                        Image morphing

                        Digital Image ProcessingDigital Image Processing

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                        Fundamental Steps in DIP

                        methods whose input and output are images

                        methods whose inputs are images but whose outputs are attributes extracted from those images

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Outputs are images

                        bull image acquisition

                        bull image filtering and enhancement

                        bull image restoration

                        bull color image processing

                        bull wavelets and multiresolution processing

                        bull compression

                        bull morphological processing

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Outputs are attributes

                        bull morphological processing

                        bull segmentation

                        bull representation and description

                        bull object recognition

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Image acquisition - may involve preprocessing such as scaling

                        Image enhancement

                        bull manipulating an image so that the result is more suitable than the original for a specific operation

                        bull enhancement is problem oriented

                        bull there is no general sbquotheoryrsquo of image enhancement

                        bull enhancement use subjective methods for image emprovement

                        bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Image restoration

                        bull improving the appearance of an image

                        bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                        Color image processing

                        bull fundamental concept in color models

                        bull basic color processing in a digital domain

                        Wavelets and multiresolution processing

                        representing images in various degree of resolution

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Compression

                        reducing the storage required to save an image or the bandwidth required to transmit it

                        Morphological processing

                        bull tools for extracting image components that are useful in the representation and description of shape

                        bull a transition from processes that output images to processes that outputimage attributes

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Segmentation

                        bull partitioning an image into its constituents parts or objects

                        bull autonomous segmentation is one of the most difficult tasks of DIP

                        bull the more accurate the segmentation the more likley recognition is to succeed

                        Representation and description (almost always follows segmentation)

                        bull segmentation produces either the boundary of a region or all the poits in the region itself

                        bull converting the data produced by segmentation to a form suitable for computer processing

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        bull boundary representation the focus is on external shape characteristics such as corners or inflections

                        bull complete region the focus is on internal properties such as texture or skeletal shape

                        bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                        Object recognition

                        the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                        Knowledge database

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Simplified diagramof a cross sectionof the human eye

                        Digital Image ProcessingDigital Image Processing

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                        Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                        The cornea is a tough transparent tissue that covers the anterior surface of the eye

                        Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                        The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                        The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                        Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                        Fovea = the place where the image of the object of interest falls on

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                        Blind spot region without receptors

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Image formation in the eye

                        Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                        Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                        distance between lens and retina along visual axix = 17 mm

                        range of focal length = 14 mm to 17 mm

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Digital Image ProcessingDigital Image Processing

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                        Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Optical illusions

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                        gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                        quantities that describe the quality of a chromatic light source radiance

                        the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                        measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                        brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        the physical meaning is determined by the source of the image

                        ( )f D f x y

                        Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                        f(xy) ndash characterized by two components

                        i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                        r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                        ( ) ( ) ( )

                        0 ( ) 0 ( ) 1

                        f x y i x y r x y

                        i x y r x y

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        r(xy)=0 - total absorption r(xy)=1 - total reflectance

                        i(xy) ndash determined by the illumination source

                        r(xy) ndash determined by the characteristics of the imaged objects

                        is called gray (or intensity) scale

                        In practice

                        min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                        indoor values without additional illuminationmin max10 1000L L

                        black whitemin max0 1 0 1 0 1L L L L l l L

                        min maxL L

                        Digital Image ProcessingDigital Image Processing

                        Week 1Week 1

                        Digital Image Processing

                        Week 1

                        Image Sampling and Quantization

                        - the output of the sensors is a continuous voltage waveform related to the sensed

                        scene

                        converting a continuous image f to digital form

                        - digitizing (x y) is called sampling

                        - digitizing f(x y) is called quantization

                        Digital Image Processing

                        Week 1

                        Digital Image Processing

                        Week 1

                        Continuous image projected onto a sensor array Result of image sampling and quantization

                        Digital Image Processing

                        Week 1

                        Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                        (00) (01) (0 1)(10) (11) (1 1)

                        ( )

                        ( 10) ( 11) ( 1 1)

                        f f f Nf f f N

                        f x y

                        f M f M f M N

                        image element pixel

                        00 01 0 1

                        10 11 1 1

                        10 11 1 1

                        ( ) ( )

                        N

                        i jN M N

                        i j

                        M M M N

                        a a aa f x i y j f i ja a a

                        Aa

                        a a a

                        f(00) ndash the upper left corner of the image

                        Digital Image Processing

                        Week 1

                        M N ge 0 L=2k

                        [0 1]i j i ja a L

                        Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                        Digital Image Processing

                        Week 1

                        Digital Image Processing

                        Week 1

                        Number of bits required to store a digitized image

                        for 2 b M N k M N b N k

                        When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                        Digital Image Processing

                        Week 1

                        Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                        Measures line pairs per unit distance dots (pixels) per unit distance

                        Image resolution = the largest number of discernible line pairs per unit distance

                        (eg 100 line pairs per mm)

                        Dots per unit distance are commonly used in printing and publishing

                        In US the measure is expressed in dots per inch (dpi)

                        (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                        Intensity resolution ndash the smallest discernible change in intensity level

                        The number of intensity levels (L) is determined by hardware considerations

                        L=2k ndash most common k = 8

                        Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                        Digital Image Processing

                        Week 1

                        Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                        150 dpi (lower left) 72 dpi (lower right)

                        Digital Image Processing

                        Week 1

                        Reducing the number of gray levels 256 128 64 32

                        Digital Image Processing

                        Week 1

                        Reducing the number of gray levels 16 8 4 2

                        Digital Image Processing

                        Week 1

                        Image Interpolation - used in zooming shrinking rotating and geometric corrections

                        Shrinking zooming ndash image resizing ndash image resampling methods

                        Interpolation is the process of using known data to estimate values at unknown locations

                        Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                        750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                        same spacing as the original and then shrink it so that it fits exactly over the original

                        image The pixel spacing in the 750 times 750 grid will be less than in the original image

                        Problem assignment of intensity-level in the new 750 times 750 grid

                        Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                        intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                        This technique has the tendency to produce undesirable effects like severe distortion of

                        straight edges

                        Digital Image Processing

                        Week 1

                        Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                        where the four coefficients are determined from the 4 equations in 4 unknowns that can

                        be written using the 4 nearest neighbors of point (x y)

                        Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                        modest increase in computational effort

                        Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                        nearest neighbors of the point 3 3

                        0 0

                        ( ) i ji j

                        i jv x y c x y

                        The coefficients cij are obtained solving a 16x16 linear system

                        intensity levels of the 16 nearest neighbors of 3 3

                        0 0

                        ( )i ji j

                        i jc x y x y

                        Digital Image Processing

                        Week 1

                        Generally bicubic interpolation does a better job of preserving fine detail than the

                        bilinear technique Bicubic interpolation is the standard used in commercial image editing

                        programs such as Adobe Photoshop and Corel Photopaint

                        Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                        the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                        then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                        neighbor interpolation was used (both for shrinking and zooming)

                        Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                        interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                        from 1250 dpi to 150 dpi (instead of 72 dpi)

                        Digital Image Processing

                        Week 1

                        Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                        Digital Image Processing

                        Week 1

                        Neighbors of a Pixel

                        A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                        This set of pixels called the 4-neighbors of p denoted by N4 (p)

                        The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                        and are denoted ND(p)

                        The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                        N8 (p)

                        If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                        fall outside the image

                        Digital Image Processing

                        Week 1

                        Adjacency Connectivity Regions Boundaries

                        Denote by V the set of intensity levels used to define adjacency

                        - in a binary image V 01 (V=0 V=1)

                        - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                        We consider 3 types of adjacency

                        (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                        (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                        (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                        m-adjacent if

                        4( )q N p or

                        ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                        Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                        ambiguities that often arise when 8-adjacency is used Consider the example

                        Digital Image Processing

                        Week 1

                        binary image

                        0 1 1 0 1 1 0 1 1

                        1 0 1 0 0 1 0 0 1 0

                        0 0 1 0 0 1 0 0 1

                        V

                        The three pixels at the top (first line) in the above example show multiple (ambiguous)

                        8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                        m-adjacency

                        Digital Image Processing

                        Week 1

                        A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                        is a sequence of distinct pixels with coordinates

                        and are adjacent 0 0 1 1

                        1 1

                        ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                        n n

                        i i i i

                        x y x y x y x y s tx y x y i n

                        The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                        Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                        Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                        in S if there exists a path between them consisting only of pixels from S

                        S is a connected set if there is a path in S between any 2 pixels in S

                        Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                        Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                        that are not adjacent are said to be disjoint When referring to regions only 4- and

                        8-adjacency are considered

                        Digital Image Processing

                        Week 1

                        Suppose that an image contains K disjoint regions 1 kR k K none of which

                        touches the image border

                        the complement of 1

                        ( )K

                        cu k u u

                        k

                        R R R R

                        We call all the points in Ru the foreground of the image and the points in ( )cuR the

                        background of the image

                        The boundary (border or contour) of a region R is the set of points that are adjacent to

                        points in the complement of R (R)c The border of an image is the set of pixels in the

                        region that have at least one background neighbor This definition is referred to as the

                        inner border to distinguish it from the notion of outer border which is the corresponding

                        border in the background

                        Digital Image Processing

                        Week 1

                        Distance measures

                        For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                        function or metric if

                        (a) D(p q) ge 0 D(p q) = 0 iff p=q

                        (b) D(p q) = D(q p)

                        (c) D(p z) le D(p q) + D(q z)

                        The Euclidean distance between p and q is defined as 1

                        2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                        The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                        centered at (x y)

                        Digital Image Processing

                        Week 1

                        The D4 distance (also called city-block distance) between p and q is defined as

                        4( ) | | | |D p q x s y t

                        The pixels q for which 4( )D p q r form a diamond centered at (xy)

                        4

                        22 1 2

                        2 2 1 0 1 22 1 2

                        2

                        D

                        The pixels with D4 = 1 are the 4-neighbors of (x y)

                        The D8 distance (called the chessboard distance) between p and q is defined as

                        8( ) max| | | |D p q x s y t

                        The pixels q for which 8( )D p q r form a square centered at (x y)

                        Digital Image Processing

                        Week 1

                        8

                        2 2 2 2 22 1 1 1 2

                        2 2 1 0 1 22 1 1 1 22 2 2 2 2

                        D

                        The pixels with D8 = 1 are the 8-neighbors of (x y)

                        D4 and D8 distances are independent of any paths that might exist between p and q

                        because these distances involve only the coordinates of the point

                        Digital Image Processing

                        Week 1

                        Array versus Matrix Operations

                        An array operation involving one or more images is carried out on a pixel-by-pixel basis

                        11 12 11 12

                        21 22 21 22

                        a a b ba a b b

                        Array product

                        11 12 11 12 11 11 12 12

                        21 22 21 22 21 21 22 21

                        a a b b a b a ba a b b a b a b

                        Matrix product

                        11 12 11 12 11 11 12 21 11 12 12 21

                        21 22 21 22 21 11 22 21 21 12 22 22

                        a a b b a b a b a b a ba a b b a b a b a b a b

                        We assume array operations unless stated otherwise

                        Digital Image Processing

                        Week 1

                        Linear versus Nonlinear Operations

                        One of the most important classifications of image-processing methods is whether it is

                        linear or nonlinear

                        ( ) ( )H f x y g x y

                        H is said to be a linear operator if

                        images1 2 1 2

                        1 2

                        ( ) ( ) ( ) ( )

                        H a f x y b f x y a H f x y b H f x y

                        a b f f

                        Example of nonlinear operator

                        the maximum value of the pixels of image max ( )H f f x y f

                        1 2

                        0 2 6 5 1 1

                        2 3 4 7f f a b

                        Digital Image Processing

                        Week 1

                        1 2

                        0 2 6 5 6 3max max 1 ( 1) max 2

                        2 3 4 7 2 4a f b f

                        0 2 6 51 max ( 1) max 3 ( 1)7 4

                        2 3 4 7

                        Arithmetic Operations in Image Processing

                        Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                        f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                        For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                        value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                        The two random variables are uncorrelated when their covariance is 0

                        Digital Image Processing

                        Week 1

                        Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                        used in image enhancement)

                        1

                        1( ) ( )K

                        ii

                        g x y g x yK

                        If the noise satisfies the properties stated above we have

                        2 2( ) ( )

                        1( ) ( ) g x y x yE g x y f x yK

                        ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                        and g respectively The standard deviation (square root of the variance) at any point in

                        the average image is

                        ( ) ( )1

                        g x y x yK

                        Digital Image Processing

                        Week 1

                        As K increases the variability (as measured by the variance or the standard deviation) of

                        the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                        means that ( )g x y approaches f(x y) as the number of noisy images used in the

                        averaging process increases

                        An important application of image averaging is in the field of astronomy where imaging

                        under very low light levels frequently causes sensor noise to render single images

                        virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                        was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                        64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                        images respectively

                        Digital Image Processing

                        Week 1

                        Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                        100 noisy images

                        a b c d e f

                        Digital Image Processing

                        Week 1

                        A frequent application of image subtraction is in the enhancement of differences between

                        images

                        (a) (b) (c)

                        Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                        significant bit of each pixel (c) the difference between the two images

                        Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                        Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                        images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                        difference between images (a) and (b)

                        Digital Image Processing

                        Week 1

                        Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                        h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                        intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                        source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                        bloodstream taking a series of images called live images (denoted f(x y)) of the same

                        anatomical region as h(x y) and subtracting the mask from the series of incoming live

                        images after injection of the contrast medium

                        In g(x y) we can find the differences between h and f as enhanced detail

                        Images being captured at TV rates we obtain a movie showing how the contrast medium

                        propagates through the various arteries in the area being observed

                        Digital Image Processing

                        Week 1

                        a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                        Digital Image Processing

                        Week 1

                        An important application of image multiplication (and division) is shading correction

                        Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                        f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                        When the shading function is known

                        ( )( )( )

                        g x yf x yh x y

                        h(x y) is unknown but we have access to the imaging system we can obtain an

                        approximation to the shading function by imaging a target of constant intensity When the

                        sensor is not available often the shading pattern can be estimated from the image

                        Digital Image Processing

                        Week 1

                        (a) (b) (c)

                        Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                        Digital Image Processing

                        Week 1

                        Another use of image multiplication is in masking also called region of interest (ROI)

                        operations The process consists of multiplying a given image by a mask image that has

                        1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                        image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                        (a) (b) (c)

                        Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                        Digital Image Processing

                        Week 1

                        In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                        min( )mf f f

                        0 ( 255)max( )

                        ms

                        m

                        ff K K K

                        f

                        Digital Image Processing

                        Week 1

                        Spatial Operations

                        - are performed directly on the pixels of a given image

                        There are three categories of spatial operations

                        single-pixel operations

                        neighborhood operations

                        geometric spatial transformations

                        Single-pixel operations

                        - change the values of intensity for the individual pixels ( )s T z

                        where z is the intensity of a pixel in the original image and s is the intensity of the

                        corresponding pixel in the processed image

                        Digital Image Processing

                        Week 1

                        Neighborhood operations

                        Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                        in an image f Neighborhood processing generates new intensity level at point (x y)

                        based on the values of the intensities of the points in Sxy For example if Sxy is a

                        rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                        intensity by computing the average value of the pixels in Sxy

                        ( )

                        1( ) ( )xyr c S

                        g x y f r cm n

                        The net effect is to perform local blurring in the original image This type of process is

                        used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                        largest region of an image

                        Digital Image Processing

                        Week 1

                        Geometric spatial transformations and image registration

                        - modify the spatial relationship between pixels in an image

                        - these transformations are often called rubber-sheet transformations (analogous to

                        printing an image on a sheet of rubber and then stretching the sheet according to a

                        predefined set of rules

                        A geometric transformation consists of 2 basic operations

                        1 a spatial transformation of coordinates

                        2 intensity interpolation that assign intensity values to the spatial transformed

                        pixels

                        The coordinate system transformation ( ) [( )]x y T v w

                        (v w) ndash pixel coordinates in the original image

                        (x y) ndash pixel coordinates in the transformed image

                        Digital Image Processing

                        Week 1

                        [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                        Affine transform

                        11 1211 21 31

                        21 2212 22 33

                        31 32

                        0[ 1] [ 1] [ 1] 0

                        1

                        t tx t v t w t

                        x y v w T v w t ty t v t w t

                        t t

                        (AT)

                        This transform can scale rotate translate or shear a set of coordinate points depending

                        on the elements of the matrix T If we want to resize an image rotate it and move the

                        result to some location we simply form a 3x3 matrix equal to the matrix product of the

                        scaling rotation and translation matrices from Table 1

                        Digital Image Processing

                        Week 1

                        Affine transformations

                        Digital Image Processing

                        Week 1

                        The preceding transformations relocate pixels on an image to new locations To complete

                        the process we have to assign intensity values to those locations This task is done by

                        using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                        In practice we can use equation (AT) in two basic ways

                        forward mapping scan the pixels of the input image (v w) compute the new spatial

                        location (x y) of the corresponding pixel in the new image using (AT) directly

                        Problems

                        - intensity assignment when 2 or more pixels in the original image are transformed to

                        the same location in the output image

                        - some output locations have no correspondent in the original image (no intensity

                        assignment)

                        Digital Image Processing

                        Week 1

                        inverse mapping scans the output pixel locations and at each location (x y)

                        computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                        It then interpolates among the nearest input pixels to determine the intensity of the output

                        pixel value

                        Inverse mappings are more efficient to implement than forward mappings and are used in

                        numerous commercial implementations of spatial transformations (MATLAB for ex)

                        Digital Image Processing

                        Week 1

                        Digital Image Processing

                        Week 1

                        Image registration ndash align two or more images of the same scene

                        In image registration we have available the input and output images but the specific

                        transformation that produced the output image from the input is generally unknown

                        The problem is to estimate the transformation function and then use it to register the two

                        images

                        - it may be of interest to align (register) two or more image taken at approximately the

                        same time but using different imaging systems (MRI scanner and a PET scanner)

                        - align images of a given location taken by the same instrument at different moments

                        of time (satellite images)

                        Solving the problem using tie points (also called control points) which are

                        corresponding points whose locations are known precisely in the input and reference

                        image

                        Digital Image Processing

                        Week 1

                        How to select tie points

                        - interactively selecting them

                        - use of algorithms that try to detect these points

                        - some imaging systems have physical artifacts (small metallic objects) embedded in

                        the imaging sensors These objects produce a set of known points (called reseau

                        marks) directly on all images captured by the system which can be used as guides

                        for establishing tie points

                        The problem of estimating the transformation is one of modeling Suppose we have a set

                        of 4 tie points both on the input image and the reference image A simple model based on

                        a bilinear approximation is given by

                        1 2 3 4

                        5 6 7 8

                        x c v c w c v w cy c v c w c v w c

                        (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                        Digital Image Processing

                        Week 1

                        When 4 tie points are insufficient to obtain satisfactory registration an approach used

                        frequently is to select a larger number of tie points and using this new set of tie points

                        subdivide the image in rectangular regions marked by groups of 4 tie points On the

                        subregions marked by 4 tie points we applied the transformation model described above

                        The number of tie points and the sophistication of the model required to solve the register

                        problem depend on the severity of the geometrical distortion

                        Digital Image Processing

                        Week 1

                        a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                        Digital Image Processing

                        Week 1

                        Probabilistic Methods

                        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                        p(zk) = the probability that the intensity level zk occurs in the given image

                        ( ) kk

                        np zM N

                        nk = the number of times that intensity zk occurs in the image (MN is the total number of

                        pixels in the image) 1

                        0( ) 1

                        L

                        kk

                        p z

                        The mean (average) intensity of an image is given by 1

                        0( )

                        L

                        k kk

                        m z p z

                        Digital Image Processing

                        Week 1

                        The variance of the intensities is 1

                        2 2

                        0( ) ( )

                        L

                        k kk

                        z m p z

                        The variance is a measure of the spread of the values of z about the mean so it is a

                        measure of image contrast Usually for measuring image contrast the standard deviation

                        ( ) is used

                        The n-th moment of a random variable z about the mean is defined as 1

                        0( ) ( ) ( )

                        Ln

                        n k kk

                        z z m p z

                        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                        3( ) 0z the intensities are biased to values higher than the mean

                        ( 3( ) 0z the intensities are biased to values lower than the mean

                        Digital Image Processing

                        Week 1

                        3( ) 0z the intensities are distributed approximately equally on both side of the

                        mean

                        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                        Figure 1(a) ndash standard deviation 143 (variance = 2045)

                        Figure 1(b) ndash standard deviation 316 (variance = 9986)

                        Figure 1(c) ndash standard deviation 492 (variance = 24206)

                        Digital Image Processing

                        Week 1

                        Intensity Transformations and Spatial Filtering

                        ( ) ( )g x y T f x y

                        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                        neighborhood of (x y)

                        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                        and much smaller in size than the image

                        Digital Image Processing

                        Week 1

                        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                        called spatial filter (spatial mask kernel template or window)

                        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                        ( )s T r

                        s and r are denoting respectively the intensity of g and f at (x y)

                        Figure 2 left - T produces an output image of higher contrast than the original by

                        darkening the intensity levels below k and brightening the levels above k ndash this technique

                        is called contrast stretching

                        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                        Digital Image Processing

                        Week 1

                        Figure 2 right - T produces a binary output image A mapping of this form is called

                        thresholding function

                        Some Basic Intensity Transformation Functions

                        Image Negatives

                        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                        - equivalent of a photographic negative

                        - technique suited for enhancing white or gray detail embedded in dark regions of an

                        image

                        Digital Image Processing

                        Week 1

                        Original Negative image

                        Digital Image Processing

                        Week 1

                        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                        Some basic intensity transformation functions

                        Digital Image Processing

                        Week 1

                        This transformation maps a narrow range of low intensity values in the input into a wider

                        range An operator of this type is used to expand the values of dark pixels in an image

                        while compressing the higher-level values The opposite is true for the inverse log

                        transformation The log functions compress the dynamic range of images with large

                        variations in pixel values

                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                        Digital Image Processing

                        Week 1

                        Power-Law (Gamma) Transformations

                        - positive constants( ) ( ( ) )s T r c r c s c r

                        Plots of gamma transformation for different values of γ (c=1)

                        Digital Image Processing

                        Week 1

                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                        of output values with the opposite being true for higher values of input values The

                        curves with 1 have the opposite effect of those generated with values of 1

                        1c - identity transformation

                        A variety of devices used for image capture printing and display respond according to a

                        power law The process used to correct these power-law response phenomena is called

                        gamma correction

                        Digital Image Processing

                        Week 1

                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                        Digital Image Processing

                        Week 1

                        Piecewise-Linear Transformations Functions

                        Contrast stretching

                        - a process that expands the range of intensity levels in an image so it spans the full

                        intensity range of the recording tool or display device

                        a b c d Fig5

                        Digital Image Processing

                        Week 1

                        11

                        1

                        2 1 1 21 2

                        2 1 2 1

                        22

                        2

                        [0 ]

                        ( ) ( )( ) [ ]( ) ( )

                        ( 1 ) [ 1]( 1 )

                        s r r rrs r r s r rT r r r r

                        r r r rs L r r r L

                        L r

                        Digital Image Processing

                        Week 1

                        1 1 2 2r s r s identity transformation (no change)

                        1 2 1 2 0 1r r s s L thresholding function

                        Figure 5(b) shows an 8-bit image with low contrast

                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                        in the image respectively Thus the transformation function stretched the levels linearly

                        from their original range to the full range [0 L-1]

                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                        2 2 1r s m L where m is the mean gray level in the image

                        The original image on which these results are based is a scanning electron microscope

                        image of pollen magnified approximately 700 times

                        Digital Image Processing

                        Week 1

                        Intensity-level slicing

                        - highlighting a specific range of intensities in an image

                        There are two approaches for intensity-level slicing

                        1 display in one value (white for example) all the values in the range of interest and in

                        another (say black) all other intensities (Figure 311 (a))

                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                        intensities in the image (Figure 311 (b))

                        Digital Image Processing

                        Week 1

                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                        the top of the scale of intensities This type of enhancement produces a binary image

                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                        Highlights range [A B] and preserves all other intensities

                        Digital Image Processing

                        Week 1

                        which is useful for studying the shape of the flow of the contrast substance (to detect

                        blockageshellip)

                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                        image around the mean intensity was set to black the other intensities remain unchanged

                        Fig 6 - Aortic angiogram and intensity sliced versions

                        Digital Image Processing

                        Week 1

                        Bit-plane slicing

                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                        This technique highlights the contribution made to the whole image appearances by each

                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                        Digital Image Processing

                        Week 1

                        Digital Image Processing

                        Week 1

                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                        • DIP 1 2017
                        • DIP 02 (2017)

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          A digital pictureproduced in 1921from a coded tapeby a telegraph printer withspecial type faces(McFarlane)

                          A digital picturemade in 1922from a tapepunched after the signals hadcrossed the Atlantic twice(McFarlane)

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          1964 Jet Propulsion Laboratory (Pasadena California) processed pictures of the moon transmitted by Ranger 7 (corrected image distortions)

                          The first picture of the moon by a US spacecraft Ranger 7 took this image July 31 1964 about 17 minutes before impacting the lunar surface(Courtesy of NASA)

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

                          1970s ndash invention of CAT (computerized axial tomography)

                          CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          loz geographers use DIP to study pollution patterns from aerial and satellite imagery

                          loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

                          loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

                          loz astronomy biology nuclear medicine law enforcement industry

                          DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

                          loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Examples of Fields that Use DIP

                          Images can be classified according to their sources (visual X-ray hellip)

                          Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Electromagnetic waves can be thought as propagating sinusoidal

                          waves of different wavelength or as a stream of massless particles

                          each moving in a wavelike pattern with the speed of light Each

                          massless particle contains a certain amount (bundle) of energy Each

                          bundle of energy is called a photon If spectral bands are grouped

                          according to energy per photon we obtain the spectrum shown in the

                          image above ranging from gamma-rays (highest energy) to radio

                          waves (lowest energy)

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Gamma-Ray Imaging

                          Nuclear medicine astronomical observations

                          Nuclear medicine

                          the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

                          Images are produced from the emissions collected by gamma-ray detectors

                          Images of this sort are used to locate sites of bone pathology (infections tumors)

                          PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Examples of gamma-ray imaging

                          Bone scan PET image

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          X-ray imaging

                          Medical diagnosticindustry astronomy

                          A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                          The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Angiography = contrast-enhancement radiography

                          Angiograms = images of blood vessels

                          A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                          X-rays are used in CAT (computerized axial tomography)

                          X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                          Industrial CAT scans are useful when the parts can be penetreted by X-rays

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Examples of X-ray imaging

                          Chest X-rayAortic angiogram

                          Head CT Cygnus LoopCircuit boards

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Imaging in the Ultraviolet Band

                          Litography industrial inspection microscopy biological imaging astronomical observations

                          Ultraviolet light is used in fluorescence microscopy

                          Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                          other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                          and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Imaging in the Visible and Infrared Bands

                          Light microscopy astronomy remote sensing industry law enforcement

                          LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                          Weather observations and prediction produce major applications of multispectral image from satellites

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Satellite images of Washington DC area in spectral bands of the Table 1

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Examples of light microscopy

                          Taxol (anticancer agent)magnified 250X

                          Cholesterol(40X)

                          Microprocessor(60X)

                          Nickel oxidethin film(600X)

                          Surface of audio CD(1750X)

                          Organicsuperconductor(450X)

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Automated visual inspection of manufactured goods

                          a bc de f

                          a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Imaging in the Microwave Band

                          The dominant aplication of imaging in the microwave band ndash radar

                          bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                          bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                          bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                          An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Spaceborne radar image of mountains in southeast Tibet

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Imaging in the Radio Band

                          medicine astronomy

                          MRI = Magnetic Resonance Imaging

                          This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                          Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                          The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          MRI images of a human knee (left) and spine (right)

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Images of the Crab Pulsar covering the electromagnetic spectrum

                          Gamma X-ray Optical Infrared Radio

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Other Imaging Modalities

                          acoustic imaging electron microscopy synthetic (computer-generated) imaging

                          Imaging using sound geological explorations industry medicine

                          Mineral and oil exploration

                          For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

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                          Biometry - iris

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                          Week 1Week 1

                          Biometry - fingerprint

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                          Week 1Week 1

                          Face detection and recognition

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Gender identification

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                          Image morphing

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                          Fundamental Steps in DIP

                          methods whose input and output are images

                          methods whose inputs are images but whose outputs are attributes extracted from those images

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Outputs are images

                          bull image acquisition

                          bull image filtering and enhancement

                          bull image restoration

                          bull color image processing

                          bull wavelets and multiresolution processing

                          bull compression

                          bull morphological processing

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                          Week 1Week 1

                          Outputs are attributes

                          bull morphological processing

                          bull segmentation

                          bull representation and description

                          bull object recognition

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                          Week 1Week 1

                          Image acquisition - may involve preprocessing such as scaling

                          Image enhancement

                          bull manipulating an image so that the result is more suitable than the original for a specific operation

                          bull enhancement is problem oriented

                          bull there is no general sbquotheoryrsquo of image enhancement

                          bull enhancement use subjective methods for image emprovement

                          bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

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                          Week 1Week 1

                          Image restoration

                          bull improving the appearance of an image

                          bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                          Color image processing

                          bull fundamental concept in color models

                          bull basic color processing in a digital domain

                          Wavelets and multiresolution processing

                          representing images in various degree of resolution

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Compression

                          reducing the storage required to save an image or the bandwidth required to transmit it

                          Morphological processing

                          bull tools for extracting image components that are useful in the representation and description of shape

                          bull a transition from processes that output images to processes that outputimage attributes

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Segmentation

                          bull partitioning an image into its constituents parts or objects

                          bull autonomous segmentation is one of the most difficult tasks of DIP

                          bull the more accurate the segmentation the more likley recognition is to succeed

                          Representation and description (almost always follows segmentation)

                          bull segmentation produces either the boundary of a region or all the poits in the region itself

                          bull converting the data produced by segmentation to a form suitable for computer processing

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          bull boundary representation the focus is on external shape characteristics such as corners or inflections

                          bull complete region the focus is on internal properties such as texture or skeletal shape

                          bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                          Object recognition

                          the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                          Knowledge database

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Simplified diagramof a cross sectionof the human eye

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                          The cornea is a tough transparent tissue that covers the anterior surface of the eye

                          Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                          The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                          The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                          Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                          Fovea = the place where the image of the object of interest falls on

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                          Blind spot region without receptors

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Image formation in the eye

                          Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                          Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                          distance between lens and retina along visual axix = 17 mm

                          range of focal length = 14 mm to 17 mm

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Optical illusions

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                          gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                          quantities that describe the quality of a chromatic light source radiance

                          the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                          measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                          brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          the physical meaning is determined by the source of the image

                          ( )f D f x y

                          Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                          f(xy) ndash characterized by two components

                          i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                          r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                          ( ) ( ) ( )

                          0 ( ) 0 ( ) 1

                          f x y i x y r x y

                          i x y r x y

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          r(xy)=0 - total absorption r(xy)=1 - total reflectance

                          i(xy) ndash determined by the illumination source

                          r(xy) ndash determined by the characteristics of the imaged objects

                          is called gray (or intensity) scale

                          In practice

                          min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                          indoor values without additional illuminationmin max10 1000L L

                          black whitemin max0 1 0 1 0 1L L L L l l L

                          min maxL L

                          Digital Image ProcessingDigital Image Processing

                          Week 1Week 1

                          Digital Image Processing

                          Week 1

                          Image Sampling and Quantization

                          - the output of the sensors is a continuous voltage waveform related to the sensed

                          scene

                          converting a continuous image f to digital form

                          - digitizing (x y) is called sampling

                          - digitizing f(x y) is called quantization

                          Digital Image Processing

                          Week 1

                          Digital Image Processing

                          Week 1

                          Continuous image projected onto a sensor array Result of image sampling and quantization

                          Digital Image Processing

                          Week 1

                          Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                          (00) (01) (0 1)(10) (11) (1 1)

                          ( )

                          ( 10) ( 11) ( 1 1)

                          f f f Nf f f N

                          f x y

                          f M f M f M N

                          image element pixel

                          00 01 0 1

                          10 11 1 1

                          10 11 1 1

                          ( ) ( )

                          N

                          i jN M N

                          i j

                          M M M N

                          a a aa f x i y j f i ja a a

                          Aa

                          a a a

                          f(00) ndash the upper left corner of the image

                          Digital Image Processing

                          Week 1

                          M N ge 0 L=2k

                          [0 1]i j i ja a L

                          Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                          Digital Image Processing

                          Week 1

                          Digital Image Processing

                          Week 1

                          Number of bits required to store a digitized image

                          for 2 b M N k M N b N k

                          When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                          Digital Image Processing

                          Week 1

                          Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                          Measures line pairs per unit distance dots (pixels) per unit distance

                          Image resolution = the largest number of discernible line pairs per unit distance

                          (eg 100 line pairs per mm)

                          Dots per unit distance are commonly used in printing and publishing

                          In US the measure is expressed in dots per inch (dpi)

                          (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                          Intensity resolution ndash the smallest discernible change in intensity level

                          The number of intensity levels (L) is determined by hardware considerations

                          L=2k ndash most common k = 8

                          Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                          Digital Image Processing

                          Week 1

                          Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                          150 dpi (lower left) 72 dpi (lower right)

                          Digital Image Processing

                          Week 1

                          Reducing the number of gray levels 256 128 64 32

                          Digital Image Processing

                          Week 1

                          Reducing the number of gray levels 16 8 4 2

                          Digital Image Processing

                          Week 1

                          Image Interpolation - used in zooming shrinking rotating and geometric corrections

                          Shrinking zooming ndash image resizing ndash image resampling methods

                          Interpolation is the process of using known data to estimate values at unknown locations

                          Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                          750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                          same spacing as the original and then shrink it so that it fits exactly over the original

                          image The pixel spacing in the 750 times 750 grid will be less than in the original image

                          Problem assignment of intensity-level in the new 750 times 750 grid

                          Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                          intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                          This technique has the tendency to produce undesirable effects like severe distortion of

                          straight edges

                          Digital Image Processing

                          Week 1

                          Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                          where the four coefficients are determined from the 4 equations in 4 unknowns that can

                          be written using the 4 nearest neighbors of point (x y)

                          Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                          modest increase in computational effort

                          Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                          nearest neighbors of the point 3 3

                          0 0

                          ( ) i ji j

                          i jv x y c x y

                          The coefficients cij are obtained solving a 16x16 linear system

                          intensity levels of the 16 nearest neighbors of 3 3

                          0 0

                          ( )i ji j

                          i jc x y x y

                          Digital Image Processing

                          Week 1

                          Generally bicubic interpolation does a better job of preserving fine detail than the

                          bilinear technique Bicubic interpolation is the standard used in commercial image editing

                          programs such as Adobe Photoshop and Corel Photopaint

                          Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                          the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                          then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                          neighbor interpolation was used (both for shrinking and zooming)

                          Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                          interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                          from 1250 dpi to 150 dpi (instead of 72 dpi)

                          Digital Image Processing

                          Week 1

                          Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                          Digital Image Processing

                          Week 1

                          Neighbors of a Pixel

                          A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                          This set of pixels called the 4-neighbors of p denoted by N4 (p)

                          The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                          and are denoted ND(p)

                          The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                          N8 (p)

                          If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                          fall outside the image

                          Digital Image Processing

                          Week 1

                          Adjacency Connectivity Regions Boundaries

                          Denote by V the set of intensity levels used to define adjacency

                          - in a binary image V 01 (V=0 V=1)

                          - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                          We consider 3 types of adjacency

                          (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                          (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                          (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                          m-adjacent if

                          4( )q N p or

                          ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                          Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                          ambiguities that often arise when 8-adjacency is used Consider the example

                          Digital Image Processing

                          Week 1

                          binary image

                          0 1 1 0 1 1 0 1 1

                          1 0 1 0 0 1 0 0 1 0

                          0 0 1 0 0 1 0 0 1

                          V

                          The three pixels at the top (first line) in the above example show multiple (ambiguous)

                          8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                          m-adjacency

                          Digital Image Processing

                          Week 1

                          A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                          is a sequence of distinct pixels with coordinates

                          and are adjacent 0 0 1 1

                          1 1

                          ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                          n n

                          i i i i

                          x y x y x y x y s tx y x y i n

                          The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                          Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                          Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                          in S if there exists a path between them consisting only of pixels from S

                          S is a connected set if there is a path in S between any 2 pixels in S

                          Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                          Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                          that are not adjacent are said to be disjoint When referring to regions only 4- and

                          8-adjacency are considered

                          Digital Image Processing

                          Week 1

                          Suppose that an image contains K disjoint regions 1 kR k K none of which

                          touches the image border

                          the complement of 1

                          ( )K

                          cu k u u

                          k

                          R R R R

                          We call all the points in Ru the foreground of the image and the points in ( )cuR the

                          background of the image

                          The boundary (border or contour) of a region R is the set of points that are adjacent to

                          points in the complement of R (R)c The border of an image is the set of pixels in the

                          region that have at least one background neighbor This definition is referred to as the

                          inner border to distinguish it from the notion of outer border which is the corresponding

                          border in the background

                          Digital Image Processing

                          Week 1

                          Distance measures

                          For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                          function or metric if

                          (a) D(p q) ge 0 D(p q) = 0 iff p=q

                          (b) D(p q) = D(q p)

                          (c) D(p z) le D(p q) + D(q z)

                          The Euclidean distance between p and q is defined as 1

                          2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                          The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                          centered at (x y)

                          Digital Image Processing

                          Week 1

                          The D4 distance (also called city-block distance) between p and q is defined as

                          4( ) | | | |D p q x s y t

                          The pixels q for which 4( )D p q r form a diamond centered at (xy)

                          4

                          22 1 2

                          2 2 1 0 1 22 1 2

                          2

                          D

                          The pixels with D4 = 1 are the 4-neighbors of (x y)

                          The D8 distance (called the chessboard distance) between p and q is defined as

                          8( ) max| | | |D p q x s y t

                          The pixels q for which 8( )D p q r form a square centered at (x y)

                          Digital Image Processing

                          Week 1

                          8

                          2 2 2 2 22 1 1 1 2

                          2 2 1 0 1 22 1 1 1 22 2 2 2 2

                          D

                          The pixels with D8 = 1 are the 8-neighbors of (x y)

                          D4 and D8 distances are independent of any paths that might exist between p and q

                          because these distances involve only the coordinates of the point

                          Digital Image Processing

                          Week 1

                          Array versus Matrix Operations

                          An array operation involving one or more images is carried out on a pixel-by-pixel basis

                          11 12 11 12

                          21 22 21 22

                          a a b ba a b b

                          Array product

                          11 12 11 12 11 11 12 12

                          21 22 21 22 21 21 22 21

                          a a b b a b a ba a b b a b a b

                          Matrix product

                          11 12 11 12 11 11 12 21 11 12 12 21

                          21 22 21 22 21 11 22 21 21 12 22 22

                          a a b b a b a b a b a ba a b b a b a b a b a b

                          We assume array operations unless stated otherwise

                          Digital Image Processing

                          Week 1

                          Linear versus Nonlinear Operations

                          One of the most important classifications of image-processing methods is whether it is

                          linear or nonlinear

                          ( ) ( )H f x y g x y

                          H is said to be a linear operator if

                          images1 2 1 2

                          1 2

                          ( ) ( ) ( ) ( )

                          H a f x y b f x y a H f x y b H f x y

                          a b f f

                          Example of nonlinear operator

                          the maximum value of the pixels of image max ( )H f f x y f

                          1 2

                          0 2 6 5 1 1

                          2 3 4 7f f a b

                          Digital Image Processing

                          Week 1

                          1 2

                          0 2 6 5 6 3max max 1 ( 1) max 2

                          2 3 4 7 2 4a f b f

                          0 2 6 51 max ( 1) max 3 ( 1)7 4

                          2 3 4 7

                          Arithmetic Operations in Image Processing

                          Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                          f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                          For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                          value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                          The two random variables are uncorrelated when their covariance is 0

                          Digital Image Processing

                          Week 1

                          Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                          used in image enhancement)

                          1

                          1( ) ( )K

                          ii

                          g x y g x yK

                          If the noise satisfies the properties stated above we have

                          2 2( ) ( )

                          1( ) ( ) g x y x yE g x y f x yK

                          ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                          and g respectively The standard deviation (square root of the variance) at any point in

                          the average image is

                          ( ) ( )1

                          g x y x yK

                          Digital Image Processing

                          Week 1

                          As K increases the variability (as measured by the variance or the standard deviation) of

                          the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                          means that ( )g x y approaches f(x y) as the number of noisy images used in the

                          averaging process increases

                          An important application of image averaging is in the field of astronomy where imaging

                          under very low light levels frequently causes sensor noise to render single images

                          virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                          was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                          64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                          images respectively

                          Digital Image Processing

                          Week 1

                          Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                          100 noisy images

                          a b c d e f

                          Digital Image Processing

                          Week 1

                          A frequent application of image subtraction is in the enhancement of differences between

                          images

                          (a) (b) (c)

                          Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                          significant bit of each pixel (c) the difference between the two images

                          Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                          Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                          images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                          difference between images (a) and (b)

                          Digital Image Processing

                          Week 1

                          Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                          h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                          intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                          source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                          bloodstream taking a series of images called live images (denoted f(x y)) of the same

                          anatomical region as h(x y) and subtracting the mask from the series of incoming live

                          images after injection of the contrast medium

                          In g(x y) we can find the differences between h and f as enhanced detail

                          Images being captured at TV rates we obtain a movie showing how the contrast medium

                          propagates through the various arteries in the area being observed

                          Digital Image Processing

                          Week 1

                          a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                          Digital Image Processing

                          Week 1

                          An important application of image multiplication (and division) is shading correction

                          Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                          f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                          When the shading function is known

                          ( )( )( )

                          g x yf x yh x y

                          h(x y) is unknown but we have access to the imaging system we can obtain an

                          approximation to the shading function by imaging a target of constant intensity When the

                          sensor is not available often the shading pattern can be estimated from the image

                          Digital Image Processing

                          Week 1

                          (a) (b) (c)

                          Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                          Digital Image Processing

                          Week 1

                          Another use of image multiplication is in masking also called region of interest (ROI)

                          operations The process consists of multiplying a given image by a mask image that has

                          1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                          image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                          (a) (b) (c)

                          Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                          Digital Image Processing

                          Week 1

                          In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                          min( )mf f f

                          0 ( 255)max( )

                          ms

                          m

                          ff K K K

                          f

                          Digital Image Processing

                          Week 1

                          Spatial Operations

                          - are performed directly on the pixels of a given image

                          There are three categories of spatial operations

                          single-pixel operations

                          neighborhood operations

                          geometric spatial transformations

                          Single-pixel operations

                          - change the values of intensity for the individual pixels ( )s T z

                          where z is the intensity of a pixel in the original image and s is the intensity of the

                          corresponding pixel in the processed image

                          Digital Image Processing

                          Week 1

                          Neighborhood operations

                          Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                          in an image f Neighborhood processing generates new intensity level at point (x y)

                          based on the values of the intensities of the points in Sxy For example if Sxy is a

                          rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                          intensity by computing the average value of the pixels in Sxy

                          ( )

                          1( ) ( )xyr c S

                          g x y f r cm n

                          The net effect is to perform local blurring in the original image This type of process is

                          used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                          largest region of an image

                          Digital Image Processing

                          Week 1

                          Geometric spatial transformations and image registration

                          - modify the spatial relationship between pixels in an image

                          - these transformations are often called rubber-sheet transformations (analogous to

                          printing an image on a sheet of rubber and then stretching the sheet according to a

                          predefined set of rules

                          A geometric transformation consists of 2 basic operations

                          1 a spatial transformation of coordinates

                          2 intensity interpolation that assign intensity values to the spatial transformed

                          pixels

                          The coordinate system transformation ( ) [( )]x y T v w

                          (v w) ndash pixel coordinates in the original image

                          (x y) ndash pixel coordinates in the transformed image

                          Digital Image Processing

                          Week 1

                          [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                          Affine transform

                          11 1211 21 31

                          21 2212 22 33

                          31 32

                          0[ 1] [ 1] [ 1] 0

                          1

                          t tx t v t w t

                          x y v w T v w t ty t v t w t

                          t t

                          (AT)

                          This transform can scale rotate translate or shear a set of coordinate points depending

                          on the elements of the matrix T If we want to resize an image rotate it and move the

                          result to some location we simply form a 3x3 matrix equal to the matrix product of the

                          scaling rotation and translation matrices from Table 1

                          Digital Image Processing

                          Week 1

                          Affine transformations

                          Digital Image Processing

                          Week 1

                          The preceding transformations relocate pixels on an image to new locations To complete

                          the process we have to assign intensity values to those locations This task is done by

                          using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                          In practice we can use equation (AT) in two basic ways

                          forward mapping scan the pixels of the input image (v w) compute the new spatial

                          location (x y) of the corresponding pixel in the new image using (AT) directly

                          Problems

                          - intensity assignment when 2 or more pixels in the original image are transformed to

                          the same location in the output image

                          - some output locations have no correspondent in the original image (no intensity

                          assignment)

                          Digital Image Processing

                          Week 1

                          inverse mapping scans the output pixel locations and at each location (x y)

                          computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                          It then interpolates among the nearest input pixels to determine the intensity of the output

                          pixel value

                          Inverse mappings are more efficient to implement than forward mappings and are used in

                          numerous commercial implementations of spatial transformations (MATLAB for ex)

                          Digital Image Processing

                          Week 1

                          Digital Image Processing

                          Week 1

                          Image registration ndash align two or more images of the same scene

                          In image registration we have available the input and output images but the specific

                          transformation that produced the output image from the input is generally unknown

                          The problem is to estimate the transformation function and then use it to register the two

                          images

                          - it may be of interest to align (register) two or more image taken at approximately the

                          same time but using different imaging systems (MRI scanner and a PET scanner)

                          - align images of a given location taken by the same instrument at different moments

                          of time (satellite images)

                          Solving the problem using tie points (also called control points) which are

                          corresponding points whose locations are known precisely in the input and reference

                          image

                          Digital Image Processing

                          Week 1

                          How to select tie points

                          - interactively selecting them

                          - use of algorithms that try to detect these points

                          - some imaging systems have physical artifacts (small metallic objects) embedded in

                          the imaging sensors These objects produce a set of known points (called reseau

                          marks) directly on all images captured by the system which can be used as guides

                          for establishing tie points

                          The problem of estimating the transformation is one of modeling Suppose we have a set

                          of 4 tie points both on the input image and the reference image A simple model based on

                          a bilinear approximation is given by

                          1 2 3 4

                          5 6 7 8

                          x c v c w c v w cy c v c w c v w c

                          (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                          Digital Image Processing

                          Week 1

                          When 4 tie points are insufficient to obtain satisfactory registration an approach used

                          frequently is to select a larger number of tie points and using this new set of tie points

                          subdivide the image in rectangular regions marked by groups of 4 tie points On the

                          subregions marked by 4 tie points we applied the transformation model described above

                          The number of tie points and the sophistication of the model required to solve the register

                          problem depend on the severity of the geometrical distortion

                          Digital Image Processing

                          Week 1

                          a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                          Digital Image Processing

                          Week 1

                          Probabilistic Methods

                          zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                          p(zk) = the probability that the intensity level zk occurs in the given image

                          ( ) kk

                          np zM N

                          nk = the number of times that intensity zk occurs in the image (MN is the total number of

                          pixels in the image) 1

                          0( ) 1

                          L

                          kk

                          p z

                          The mean (average) intensity of an image is given by 1

                          0( )

                          L

                          k kk

                          m z p z

                          Digital Image Processing

                          Week 1

                          The variance of the intensities is 1

                          2 2

                          0( ) ( )

                          L

                          k kk

                          z m p z

                          The variance is a measure of the spread of the values of z about the mean so it is a

                          measure of image contrast Usually for measuring image contrast the standard deviation

                          ( ) is used

                          The n-th moment of a random variable z about the mean is defined as 1

                          0( ) ( ) ( )

                          Ln

                          n k kk

                          z z m p z

                          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                          3( ) 0z the intensities are biased to values higher than the mean

                          ( 3( ) 0z the intensities are biased to values lower than the mean

                          Digital Image Processing

                          Week 1

                          3( ) 0z the intensities are distributed approximately equally on both side of the

                          mean

                          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                          Figure 1(a) ndash standard deviation 143 (variance = 2045)

                          Figure 1(b) ndash standard deviation 316 (variance = 9986)

                          Figure 1(c) ndash standard deviation 492 (variance = 24206)

                          Digital Image Processing

                          Week 1

                          Intensity Transformations and Spatial Filtering

                          ( ) ( )g x y T f x y

                          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                          neighborhood of (x y)

                          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                          and much smaller in size than the image

                          Digital Image Processing

                          Week 1

                          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                          called spatial filter (spatial mask kernel template or window)

                          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                          ( )s T r

                          s and r are denoting respectively the intensity of g and f at (x y)

                          Figure 2 left - T produces an output image of higher contrast than the original by

                          darkening the intensity levels below k and brightening the levels above k ndash this technique

                          is called contrast stretching

                          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                          Digital Image Processing

                          Week 1

                          Figure 2 right - T produces a binary output image A mapping of this form is called

                          thresholding function

                          Some Basic Intensity Transformation Functions

                          Image Negatives

                          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                          - equivalent of a photographic negative

                          - technique suited for enhancing white or gray detail embedded in dark regions of an

                          image

                          Digital Image Processing

                          Week 1

                          Original Negative image

                          Digital Image Processing

                          Week 1

                          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                          Some basic intensity transformation functions

                          Digital Image Processing

                          Week 1

                          This transformation maps a narrow range of low intensity values in the input into a wider

                          range An operator of this type is used to expand the values of dark pixels in an image

                          while compressing the higher-level values The opposite is true for the inverse log

                          transformation The log functions compress the dynamic range of images with large

                          variations in pixel values

                          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                          Digital Image Processing

                          Week 1

                          Power-Law (Gamma) Transformations

                          - positive constants( ) ( ( ) )s T r c r c s c r

                          Plots of gamma transformation for different values of γ (c=1)

                          Digital Image Processing

                          Week 1

                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                          of output values with the opposite being true for higher values of input values The

                          curves with 1 have the opposite effect of those generated with values of 1

                          1c - identity transformation

                          A variety of devices used for image capture printing and display respond according to a

                          power law The process used to correct these power-law response phenomena is called

                          gamma correction

                          Digital Image Processing

                          Week 1

                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                          Digital Image Processing

                          Week 1

                          Piecewise-Linear Transformations Functions

                          Contrast stretching

                          - a process that expands the range of intensity levels in an image so it spans the full

                          intensity range of the recording tool or display device

                          a b c d Fig5

                          Digital Image Processing

                          Week 1

                          11

                          1

                          2 1 1 21 2

                          2 1 2 1

                          22

                          2

                          [0 ]

                          ( ) ( )( ) [ ]( ) ( )

                          ( 1 ) [ 1]( 1 )

                          s r r rrs r r s r rT r r r r

                          r r r rs L r r r L

                          L r

                          Digital Image Processing

                          Week 1

                          1 1 2 2r s r s identity transformation (no change)

                          1 2 1 2 0 1r r s s L thresholding function

                          Figure 5(b) shows an 8-bit image with low contrast

                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                          in the image respectively Thus the transformation function stretched the levels linearly

                          from their original range to the full range [0 L-1]

                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                          2 2 1r s m L where m is the mean gray level in the image

                          The original image on which these results are based is a scanning electron microscope

                          image of pollen magnified approximately 700 times

                          Digital Image Processing

                          Week 1

                          Intensity-level slicing

                          - highlighting a specific range of intensities in an image

                          There are two approaches for intensity-level slicing

                          1 display in one value (white for example) all the values in the range of interest and in

                          another (say black) all other intensities (Figure 311 (a))

                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                          intensities in the image (Figure 311 (b))

                          Digital Image Processing

                          Week 1

                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                          the top of the scale of intensities This type of enhancement produces a binary image

                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                          Highlights range [A B] and preserves all other intensities

                          Digital Image Processing

                          Week 1

                          which is useful for studying the shape of the flow of the contrast substance (to detect

                          blockageshellip)

                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                          image around the mean intensity was set to black the other intensities remain unchanged

                          Fig 6 - Aortic angiogram and intensity sliced versions

                          Digital Image Processing

                          Week 1

                          Bit-plane slicing

                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                          This technique highlights the contribution made to the whole image appearances by each

                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                          Digital Image Processing

                          Week 1

                          Digital Image Processing

                          Week 1

                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                          • DIP 1 2017
                          • DIP 02 (2017)

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            1964 Jet Propulsion Laboratory (Pasadena California) processed pictures of the moon transmitted by Ranger 7 (corrected image distortions)

                            The first picture of the moon by a US spacecraft Ranger 7 took this image July 31 1964 about 17 minutes before impacting the lunar surface(Courtesy of NASA)

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

                            1970s ndash invention of CAT (computerized axial tomography)

                            CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            loz geographers use DIP to study pollution patterns from aerial and satellite imagery

                            loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

                            loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

                            loz astronomy biology nuclear medicine law enforcement industry

                            DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

                            loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Examples of Fields that Use DIP

                            Images can be classified according to their sources (visual X-ray hellip)

                            Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Electromagnetic waves can be thought as propagating sinusoidal

                            waves of different wavelength or as a stream of massless particles

                            each moving in a wavelike pattern with the speed of light Each

                            massless particle contains a certain amount (bundle) of energy Each

                            bundle of energy is called a photon If spectral bands are grouped

                            according to energy per photon we obtain the spectrum shown in the

                            image above ranging from gamma-rays (highest energy) to radio

                            waves (lowest energy)

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Gamma-Ray Imaging

                            Nuclear medicine astronomical observations

                            Nuclear medicine

                            the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

                            Images are produced from the emissions collected by gamma-ray detectors

                            Images of this sort are used to locate sites of bone pathology (infections tumors)

                            PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Examples of gamma-ray imaging

                            Bone scan PET image

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            X-ray imaging

                            Medical diagnosticindustry astronomy

                            A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                            The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Angiography = contrast-enhancement radiography

                            Angiograms = images of blood vessels

                            A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                            X-rays are used in CAT (computerized axial tomography)

                            X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                            Industrial CAT scans are useful when the parts can be penetreted by X-rays

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Examples of X-ray imaging

                            Chest X-rayAortic angiogram

                            Head CT Cygnus LoopCircuit boards

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Imaging in the Ultraviolet Band

                            Litography industrial inspection microscopy biological imaging astronomical observations

                            Ultraviolet light is used in fluorescence microscopy

                            Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                            other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                            and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Imaging in the Visible and Infrared Bands

                            Light microscopy astronomy remote sensing industry law enforcement

                            LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                            Weather observations and prediction produce major applications of multispectral image from satellites

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Satellite images of Washington DC area in spectral bands of the Table 1

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Examples of light microscopy

                            Taxol (anticancer agent)magnified 250X

                            Cholesterol(40X)

                            Microprocessor(60X)

                            Nickel oxidethin film(600X)

                            Surface of audio CD(1750X)

                            Organicsuperconductor(450X)

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Automated visual inspection of manufactured goods

                            a bc de f

                            a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Imaging in the Microwave Band

                            The dominant aplication of imaging in the microwave band ndash radar

                            bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                            bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                            bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                            An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Spaceborne radar image of mountains in southeast Tibet

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Imaging in the Radio Band

                            medicine astronomy

                            MRI = Magnetic Resonance Imaging

                            This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                            Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                            The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            MRI images of a human knee (left) and spine (right)

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Images of the Crab Pulsar covering the electromagnetic spectrum

                            Gamma X-ray Optical Infrared Radio

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Other Imaging Modalities

                            acoustic imaging electron microscopy synthetic (computer-generated) imaging

                            Imaging using sound geological explorations industry medicine

                            Mineral and oil exploration

                            For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Biometry - iris

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Biometry - fingerprint

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Face detection and recognition

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Gender identification

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Image morphing

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Fundamental Steps in DIP

                            methods whose input and output are images

                            methods whose inputs are images but whose outputs are attributes extracted from those images

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Outputs are images

                            bull image acquisition

                            bull image filtering and enhancement

                            bull image restoration

                            bull color image processing

                            bull wavelets and multiresolution processing

                            bull compression

                            bull morphological processing

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Outputs are attributes

                            bull morphological processing

                            bull segmentation

                            bull representation and description

                            bull object recognition

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Image acquisition - may involve preprocessing such as scaling

                            Image enhancement

                            bull manipulating an image so that the result is more suitable than the original for a specific operation

                            bull enhancement is problem oriented

                            bull there is no general sbquotheoryrsquo of image enhancement

                            bull enhancement use subjective methods for image emprovement

                            bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Image restoration

                            bull improving the appearance of an image

                            bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                            Color image processing

                            bull fundamental concept in color models

                            bull basic color processing in a digital domain

                            Wavelets and multiresolution processing

                            representing images in various degree of resolution

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Compression

                            reducing the storage required to save an image or the bandwidth required to transmit it

                            Morphological processing

                            bull tools for extracting image components that are useful in the representation and description of shape

                            bull a transition from processes that output images to processes that outputimage attributes

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Segmentation

                            bull partitioning an image into its constituents parts or objects

                            bull autonomous segmentation is one of the most difficult tasks of DIP

                            bull the more accurate the segmentation the more likley recognition is to succeed

                            Representation and description (almost always follows segmentation)

                            bull segmentation produces either the boundary of a region or all the poits in the region itself

                            bull converting the data produced by segmentation to a form suitable for computer processing

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            bull boundary representation the focus is on external shape characteristics such as corners or inflections

                            bull complete region the focus is on internal properties such as texture or skeletal shape

                            bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                            Object recognition

                            the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                            Knowledge database

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Simplified diagramof a cross sectionof the human eye

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                            The cornea is a tough transparent tissue that covers the anterior surface of the eye

                            Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                            The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                            The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                            Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                            Fovea = the place where the image of the object of interest falls on

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                            Blind spot region without receptors

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Image formation in the eye

                            Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                            Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                            distance between lens and retina along visual axix = 17 mm

                            range of focal length = 14 mm to 17 mm

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Optical illusions

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                            gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                            quantities that describe the quality of a chromatic light source radiance

                            the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                            measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                            brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            the physical meaning is determined by the source of the image

                            ( )f D f x y

                            Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                            f(xy) ndash characterized by two components

                            i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                            r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                            ( ) ( ) ( )

                            0 ( ) 0 ( ) 1

                            f x y i x y r x y

                            i x y r x y

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            r(xy)=0 - total absorption r(xy)=1 - total reflectance

                            i(xy) ndash determined by the illumination source

                            r(xy) ndash determined by the characteristics of the imaged objects

                            is called gray (or intensity) scale

                            In practice

                            min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                            indoor values without additional illuminationmin max10 1000L L

                            black whitemin max0 1 0 1 0 1L L L L l l L

                            min maxL L

                            Digital Image ProcessingDigital Image Processing

                            Week 1Week 1

                            Digital Image Processing

                            Week 1

                            Image Sampling and Quantization

                            - the output of the sensors is a continuous voltage waveform related to the sensed

                            scene

                            converting a continuous image f to digital form

                            - digitizing (x y) is called sampling

                            - digitizing f(x y) is called quantization

                            Digital Image Processing

                            Week 1

                            Digital Image Processing

                            Week 1

                            Continuous image projected onto a sensor array Result of image sampling and quantization

                            Digital Image Processing

                            Week 1

                            Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                            (00) (01) (0 1)(10) (11) (1 1)

                            ( )

                            ( 10) ( 11) ( 1 1)

                            f f f Nf f f N

                            f x y

                            f M f M f M N

                            image element pixel

                            00 01 0 1

                            10 11 1 1

                            10 11 1 1

                            ( ) ( )

                            N

                            i jN M N

                            i j

                            M M M N

                            a a aa f x i y j f i ja a a

                            Aa

                            a a a

                            f(00) ndash the upper left corner of the image

                            Digital Image Processing

                            Week 1

                            M N ge 0 L=2k

                            [0 1]i j i ja a L

                            Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                            Digital Image Processing

                            Week 1

                            Digital Image Processing

                            Week 1

                            Number of bits required to store a digitized image

                            for 2 b M N k M N b N k

                            When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                            Digital Image Processing

                            Week 1

                            Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                            Measures line pairs per unit distance dots (pixels) per unit distance

                            Image resolution = the largest number of discernible line pairs per unit distance

                            (eg 100 line pairs per mm)

                            Dots per unit distance are commonly used in printing and publishing

                            In US the measure is expressed in dots per inch (dpi)

                            (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                            Intensity resolution ndash the smallest discernible change in intensity level

                            The number of intensity levels (L) is determined by hardware considerations

                            L=2k ndash most common k = 8

                            Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                            Digital Image Processing

                            Week 1

                            Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                            150 dpi (lower left) 72 dpi (lower right)

                            Digital Image Processing

                            Week 1

                            Reducing the number of gray levels 256 128 64 32

                            Digital Image Processing

                            Week 1

                            Reducing the number of gray levels 16 8 4 2

                            Digital Image Processing

                            Week 1

                            Image Interpolation - used in zooming shrinking rotating and geometric corrections

                            Shrinking zooming ndash image resizing ndash image resampling methods

                            Interpolation is the process of using known data to estimate values at unknown locations

                            Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                            750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                            same spacing as the original and then shrink it so that it fits exactly over the original

                            image The pixel spacing in the 750 times 750 grid will be less than in the original image

                            Problem assignment of intensity-level in the new 750 times 750 grid

                            Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                            intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                            This technique has the tendency to produce undesirable effects like severe distortion of

                            straight edges

                            Digital Image Processing

                            Week 1

                            Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                            where the four coefficients are determined from the 4 equations in 4 unknowns that can

                            be written using the 4 nearest neighbors of point (x y)

                            Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                            modest increase in computational effort

                            Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                            nearest neighbors of the point 3 3

                            0 0

                            ( ) i ji j

                            i jv x y c x y

                            The coefficients cij are obtained solving a 16x16 linear system

                            intensity levels of the 16 nearest neighbors of 3 3

                            0 0

                            ( )i ji j

                            i jc x y x y

                            Digital Image Processing

                            Week 1

                            Generally bicubic interpolation does a better job of preserving fine detail than the

                            bilinear technique Bicubic interpolation is the standard used in commercial image editing

                            programs such as Adobe Photoshop and Corel Photopaint

                            Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                            the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                            then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                            neighbor interpolation was used (both for shrinking and zooming)

                            Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                            interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                            from 1250 dpi to 150 dpi (instead of 72 dpi)

                            Digital Image Processing

                            Week 1

                            Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                            Digital Image Processing

                            Week 1

                            Neighbors of a Pixel

                            A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                            This set of pixels called the 4-neighbors of p denoted by N4 (p)

                            The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                            and are denoted ND(p)

                            The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                            N8 (p)

                            If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                            fall outside the image

                            Digital Image Processing

                            Week 1

                            Adjacency Connectivity Regions Boundaries

                            Denote by V the set of intensity levels used to define adjacency

                            - in a binary image V 01 (V=0 V=1)

                            - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                            We consider 3 types of adjacency

                            (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                            (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                            (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                            m-adjacent if

                            4( )q N p or

                            ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                            Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                            ambiguities that often arise when 8-adjacency is used Consider the example

                            Digital Image Processing

                            Week 1

                            binary image

                            0 1 1 0 1 1 0 1 1

                            1 0 1 0 0 1 0 0 1 0

                            0 0 1 0 0 1 0 0 1

                            V

                            The three pixels at the top (first line) in the above example show multiple (ambiguous)

                            8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                            m-adjacency

                            Digital Image Processing

                            Week 1

                            A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                            is a sequence of distinct pixels with coordinates

                            and are adjacent 0 0 1 1

                            1 1

                            ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                            n n

                            i i i i

                            x y x y x y x y s tx y x y i n

                            The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                            Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                            Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                            in S if there exists a path between them consisting only of pixels from S

                            S is a connected set if there is a path in S between any 2 pixels in S

                            Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                            Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                            that are not adjacent are said to be disjoint When referring to regions only 4- and

                            8-adjacency are considered

                            Digital Image Processing

                            Week 1

                            Suppose that an image contains K disjoint regions 1 kR k K none of which

                            touches the image border

                            the complement of 1

                            ( )K

                            cu k u u

                            k

                            R R R R

                            We call all the points in Ru the foreground of the image and the points in ( )cuR the

                            background of the image

                            The boundary (border or contour) of a region R is the set of points that are adjacent to

                            points in the complement of R (R)c The border of an image is the set of pixels in the

                            region that have at least one background neighbor This definition is referred to as the

                            inner border to distinguish it from the notion of outer border which is the corresponding

                            border in the background

                            Digital Image Processing

                            Week 1

                            Distance measures

                            For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                            function or metric if

                            (a) D(p q) ge 0 D(p q) = 0 iff p=q

                            (b) D(p q) = D(q p)

                            (c) D(p z) le D(p q) + D(q z)

                            The Euclidean distance between p and q is defined as 1

                            2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                            The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                            centered at (x y)

                            Digital Image Processing

                            Week 1

                            The D4 distance (also called city-block distance) between p and q is defined as

                            4( ) | | | |D p q x s y t

                            The pixels q for which 4( )D p q r form a diamond centered at (xy)

                            4

                            22 1 2

                            2 2 1 0 1 22 1 2

                            2

                            D

                            The pixels with D4 = 1 are the 4-neighbors of (x y)

                            The D8 distance (called the chessboard distance) between p and q is defined as

                            8( ) max| | | |D p q x s y t

                            The pixels q for which 8( )D p q r form a square centered at (x y)

                            Digital Image Processing

                            Week 1

                            8

                            2 2 2 2 22 1 1 1 2

                            2 2 1 0 1 22 1 1 1 22 2 2 2 2

                            D

                            The pixels with D8 = 1 are the 8-neighbors of (x y)

                            D4 and D8 distances are independent of any paths that might exist between p and q

                            because these distances involve only the coordinates of the point

                            Digital Image Processing

                            Week 1

                            Array versus Matrix Operations

                            An array operation involving one or more images is carried out on a pixel-by-pixel basis

                            11 12 11 12

                            21 22 21 22

                            a a b ba a b b

                            Array product

                            11 12 11 12 11 11 12 12

                            21 22 21 22 21 21 22 21

                            a a b b a b a ba a b b a b a b

                            Matrix product

                            11 12 11 12 11 11 12 21 11 12 12 21

                            21 22 21 22 21 11 22 21 21 12 22 22

                            a a b b a b a b a b a ba a b b a b a b a b a b

                            We assume array operations unless stated otherwise

                            Digital Image Processing

                            Week 1

                            Linear versus Nonlinear Operations

                            One of the most important classifications of image-processing methods is whether it is

                            linear or nonlinear

                            ( ) ( )H f x y g x y

                            H is said to be a linear operator if

                            images1 2 1 2

                            1 2

                            ( ) ( ) ( ) ( )

                            H a f x y b f x y a H f x y b H f x y

                            a b f f

                            Example of nonlinear operator

                            the maximum value of the pixels of image max ( )H f f x y f

                            1 2

                            0 2 6 5 1 1

                            2 3 4 7f f a b

                            Digital Image Processing

                            Week 1

                            1 2

                            0 2 6 5 6 3max max 1 ( 1) max 2

                            2 3 4 7 2 4a f b f

                            0 2 6 51 max ( 1) max 3 ( 1)7 4

                            2 3 4 7

                            Arithmetic Operations in Image Processing

                            Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                            f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                            For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                            value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                            The two random variables are uncorrelated when their covariance is 0

                            Digital Image Processing

                            Week 1

                            Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                            used in image enhancement)

                            1

                            1( ) ( )K

                            ii

                            g x y g x yK

                            If the noise satisfies the properties stated above we have

                            2 2( ) ( )

                            1( ) ( ) g x y x yE g x y f x yK

                            ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                            and g respectively The standard deviation (square root of the variance) at any point in

                            the average image is

                            ( ) ( )1

                            g x y x yK

                            Digital Image Processing

                            Week 1

                            As K increases the variability (as measured by the variance or the standard deviation) of

                            the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                            means that ( )g x y approaches f(x y) as the number of noisy images used in the

                            averaging process increases

                            An important application of image averaging is in the field of astronomy where imaging

                            under very low light levels frequently causes sensor noise to render single images

                            virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                            was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                            64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                            images respectively

                            Digital Image Processing

                            Week 1

                            Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                            100 noisy images

                            a b c d e f

                            Digital Image Processing

                            Week 1

                            A frequent application of image subtraction is in the enhancement of differences between

                            images

                            (a) (b) (c)

                            Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                            significant bit of each pixel (c) the difference between the two images

                            Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                            Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                            images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                            difference between images (a) and (b)

                            Digital Image Processing

                            Week 1

                            Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                            h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                            intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                            source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                            bloodstream taking a series of images called live images (denoted f(x y)) of the same

                            anatomical region as h(x y) and subtracting the mask from the series of incoming live

                            images after injection of the contrast medium

                            In g(x y) we can find the differences between h and f as enhanced detail

                            Images being captured at TV rates we obtain a movie showing how the contrast medium

                            propagates through the various arteries in the area being observed

                            Digital Image Processing

                            Week 1

                            a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                            Digital Image Processing

                            Week 1

                            An important application of image multiplication (and division) is shading correction

                            Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                            f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                            When the shading function is known

                            ( )( )( )

                            g x yf x yh x y

                            h(x y) is unknown but we have access to the imaging system we can obtain an

                            approximation to the shading function by imaging a target of constant intensity When the

                            sensor is not available often the shading pattern can be estimated from the image

                            Digital Image Processing

                            Week 1

                            (a) (b) (c)

                            Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                            Digital Image Processing

                            Week 1

                            Another use of image multiplication is in masking also called region of interest (ROI)

                            operations The process consists of multiplying a given image by a mask image that has

                            1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                            image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                            (a) (b) (c)

                            Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                            Digital Image Processing

                            Week 1

                            In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                            min( )mf f f

                            0 ( 255)max( )

                            ms

                            m

                            ff K K K

                            f

                            Digital Image Processing

                            Week 1

                            Spatial Operations

                            - are performed directly on the pixels of a given image

                            There are three categories of spatial operations

                            single-pixel operations

                            neighborhood operations

                            geometric spatial transformations

                            Single-pixel operations

                            - change the values of intensity for the individual pixels ( )s T z

                            where z is the intensity of a pixel in the original image and s is the intensity of the

                            corresponding pixel in the processed image

                            Digital Image Processing

                            Week 1

                            Neighborhood operations

                            Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                            in an image f Neighborhood processing generates new intensity level at point (x y)

                            based on the values of the intensities of the points in Sxy For example if Sxy is a

                            rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                            intensity by computing the average value of the pixels in Sxy

                            ( )

                            1( ) ( )xyr c S

                            g x y f r cm n

                            The net effect is to perform local blurring in the original image This type of process is

                            used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                            largest region of an image

                            Digital Image Processing

                            Week 1

                            Geometric spatial transformations and image registration

                            - modify the spatial relationship between pixels in an image

                            - these transformations are often called rubber-sheet transformations (analogous to

                            printing an image on a sheet of rubber and then stretching the sheet according to a

                            predefined set of rules

                            A geometric transformation consists of 2 basic operations

                            1 a spatial transformation of coordinates

                            2 intensity interpolation that assign intensity values to the spatial transformed

                            pixels

                            The coordinate system transformation ( ) [( )]x y T v w

                            (v w) ndash pixel coordinates in the original image

                            (x y) ndash pixel coordinates in the transformed image

                            Digital Image Processing

                            Week 1

                            [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                            Affine transform

                            11 1211 21 31

                            21 2212 22 33

                            31 32

                            0[ 1] [ 1] [ 1] 0

                            1

                            t tx t v t w t

                            x y v w T v w t ty t v t w t

                            t t

                            (AT)

                            This transform can scale rotate translate or shear a set of coordinate points depending

                            on the elements of the matrix T If we want to resize an image rotate it and move the

                            result to some location we simply form a 3x3 matrix equal to the matrix product of the

                            scaling rotation and translation matrices from Table 1

                            Digital Image Processing

                            Week 1

                            Affine transformations

                            Digital Image Processing

                            Week 1

                            The preceding transformations relocate pixels on an image to new locations To complete

                            the process we have to assign intensity values to those locations This task is done by

                            using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                            In practice we can use equation (AT) in two basic ways

                            forward mapping scan the pixels of the input image (v w) compute the new spatial

                            location (x y) of the corresponding pixel in the new image using (AT) directly

                            Problems

                            - intensity assignment when 2 or more pixels in the original image are transformed to

                            the same location in the output image

                            - some output locations have no correspondent in the original image (no intensity

                            assignment)

                            Digital Image Processing

                            Week 1

                            inverse mapping scans the output pixel locations and at each location (x y)

                            computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                            It then interpolates among the nearest input pixels to determine the intensity of the output

                            pixel value

                            Inverse mappings are more efficient to implement than forward mappings and are used in

                            numerous commercial implementations of spatial transformations (MATLAB for ex)

                            Digital Image Processing

                            Week 1

                            Digital Image Processing

                            Week 1

                            Image registration ndash align two or more images of the same scene

                            In image registration we have available the input and output images but the specific

                            transformation that produced the output image from the input is generally unknown

                            The problem is to estimate the transformation function and then use it to register the two

                            images

                            - it may be of interest to align (register) two or more image taken at approximately the

                            same time but using different imaging systems (MRI scanner and a PET scanner)

                            - align images of a given location taken by the same instrument at different moments

                            of time (satellite images)

                            Solving the problem using tie points (also called control points) which are

                            corresponding points whose locations are known precisely in the input and reference

                            image

                            Digital Image Processing

                            Week 1

                            How to select tie points

                            - interactively selecting them

                            - use of algorithms that try to detect these points

                            - some imaging systems have physical artifacts (small metallic objects) embedded in

                            the imaging sensors These objects produce a set of known points (called reseau

                            marks) directly on all images captured by the system which can be used as guides

                            for establishing tie points

                            The problem of estimating the transformation is one of modeling Suppose we have a set

                            of 4 tie points both on the input image and the reference image A simple model based on

                            a bilinear approximation is given by

                            1 2 3 4

                            5 6 7 8

                            x c v c w c v w cy c v c w c v w c

                            (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                            Digital Image Processing

                            Week 1

                            When 4 tie points are insufficient to obtain satisfactory registration an approach used

                            frequently is to select a larger number of tie points and using this new set of tie points

                            subdivide the image in rectangular regions marked by groups of 4 tie points On the

                            subregions marked by 4 tie points we applied the transformation model described above

                            The number of tie points and the sophistication of the model required to solve the register

                            problem depend on the severity of the geometrical distortion

                            Digital Image Processing

                            Week 1

                            a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                            Digital Image Processing

                            Week 1

                            Probabilistic Methods

                            zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                            p(zk) = the probability that the intensity level zk occurs in the given image

                            ( ) kk

                            np zM N

                            nk = the number of times that intensity zk occurs in the image (MN is the total number of

                            pixels in the image) 1

                            0( ) 1

                            L

                            kk

                            p z

                            The mean (average) intensity of an image is given by 1

                            0( )

                            L

                            k kk

                            m z p z

                            Digital Image Processing

                            Week 1

                            The variance of the intensities is 1

                            2 2

                            0( ) ( )

                            L

                            k kk

                            z m p z

                            The variance is a measure of the spread of the values of z about the mean so it is a

                            measure of image contrast Usually for measuring image contrast the standard deviation

                            ( ) is used

                            The n-th moment of a random variable z about the mean is defined as 1

                            0( ) ( ) ( )

                            Ln

                            n k kk

                            z z m p z

                            ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                            3( ) 0z the intensities are biased to values higher than the mean

                            ( 3( ) 0z the intensities are biased to values lower than the mean

                            Digital Image Processing

                            Week 1

                            3( ) 0z the intensities are distributed approximately equally on both side of the

                            mean

                            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                            Figure 1(a) ndash standard deviation 143 (variance = 2045)

                            Figure 1(b) ndash standard deviation 316 (variance = 9986)

                            Figure 1(c) ndash standard deviation 492 (variance = 24206)

                            Digital Image Processing

                            Week 1

                            Intensity Transformations and Spatial Filtering

                            ( ) ( )g x y T f x y

                            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                            neighborhood of (x y)

                            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                            and much smaller in size than the image

                            Digital Image Processing

                            Week 1

                            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                            called spatial filter (spatial mask kernel template or window)

                            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                            ( )s T r

                            s and r are denoting respectively the intensity of g and f at (x y)

                            Figure 2 left - T produces an output image of higher contrast than the original by

                            darkening the intensity levels below k and brightening the levels above k ndash this technique

                            is called contrast stretching

                            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                            Digital Image Processing

                            Week 1

                            Figure 2 right - T produces a binary output image A mapping of this form is called

                            thresholding function

                            Some Basic Intensity Transformation Functions

                            Image Negatives

                            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                            - equivalent of a photographic negative

                            - technique suited for enhancing white or gray detail embedded in dark regions of an

                            image

                            Digital Image Processing

                            Week 1

                            Original Negative image

                            Digital Image Processing

                            Week 1

                            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                            Some basic intensity transformation functions

                            Digital Image Processing

                            Week 1

                            This transformation maps a narrow range of low intensity values in the input into a wider

                            range An operator of this type is used to expand the values of dark pixels in an image

                            while compressing the higher-level values The opposite is true for the inverse log

                            transformation The log functions compress the dynamic range of images with large

                            variations in pixel values

                            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                            Digital Image Processing

                            Week 1

                            Power-Law (Gamma) Transformations

                            - positive constants( ) ( ( ) )s T r c r c s c r

                            Plots of gamma transformation for different values of γ (c=1)

                            Digital Image Processing

                            Week 1

                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                            of output values with the opposite being true for higher values of input values The

                            curves with 1 have the opposite effect of those generated with values of 1

                            1c - identity transformation

                            A variety of devices used for image capture printing and display respond according to a

                            power law The process used to correct these power-law response phenomena is called

                            gamma correction

                            Digital Image Processing

                            Week 1

                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                            Digital Image Processing

                            Week 1

                            Piecewise-Linear Transformations Functions

                            Contrast stretching

                            - a process that expands the range of intensity levels in an image so it spans the full

                            intensity range of the recording tool or display device

                            a b c d Fig5

                            Digital Image Processing

                            Week 1

                            11

                            1

                            2 1 1 21 2

                            2 1 2 1

                            22

                            2

                            [0 ]

                            ( ) ( )( ) [ ]( ) ( )

                            ( 1 ) [ 1]( 1 )

                            s r r rrs r r s r rT r r r r

                            r r r rs L r r r L

                            L r

                            Digital Image Processing

                            Week 1

                            1 1 2 2r s r s identity transformation (no change)

                            1 2 1 2 0 1r r s s L thresholding function

                            Figure 5(b) shows an 8-bit image with low contrast

                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                            in the image respectively Thus the transformation function stretched the levels linearly

                            from their original range to the full range [0 L-1]

                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                            2 2 1r s m L where m is the mean gray level in the image

                            The original image on which these results are based is a scanning electron microscope

                            image of pollen magnified approximately 700 times

                            Digital Image Processing

                            Week 1

                            Intensity-level slicing

                            - highlighting a specific range of intensities in an image

                            There are two approaches for intensity-level slicing

                            1 display in one value (white for example) all the values in the range of interest and in

                            another (say black) all other intensities (Figure 311 (a))

                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                            intensities in the image (Figure 311 (b))

                            Digital Image Processing

                            Week 1

                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                            the top of the scale of intensities This type of enhancement produces a binary image

                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                            Highlights range [A B] and preserves all other intensities

                            Digital Image Processing

                            Week 1

                            which is useful for studying the shape of the flow of the contrast substance (to detect

                            blockageshellip)

                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                            image around the mean intensity was set to black the other intensities remain unchanged

                            Fig 6 - Aortic angiogram and intensity sliced versions

                            Digital Image Processing

                            Week 1

                            Bit-plane slicing

                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                            This technique highlights the contribution made to the whole image appearances by each

                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                            Digital Image Processing

                            Week 1

                            Digital Image Processing

                            Week 1

                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                            • DIP 1 2017
                            • DIP 02 (2017)

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              1960-1970 ndash image processing techniques were used in medical image remote Earth resources observations astronomy

                              1970s ndash invention of CAT (computerized axial tomography)

                              CAT is a process in which a ring of detectors encircles an object (patient) and a X-ray source concentric with the detector ring rotates about the object The X-ray passes through the patient and the resulted waves are collected at the opposite end by the detectors As the source rotates the procedure is repeated Tomography consists of algorithms that use the sense data to construct an image that represent a ldquoslicerdquo through the object Motion of the object in a direction perpendicular to the ring of detectors produces a set of ldquoslicesrdquo which can be assembled in a 3D information of the inside of the object

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              loz geographers use DIP to study pollution patterns from aerial and satellite imagery

                              loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

                              loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

                              loz astronomy biology nuclear medicine law enforcement industry

                              DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

                              loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Examples of Fields that Use DIP

                              Images can be classified according to their sources (visual X-ray hellip)

                              Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Electromagnetic waves can be thought as propagating sinusoidal

                              waves of different wavelength or as a stream of massless particles

                              each moving in a wavelike pattern with the speed of light Each

                              massless particle contains a certain amount (bundle) of energy Each

                              bundle of energy is called a photon If spectral bands are grouped

                              according to energy per photon we obtain the spectrum shown in the

                              image above ranging from gamma-rays (highest energy) to radio

                              waves (lowest energy)

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Gamma-Ray Imaging

                              Nuclear medicine astronomical observations

                              Nuclear medicine

                              the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

                              Images are produced from the emissions collected by gamma-ray detectors

                              Images of this sort are used to locate sites of bone pathology (infections tumors)

                              PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Examples of gamma-ray imaging

                              Bone scan PET image

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              X-ray imaging

                              Medical diagnosticindustry astronomy

                              A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                              The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Angiography = contrast-enhancement radiography

                              Angiograms = images of blood vessels

                              A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                              X-rays are used in CAT (computerized axial tomography)

                              X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                              Industrial CAT scans are useful when the parts can be penetreted by X-rays

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Examples of X-ray imaging

                              Chest X-rayAortic angiogram

                              Head CT Cygnus LoopCircuit boards

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Imaging in the Ultraviolet Band

                              Litography industrial inspection microscopy biological imaging astronomical observations

                              Ultraviolet light is used in fluorescence microscopy

                              Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                              other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                              and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Imaging in the Visible and Infrared Bands

                              Light microscopy astronomy remote sensing industry law enforcement

                              LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                              Weather observations and prediction produce major applications of multispectral image from satellites

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Satellite images of Washington DC area in spectral bands of the Table 1

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Examples of light microscopy

                              Taxol (anticancer agent)magnified 250X

                              Cholesterol(40X)

                              Microprocessor(60X)

                              Nickel oxidethin film(600X)

                              Surface of audio CD(1750X)

                              Organicsuperconductor(450X)

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Automated visual inspection of manufactured goods

                              a bc de f

                              a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Imaging in the Microwave Band

                              The dominant aplication of imaging in the microwave band ndash radar

                              bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                              bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                              bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                              An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Spaceborne radar image of mountains in southeast Tibet

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Imaging in the Radio Band

                              medicine astronomy

                              MRI = Magnetic Resonance Imaging

                              This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                              Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                              The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              MRI images of a human knee (left) and spine (right)

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Images of the Crab Pulsar covering the electromagnetic spectrum

                              Gamma X-ray Optical Infrared Radio

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Other Imaging Modalities

                              acoustic imaging electron microscopy synthetic (computer-generated) imaging

                              Imaging using sound geological explorations industry medicine

                              Mineral and oil exploration

                              For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Biometry - iris

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Biometry - fingerprint

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Face detection and recognition

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Gender identification

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Image morphing

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Fundamental Steps in DIP

                              methods whose input and output are images

                              methods whose inputs are images but whose outputs are attributes extracted from those images

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Outputs are images

                              bull image acquisition

                              bull image filtering and enhancement

                              bull image restoration

                              bull color image processing

                              bull wavelets and multiresolution processing

                              bull compression

                              bull morphological processing

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Outputs are attributes

                              bull morphological processing

                              bull segmentation

                              bull representation and description

                              bull object recognition

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Image acquisition - may involve preprocessing such as scaling

                              Image enhancement

                              bull manipulating an image so that the result is more suitable than the original for a specific operation

                              bull enhancement is problem oriented

                              bull there is no general sbquotheoryrsquo of image enhancement

                              bull enhancement use subjective methods for image emprovement

                              bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Image restoration

                              bull improving the appearance of an image

                              bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                              Color image processing

                              bull fundamental concept in color models

                              bull basic color processing in a digital domain

                              Wavelets and multiresolution processing

                              representing images in various degree of resolution

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Compression

                              reducing the storage required to save an image or the bandwidth required to transmit it

                              Morphological processing

                              bull tools for extracting image components that are useful in the representation and description of shape

                              bull a transition from processes that output images to processes that outputimage attributes

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Segmentation

                              bull partitioning an image into its constituents parts or objects

                              bull autonomous segmentation is one of the most difficult tasks of DIP

                              bull the more accurate the segmentation the more likley recognition is to succeed

                              Representation and description (almost always follows segmentation)

                              bull segmentation produces either the boundary of a region or all the poits in the region itself

                              bull converting the data produced by segmentation to a form suitable for computer processing

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              bull boundary representation the focus is on external shape characteristics such as corners or inflections

                              bull complete region the focus is on internal properties such as texture or skeletal shape

                              bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                              Object recognition

                              the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                              Knowledge database

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Simplified diagramof a cross sectionof the human eye

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                              The cornea is a tough transparent tissue that covers the anterior surface of the eye

                              Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                              The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                              The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                              Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                              Fovea = the place where the image of the object of interest falls on

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                              Blind spot region without receptors

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Image formation in the eye

                              Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                              Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                              distance between lens and retina along visual axix = 17 mm

                              range of focal length = 14 mm to 17 mm

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Optical illusions

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                              gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                              quantities that describe the quality of a chromatic light source radiance

                              the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                              measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                              brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              the physical meaning is determined by the source of the image

                              ( )f D f x y

                              Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                              f(xy) ndash characterized by two components

                              i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                              r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                              ( ) ( ) ( )

                              0 ( ) 0 ( ) 1

                              f x y i x y r x y

                              i x y r x y

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              r(xy)=0 - total absorption r(xy)=1 - total reflectance

                              i(xy) ndash determined by the illumination source

                              r(xy) ndash determined by the characteristics of the imaged objects

                              is called gray (or intensity) scale

                              In practice

                              min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                              indoor values without additional illuminationmin max10 1000L L

                              black whitemin max0 1 0 1 0 1L L L L l l L

                              min maxL L

                              Digital Image ProcessingDigital Image Processing

                              Week 1Week 1

                              Digital Image Processing

                              Week 1

                              Image Sampling and Quantization

                              - the output of the sensors is a continuous voltage waveform related to the sensed

                              scene

                              converting a continuous image f to digital form

                              - digitizing (x y) is called sampling

                              - digitizing f(x y) is called quantization

                              Digital Image Processing

                              Week 1

                              Digital Image Processing

                              Week 1

                              Continuous image projected onto a sensor array Result of image sampling and quantization

                              Digital Image Processing

                              Week 1

                              Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                              (00) (01) (0 1)(10) (11) (1 1)

                              ( )

                              ( 10) ( 11) ( 1 1)

                              f f f Nf f f N

                              f x y

                              f M f M f M N

                              image element pixel

                              00 01 0 1

                              10 11 1 1

                              10 11 1 1

                              ( ) ( )

                              N

                              i jN M N

                              i j

                              M M M N

                              a a aa f x i y j f i ja a a

                              Aa

                              a a a

                              f(00) ndash the upper left corner of the image

                              Digital Image Processing

                              Week 1

                              M N ge 0 L=2k

                              [0 1]i j i ja a L

                              Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                              Digital Image Processing

                              Week 1

                              Digital Image Processing

                              Week 1

                              Number of bits required to store a digitized image

                              for 2 b M N k M N b N k

                              When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                              Digital Image Processing

                              Week 1

                              Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                              Measures line pairs per unit distance dots (pixels) per unit distance

                              Image resolution = the largest number of discernible line pairs per unit distance

                              (eg 100 line pairs per mm)

                              Dots per unit distance are commonly used in printing and publishing

                              In US the measure is expressed in dots per inch (dpi)

                              (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                              Intensity resolution ndash the smallest discernible change in intensity level

                              The number of intensity levels (L) is determined by hardware considerations

                              L=2k ndash most common k = 8

                              Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                              Digital Image Processing

                              Week 1

                              Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                              150 dpi (lower left) 72 dpi (lower right)

                              Digital Image Processing

                              Week 1

                              Reducing the number of gray levels 256 128 64 32

                              Digital Image Processing

                              Week 1

                              Reducing the number of gray levels 16 8 4 2

                              Digital Image Processing

                              Week 1

                              Image Interpolation - used in zooming shrinking rotating and geometric corrections

                              Shrinking zooming ndash image resizing ndash image resampling methods

                              Interpolation is the process of using known data to estimate values at unknown locations

                              Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                              750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                              same spacing as the original and then shrink it so that it fits exactly over the original

                              image The pixel spacing in the 750 times 750 grid will be less than in the original image

                              Problem assignment of intensity-level in the new 750 times 750 grid

                              Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                              intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                              This technique has the tendency to produce undesirable effects like severe distortion of

                              straight edges

                              Digital Image Processing

                              Week 1

                              Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                              where the four coefficients are determined from the 4 equations in 4 unknowns that can

                              be written using the 4 nearest neighbors of point (x y)

                              Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                              modest increase in computational effort

                              Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                              nearest neighbors of the point 3 3

                              0 0

                              ( ) i ji j

                              i jv x y c x y

                              The coefficients cij are obtained solving a 16x16 linear system

                              intensity levels of the 16 nearest neighbors of 3 3

                              0 0

                              ( )i ji j

                              i jc x y x y

                              Digital Image Processing

                              Week 1

                              Generally bicubic interpolation does a better job of preserving fine detail than the

                              bilinear technique Bicubic interpolation is the standard used in commercial image editing

                              programs such as Adobe Photoshop and Corel Photopaint

                              Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                              the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                              then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                              neighbor interpolation was used (both for shrinking and zooming)

                              Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                              interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                              from 1250 dpi to 150 dpi (instead of 72 dpi)

                              Digital Image Processing

                              Week 1

                              Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                              Digital Image Processing

                              Week 1

                              Neighbors of a Pixel

                              A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                              This set of pixels called the 4-neighbors of p denoted by N4 (p)

                              The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                              and are denoted ND(p)

                              The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                              N8 (p)

                              If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                              fall outside the image

                              Digital Image Processing

                              Week 1

                              Adjacency Connectivity Regions Boundaries

                              Denote by V the set of intensity levels used to define adjacency

                              - in a binary image V 01 (V=0 V=1)

                              - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                              We consider 3 types of adjacency

                              (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                              (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                              (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                              m-adjacent if

                              4( )q N p or

                              ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                              Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                              ambiguities that often arise when 8-adjacency is used Consider the example

                              Digital Image Processing

                              Week 1

                              binary image

                              0 1 1 0 1 1 0 1 1

                              1 0 1 0 0 1 0 0 1 0

                              0 0 1 0 0 1 0 0 1

                              V

                              The three pixels at the top (first line) in the above example show multiple (ambiguous)

                              8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                              m-adjacency

                              Digital Image Processing

                              Week 1

                              A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                              is a sequence of distinct pixels with coordinates

                              and are adjacent 0 0 1 1

                              1 1

                              ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                              n n

                              i i i i

                              x y x y x y x y s tx y x y i n

                              The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                              Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                              Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                              in S if there exists a path between them consisting only of pixels from S

                              S is a connected set if there is a path in S between any 2 pixels in S

                              Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                              Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                              that are not adjacent are said to be disjoint When referring to regions only 4- and

                              8-adjacency are considered

                              Digital Image Processing

                              Week 1

                              Suppose that an image contains K disjoint regions 1 kR k K none of which

                              touches the image border

                              the complement of 1

                              ( )K

                              cu k u u

                              k

                              R R R R

                              We call all the points in Ru the foreground of the image and the points in ( )cuR the

                              background of the image

                              The boundary (border or contour) of a region R is the set of points that are adjacent to

                              points in the complement of R (R)c The border of an image is the set of pixels in the

                              region that have at least one background neighbor This definition is referred to as the

                              inner border to distinguish it from the notion of outer border which is the corresponding

                              border in the background

                              Digital Image Processing

                              Week 1

                              Distance measures

                              For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                              function or metric if

                              (a) D(p q) ge 0 D(p q) = 0 iff p=q

                              (b) D(p q) = D(q p)

                              (c) D(p z) le D(p q) + D(q z)

                              The Euclidean distance between p and q is defined as 1

                              2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                              The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                              centered at (x y)

                              Digital Image Processing

                              Week 1

                              The D4 distance (also called city-block distance) between p and q is defined as

                              4( ) | | | |D p q x s y t

                              The pixels q for which 4( )D p q r form a diamond centered at (xy)

                              4

                              22 1 2

                              2 2 1 0 1 22 1 2

                              2

                              D

                              The pixels with D4 = 1 are the 4-neighbors of (x y)

                              The D8 distance (called the chessboard distance) between p and q is defined as

                              8( ) max| | | |D p q x s y t

                              The pixels q for which 8( )D p q r form a square centered at (x y)

                              Digital Image Processing

                              Week 1

                              8

                              2 2 2 2 22 1 1 1 2

                              2 2 1 0 1 22 1 1 1 22 2 2 2 2

                              D

                              The pixels with D8 = 1 are the 8-neighbors of (x y)

                              D4 and D8 distances are independent of any paths that might exist between p and q

                              because these distances involve only the coordinates of the point

                              Digital Image Processing

                              Week 1

                              Array versus Matrix Operations

                              An array operation involving one or more images is carried out on a pixel-by-pixel basis

                              11 12 11 12

                              21 22 21 22

                              a a b ba a b b

                              Array product

                              11 12 11 12 11 11 12 12

                              21 22 21 22 21 21 22 21

                              a a b b a b a ba a b b a b a b

                              Matrix product

                              11 12 11 12 11 11 12 21 11 12 12 21

                              21 22 21 22 21 11 22 21 21 12 22 22

                              a a b b a b a b a b a ba a b b a b a b a b a b

                              We assume array operations unless stated otherwise

                              Digital Image Processing

                              Week 1

                              Linear versus Nonlinear Operations

                              One of the most important classifications of image-processing methods is whether it is

                              linear or nonlinear

                              ( ) ( )H f x y g x y

                              H is said to be a linear operator if

                              images1 2 1 2

                              1 2

                              ( ) ( ) ( ) ( )

                              H a f x y b f x y a H f x y b H f x y

                              a b f f

                              Example of nonlinear operator

                              the maximum value of the pixels of image max ( )H f f x y f

                              1 2

                              0 2 6 5 1 1

                              2 3 4 7f f a b

                              Digital Image Processing

                              Week 1

                              1 2

                              0 2 6 5 6 3max max 1 ( 1) max 2

                              2 3 4 7 2 4a f b f

                              0 2 6 51 max ( 1) max 3 ( 1)7 4

                              2 3 4 7

                              Arithmetic Operations in Image Processing

                              Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                              f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                              For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                              value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                              The two random variables are uncorrelated when their covariance is 0

                              Digital Image Processing

                              Week 1

                              Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                              used in image enhancement)

                              1

                              1( ) ( )K

                              ii

                              g x y g x yK

                              If the noise satisfies the properties stated above we have

                              2 2( ) ( )

                              1( ) ( ) g x y x yE g x y f x yK

                              ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                              and g respectively The standard deviation (square root of the variance) at any point in

                              the average image is

                              ( ) ( )1

                              g x y x yK

                              Digital Image Processing

                              Week 1

                              As K increases the variability (as measured by the variance or the standard deviation) of

                              the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                              means that ( )g x y approaches f(x y) as the number of noisy images used in the

                              averaging process increases

                              An important application of image averaging is in the field of astronomy where imaging

                              under very low light levels frequently causes sensor noise to render single images

                              virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                              was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                              64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                              images respectively

                              Digital Image Processing

                              Week 1

                              Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                              100 noisy images

                              a b c d e f

                              Digital Image Processing

                              Week 1

                              A frequent application of image subtraction is in the enhancement of differences between

                              images

                              (a) (b) (c)

                              Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                              significant bit of each pixel (c) the difference between the two images

                              Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                              Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                              images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                              difference between images (a) and (b)

                              Digital Image Processing

                              Week 1

                              Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                              h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                              intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                              source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                              bloodstream taking a series of images called live images (denoted f(x y)) of the same

                              anatomical region as h(x y) and subtracting the mask from the series of incoming live

                              images after injection of the contrast medium

                              In g(x y) we can find the differences between h and f as enhanced detail

                              Images being captured at TV rates we obtain a movie showing how the contrast medium

                              propagates through the various arteries in the area being observed

                              Digital Image Processing

                              Week 1

                              a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                              Digital Image Processing

                              Week 1

                              An important application of image multiplication (and division) is shading correction

                              Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                              f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                              When the shading function is known

                              ( )( )( )

                              g x yf x yh x y

                              h(x y) is unknown but we have access to the imaging system we can obtain an

                              approximation to the shading function by imaging a target of constant intensity When the

                              sensor is not available often the shading pattern can be estimated from the image

                              Digital Image Processing

                              Week 1

                              (a) (b) (c)

                              Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                              Digital Image Processing

                              Week 1

                              Another use of image multiplication is in masking also called region of interest (ROI)

                              operations The process consists of multiplying a given image by a mask image that has

                              1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                              image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                              (a) (b) (c)

                              Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                              Digital Image Processing

                              Week 1

                              In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                              min( )mf f f

                              0 ( 255)max( )

                              ms

                              m

                              ff K K K

                              f

                              Digital Image Processing

                              Week 1

                              Spatial Operations

                              - are performed directly on the pixels of a given image

                              There are three categories of spatial operations

                              single-pixel operations

                              neighborhood operations

                              geometric spatial transformations

                              Single-pixel operations

                              - change the values of intensity for the individual pixels ( )s T z

                              where z is the intensity of a pixel in the original image and s is the intensity of the

                              corresponding pixel in the processed image

                              Digital Image Processing

                              Week 1

                              Neighborhood operations

                              Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                              in an image f Neighborhood processing generates new intensity level at point (x y)

                              based on the values of the intensities of the points in Sxy For example if Sxy is a

                              rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                              intensity by computing the average value of the pixels in Sxy

                              ( )

                              1( ) ( )xyr c S

                              g x y f r cm n

                              The net effect is to perform local blurring in the original image This type of process is

                              used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                              largest region of an image

                              Digital Image Processing

                              Week 1

                              Geometric spatial transformations and image registration

                              - modify the spatial relationship between pixels in an image

                              - these transformations are often called rubber-sheet transformations (analogous to

                              printing an image on a sheet of rubber and then stretching the sheet according to a

                              predefined set of rules

                              A geometric transformation consists of 2 basic operations

                              1 a spatial transformation of coordinates

                              2 intensity interpolation that assign intensity values to the spatial transformed

                              pixels

                              The coordinate system transformation ( ) [( )]x y T v w

                              (v w) ndash pixel coordinates in the original image

                              (x y) ndash pixel coordinates in the transformed image

                              Digital Image Processing

                              Week 1

                              [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                              Affine transform

                              11 1211 21 31

                              21 2212 22 33

                              31 32

                              0[ 1] [ 1] [ 1] 0

                              1

                              t tx t v t w t

                              x y v w T v w t ty t v t w t

                              t t

                              (AT)

                              This transform can scale rotate translate or shear a set of coordinate points depending

                              on the elements of the matrix T If we want to resize an image rotate it and move the

                              result to some location we simply form a 3x3 matrix equal to the matrix product of the

                              scaling rotation and translation matrices from Table 1

                              Digital Image Processing

                              Week 1

                              Affine transformations

                              Digital Image Processing

                              Week 1

                              The preceding transformations relocate pixels on an image to new locations To complete

                              the process we have to assign intensity values to those locations This task is done by

                              using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                              In practice we can use equation (AT) in two basic ways

                              forward mapping scan the pixels of the input image (v w) compute the new spatial

                              location (x y) of the corresponding pixel in the new image using (AT) directly

                              Problems

                              - intensity assignment when 2 or more pixels in the original image are transformed to

                              the same location in the output image

                              - some output locations have no correspondent in the original image (no intensity

                              assignment)

                              Digital Image Processing

                              Week 1

                              inverse mapping scans the output pixel locations and at each location (x y)

                              computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                              It then interpolates among the nearest input pixels to determine the intensity of the output

                              pixel value

                              Inverse mappings are more efficient to implement than forward mappings and are used in

                              numerous commercial implementations of spatial transformations (MATLAB for ex)

                              Digital Image Processing

                              Week 1

                              Digital Image Processing

                              Week 1

                              Image registration ndash align two or more images of the same scene

                              In image registration we have available the input and output images but the specific

                              transformation that produced the output image from the input is generally unknown

                              The problem is to estimate the transformation function and then use it to register the two

                              images

                              - it may be of interest to align (register) two or more image taken at approximately the

                              same time but using different imaging systems (MRI scanner and a PET scanner)

                              - align images of a given location taken by the same instrument at different moments

                              of time (satellite images)

                              Solving the problem using tie points (also called control points) which are

                              corresponding points whose locations are known precisely in the input and reference

                              image

                              Digital Image Processing

                              Week 1

                              How to select tie points

                              - interactively selecting them

                              - use of algorithms that try to detect these points

                              - some imaging systems have physical artifacts (small metallic objects) embedded in

                              the imaging sensors These objects produce a set of known points (called reseau

                              marks) directly on all images captured by the system which can be used as guides

                              for establishing tie points

                              The problem of estimating the transformation is one of modeling Suppose we have a set

                              of 4 tie points both on the input image and the reference image A simple model based on

                              a bilinear approximation is given by

                              1 2 3 4

                              5 6 7 8

                              x c v c w c v w cy c v c w c v w c

                              (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                              Digital Image Processing

                              Week 1

                              When 4 tie points are insufficient to obtain satisfactory registration an approach used

                              frequently is to select a larger number of tie points and using this new set of tie points

                              subdivide the image in rectangular regions marked by groups of 4 tie points On the

                              subregions marked by 4 tie points we applied the transformation model described above

                              The number of tie points and the sophistication of the model required to solve the register

                              problem depend on the severity of the geometrical distortion

                              Digital Image Processing

                              Week 1

                              a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                              Digital Image Processing

                              Week 1

                              Probabilistic Methods

                              zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                              p(zk) = the probability that the intensity level zk occurs in the given image

                              ( ) kk

                              np zM N

                              nk = the number of times that intensity zk occurs in the image (MN is the total number of

                              pixels in the image) 1

                              0( ) 1

                              L

                              kk

                              p z

                              The mean (average) intensity of an image is given by 1

                              0( )

                              L

                              k kk

                              m z p z

                              Digital Image Processing

                              Week 1

                              The variance of the intensities is 1

                              2 2

                              0( ) ( )

                              L

                              k kk

                              z m p z

                              The variance is a measure of the spread of the values of z about the mean so it is a

                              measure of image contrast Usually for measuring image contrast the standard deviation

                              ( ) is used

                              The n-th moment of a random variable z about the mean is defined as 1

                              0( ) ( ) ( )

                              Ln

                              n k kk

                              z z m p z

                              ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                              3( ) 0z the intensities are biased to values higher than the mean

                              ( 3( ) 0z the intensities are biased to values lower than the mean

                              Digital Image Processing

                              Week 1

                              3( ) 0z the intensities are distributed approximately equally on both side of the

                              mean

                              Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                              Figure 1(a) ndash standard deviation 143 (variance = 2045)

                              Figure 1(b) ndash standard deviation 316 (variance = 9986)

                              Figure 1(c) ndash standard deviation 492 (variance = 24206)

                              Digital Image Processing

                              Week 1

                              Intensity Transformations and Spatial Filtering

                              ( ) ( )g x y T f x y

                              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                              neighborhood of (x y)

                              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                              and much smaller in size than the image

                              Digital Image Processing

                              Week 1

                              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                              called spatial filter (spatial mask kernel template or window)

                              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                              ( )s T r

                              s and r are denoting respectively the intensity of g and f at (x y)

                              Figure 2 left - T produces an output image of higher contrast than the original by

                              darkening the intensity levels below k and brightening the levels above k ndash this technique

                              is called contrast stretching

                              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                              Digital Image Processing

                              Week 1

                              Figure 2 right - T produces a binary output image A mapping of this form is called

                              thresholding function

                              Some Basic Intensity Transformation Functions

                              Image Negatives

                              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                              - equivalent of a photographic negative

                              - technique suited for enhancing white or gray detail embedded in dark regions of an

                              image

                              Digital Image Processing

                              Week 1

                              Original Negative image

                              Digital Image Processing

                              Week 1

                              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                              Some basic intensity transformation functions

                              Digital Image Processing

                              Week 1

                              This transformation maps a narrow range of low intensity values in the input into a wider

                              range An operator of this type is used to expand the values of dark pixels in an image

                              while compressing the higher-level values The opposite is true for the inverse log

                              transformation The log functions compress the dynamic range of images with large

                              variations in pixel values

                              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                              Digital Image Processing

                              Week 1

                              Power-Law (Gamma) Transformations

                              - positive constants( ) ( ( ) )s T r c r c s c r

                              Plots of gamma transformation for different values of γ (c=1)

                              Digital Image Processing

                              Week 1

                              Power-law curves with 1 map a narrow range of dark input values into a wider range

                              of output values with the opposite being true for higher values of input values The

                              curves with 1 have the opposite effect of those generated with values of 1

                              1c - identity transformation

                              A variety of devices used for image capture printing and display respond according to a

                              power law The process used to correct these power-law response phenomena is called

                              gamma correction

                              Digital Image Processing

                              Week 1

                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                              Digital Image Processing

                              Week 1

                              Piecewise-Linear Transformations Functions

                              Contrast stretching

                              - a process that expands the range of intensity levels in an image so it spans the full

                              intensity range of the recording tool or display device

                              a b c d Fig5

                              Digital Image Processing

                              Week 1

                              11

                              1

                              2 1 1 21 2

                              2 1 2 1

                              22

                              2

                              [0 ]

                              ( ) ( )( ) [ ]( ) ( )

                              ( 1 ) [ 1]( 1 )

                              s r r rrs r r s r rT r r r r

                              r r r rs L r r r L

                              L r

                              Digital Image Processing

                              Week 1

                              1 1 2 2r s r s identity transformation (no change)

                              1 2 1 2 0 1r r s s L thresholding function

                              Figure 5(b) shows an 8-bit image with low contrast

                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                              in the image respectively Thus the transformation function stretched the levels linearly

                              from their original range to the full range [0 L-1]

                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                              2 2 1r s m L where m is the mean gray level in the image

                              The original image on which these results are based is a scanning electron microscope

                              image of pollen magnified approximately 700 times

                              Digital Image Processing

                              Week 1

                              Intensity-level slicing

                              - highlighting a specific range of intensities in an image

                              There are two approaches for intensity-level slicing

                              1 display in one value (white for example) all the values in the range of interest and in

                              another (say black) all other intensities (Figure 311 (a))

                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                              intensities in the image (Figure 311 (b))

                              Digital Image Processing

                              Week 1

                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                              the top of the scale of intensities This type of enhancement produces a binary image

                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                              Highlights range [A B] and preserves all other intensities

                              Digital Image Processing

                              Week 1

                              which is useful for studying the shape of the flow of the contrast substance (to detect

                              blockageshellip)

                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                              image around the mean intensity was set to black the other intensities remain unchanged

                              Fig 6 - Aortic angiogram and intensity sliced versions

                              Digital Image Processing

                              Week 1

                              Bit-plane slicing

                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                              This technique highlights the contribution made to the whole image appearances by each

                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                              Digital Image Processing

                              Week 1

                              Digital Image Processing

                              Week 1

                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                              • DIP 1 2017
                              • DIP 02 (2017)

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                loz geographers use DIP to study pollution patterns from aerial and satellite imagery

                                loz archeology ndash DIP allowed restoring blurred pictures that recorded rare artifacts lost or damaged after being photographed

                                loz physics ndash enhance images of experiments (high-energy plasmas electron microscopy)

                                loz astronomy biology nuclear medicine law enforcement industry

                                DIP ndash used in solving problems dealing with machine perception ndashextracting from an image information suitable for computer processing (statistical moments Fourier transform coefficients hellip)

                                loz automatic character recognition industrial machine vision for product assembly and inspection military recognizance automatic processing of fingerprints machine processing of aerial and satellite imagery for weather prediction Internet

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Examples of Fields that Use DIP

                                Images can be classified according to their sources (visual X-ray hellip)

                                Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Electromagnetic waves can be thought as propagating sinusoidal

                                waves of different wavelength or as a stream of massless particles

                                each moving in a wavelike pattern with the speed of light Each

                                massless particle contains a certain amount (bundle) of energy Each

                                bundle of energy is called a photon If spectral bands are grouped

                                according to energy per photon we obtain the spectrum shown in the

                                image above ranging from gamma-rays (highest energy) to radio

                                waves (lowest energy)

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Gamma-Ray Imaging

                                Nuclear medicine astronomical observations

                                Nuclear medicine

                                the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

                                Images are produced from the emissions collected by gamma-ray detectors

                                Images of this sort are used to locate sites of bone pathology (infections tumors)

                                PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Examples of gamma-ray imaging

                                Bone scan PET image

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                X-ray imaging

                                Medical diagnosticindustry astronomy

                                A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                                The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Angiography = contrast-enhancement radiography

                                Angiograms = images of blood vessels

                                A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                                X-rays are used in CAT (computerized axial tomography)

                                X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                                Industrial CAT scans are useful when the parts can be penetreted by X-rays

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Examples of X-ray imaging

                                Chest X-rayAortic angiogram

                                Head CT Cygnus LoopCircuit boards

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Imaging in the Ultraviolet Band

                                Litography industrial inspection microscopy biological imaging astronomical observations

                                Ultraviolet light is used in fluorescence microscopy

                                Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                                other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                                and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Imaging in the Visible and Infrared Bands

                                Light microscopy astronomy remote sensing industry law enforcement

                                LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                                Weather observations and prediction produce major applications of multispectral image from satellites

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Satellite images of Washington DC area in spectral bands of the Table 1

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Examples of light microscopy

                                Taxol (anticancer agent)magnified 250X

                                Cholesterol(40X)

                                Microprocessor(60X)

                                Nickel oxidethin film(600X)

                                Surface of audio CD(1750X)

                                Organicsuperconductor(450X)

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Automated visual inspection of manufactured goods

                                a bc de f

                                a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

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                                Imaging in the Microwave Band

                                The dominant aplication of imaging in the microwave band ndash radar

                                bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                Digital Image ProcessingDigital Image Processing

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                                Spaceborne radar image of mountains in southeast Tibet

                                Digital Image ProcessingDigital Image Processing

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                                Imaging in the Radio Band

                                medicine astronomy

                                MRI = Magnetic Resonance Imaging

                                This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                MRI images of a human knee (left) and spine (right)

                                Digital Image ProcessingDigital Image Processing

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                                Images of the Crab Pulsar covering the electromagnetic spectrum

                                Gamma X-ray Optical Infrared Radio

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Other Imaging Modalities

                                acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                Imaging using sound geological explorations industry medicine

                                Mineral and oil exploration

                                For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                Digital Image ProcessingDigital Image Processing

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                                Biometry - iris

                                Digital Image ProcessingDigital Image Processing

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                                Biometry - fingerprint

                                Digital Image ProcessingDigital Image Processing

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                                Face detection and recognition

                                Digital Image ProcessingDigital Image Processing

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                                Gender identification

                                Digital Image ProcessingDigital Image Processing

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                                Image morphing

                                Digital Image ProcessingDigital Image Processing

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                                Fundamental Steps in DIP

                                methods whose input and output are images

                                methods whose inputs are images but whose outputs are attributes extracted from those images

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Outputs are images

                                bull image acquisition

                                bull image filtering and enhancement

                                bull image restoration

                                bull color image processing

                                bull wavelets and multiresolution processing

                                bull compression

                                bull morphological processing

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Outputs are attributes

                                bull morphological processing

                                bull segmentation

                                bull representation and description

                                bull object recognition

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                                Image acquisition - may involve preprocessing such as scaling

                                Image enhancement

                                bull manipulating an image so that the result is more suitable than the original for a specific operation

                                bull enhancement is problem oriented

                                bull there is no general sbquotheoryrsquo of image enhancement

                                bull enhancement use subjective methods for image emprovement

                                bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Image restoration

                                bull improving the appearance of an image

                                bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                Color image processing

                                bull fundamental concept in color models

                                bull basic color processing in a digital domain

                                Wavelets and multiresolution processing

                                representing images in various degree of resolution

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Compression

                                reducing the storage required to save an image or the bandwidth required to transmit it

                                Morphological processing

                                bull tools for extracting image components that are useful in the representation and description of shape

                                bull a transition from processes that output images to processes that outputimage attributes

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                                Week 1Week 1

                                Segmentation

                                bull partitioning an image into its constituents parts or objects

                                bull autonomous segmentation is one of the most difficult tasks of DIP

                                bull the more accurate the segmentation the more likley recognition is to succeed

                                Representation and description (almost always follows segmentation)

                                bull segmentation produces either the boundary of a region or all the poits in the region itself

                                bull converting the data produced by segmentation to a form suitable for computer processing

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                bull complete region the focus is on internal properties such as texture or skeletal shape

                                bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                Object recognition

                                the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                Knowledge database

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                                Week 1Week 1

                                Simplified diagramof a cross sectionof the human eye

                                Digital Image ProcessingDigital Image Processing

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                                Week 1Week 1

                                Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                Fovea = the place where the image of the object of interest falls on

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                Blind spot region without receptors

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Image formation in the eye

                                Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                distance between lens and retina along visual axix = 17 mm

                                range of focal length = 14 mm to 17 mm

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Digital Image ProcessingDigital Image Processing

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                                Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Optical illusions

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                quantities that describe the quality of a chromatic light source radiance

                                the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                the physical meaning is determined by the source of the image

                                ( )f D f x y

                                Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                f(xy) ndash characterized by two components

                                i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                ( ) ( ) ( )

                                0 ( ) 0 ( ) 1

                                f x y i x y r x y

                                i x y r x y

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                i(xy) ndash determined by the illumination source

                                r(xy) ndash determined by the characteristics of the imaged objects

                                is called gray (or intensity) scale

                                In practice

                                min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                indoor values without additional illuminationmin max10 1000L L

                                black whitemin max0 1 0 1 0 1L L L L l l L

                                min maxL L

                                Digital Image ProcessingDigital Image Processing

                                Week 1Week 1

                                Digital Image Processing

                                Week 1

                                Image Sampling and Quantization

                                - the output of the sensors is a continuous voltage waveform related to the sensed

                                scene

                                converting a continuous image f to digital form

                                - digitizing (x y) is called sampling

                                - digitizing f(x y) is called quantization

                                Digital Image Processing

                                Week 1

                                Digital Image Processing

                                Week 1

                                Continuous image projected onto a sensor array Result of image sampling and quantization

                                Digital Image Processing

                                Week 1

                                Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                (00) (01) (0 1)(10) (11) (1 1)

                                ( )

                                ( 10) ( 11) ( 1 1)

                                f f f Nf f f N

                                f x y

                                f M f M f M N

                                image element pixel

                                00 01 0 1

                                10 11 1 1

                                10 11 1 1

                                ( ) ( )

                                N

                                i jN M N

                                i j

                                M M M N

                                a a aa f x i y j f i ja a a

                                Aa

                                a a a

                                f(00) ndash the upper left corner of the image

                                Digital Image Processing

                                Week 1

                                M N ge 0 L=2k

                                [0 1]i j i ja a L

                                Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                Digital Image Processing

                                Week 1

                                Digital Image Processing

                                Week 1

                                Number of bits required to store a digitized image

                                for 2 b M N k M N b N k

                                When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                Digital Image Processing

                                Week 1

                                Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                Measures line pairs per unit distance dots (pixels) per unit distance

                                Image resolution = the largest number of discernible line pairs per unit distance

                                (eg 100 line pairs per mm)

                                Dots per unit distance are commonly used in printing and publishing

                                In US the measure is expressed in dots per inch (dpi)

                                (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                Intensity resolution ndash the smallest discernible change in intensity level

                                The number of intensity levels (L) is determined by hardware considerations

                                L=2k ndash most common k = 8

                                Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                Digital Image Processing

                                Week 1

                                Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                150 dpi (lower left) 72 dpi (lower right)

                                Digital Image Processing

                                Week 1

                                Reducing the number of gray levels 256 128 64 32

                                Digital Image Processing

                                Week 1

                                Reducing the number of gray levels 16 8 4 2

                                Digital Image Processing

                                Week 1

                                Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                Shrinking zooming ndash image resizing ndash image resampling methods

                                Interpolation is the process of using known data to estimate values at unknown locations

                                Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                same spacing as the original and then shrink it so that it fits exactly over the original

                                image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                Problem assignment of intensity-level in the new 750 times 750 grid

                                Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                This technique has the tendency to produce undesirable effects like severe distortion of

                                straight edges

                                Digital Image Processing

                                Week 1

                                Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                be written using the 4 nearest neighbors of point (x y)

                                Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                modest increase in computational effort

                                Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                nearest neighbors of the point 3 3

                                0 0

                                ( ) i ji j

                                i jv x y c x y

                                The coefficients cij are obtained solving a 16x16 linear system

                                intensity levels of the 16 nearest neighbors of 3 3

                                0 0

                                ( )i ji j

                                i jc x y x y

                                Digital Image Processing

                                Week 1

                                Generally bicubic interpolation does a better job of preserving fine detail than the

                                bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                programs such as Adobe Photoshop and Corel Photopaint

                                Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                neighbor interpolation was used (both for shrinking and zooming)

                                Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                from 1250 dpi to 150 dpi (instead of 72 dpi)

                                Digital Image Processing

                                Week 1

                                Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                Digital Image Processing

                                Week 1

                                Neighbors of a Pixel

                                A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                and are denoted ND(p)

                                The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                N8 (p)

                                If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                fall outside the image

                                Digital Image Processing

                                Week 1

                                Adjacency Connectivity Regions Boundaries

                                Denote by V the set of intensity levels used to define adjacency

                                - in a binary image V 01 (V=0 V=1)

                                - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                We consider 3 types of adjacency

                                (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                m-adjacent if

                                4( )q N p or

                                ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                ambiguities that often arise when 8-adjacency is used Consider the example

                                Digital Image Processing

                                Week 1

                                binary image

                                0 1 1 0 1 1 0 1 1

                                1 0 1 0 0 1 0 0 1 0

                                0 0 1 0 0 1 0 0 1

                                V

                                The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                m-adjacency

                                Digital Image Processing

                                Week 1

                                A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                is a sequence of distinct pixels with coordinates

                                and are adjacent 0 0 1 1

                                1 1

                                ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                n n

                                i i i i

                                x y x y x y x y s tx y x y i n

                                The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                in S if there exists a path between them consisting only of pixels from S

                                S is a connected set if there is a path in S between any 2 pixels in S

                                Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                that are not adjacent are said to be disjoint When referring to regions only 4- and

                                8-adjacency are considered

                                Digital Image Processing

                                Week 1

                                Suppose that an image contains K disjoint regions 1 kR k K none of which

                                touches the image border

                                the complement of 1

                                ( )K

                                cu k u u

                                k

                                R R R R

                                We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                background of the image

                                The boundary (border or contour) of a region R is the set of points that are adjacent to

                                points in the complement of R (R)c The border of an image is the set of pixels in the

                                region that have at least one background neighbor This definition is referred to as the

                                inner border to distinguish it from the notion of outer border which is the corresponding

                                border in the background

                                Digital Image Processing

                                Week 1

                                Distance measures

                                For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                function or metric if

                                (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                (b) D(p q) = D(q p)

                                (c) D(p z) le D(p q) + D(q z)

                                The Euclidean distance between p and q is defined as 1

                                2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                centered at (x y)

                                Digital Image Processing

                                Week 1

                                The D4 distance (also called city-block distance) between p and q is defined as

                                4( ) | | | |D p q x s y t

                                The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                4

                                22 1 2

                                2 2 1 0 1 22 1 2

                                2

                                D

                                The pixels with D4 = 1 are the 4-neighbors of (x y)

                                The D8 distance (called the chessboard distance) between p and q is defined as

                                8( ) max| | | |D p q x s y t

                                The pixels q for which 8( )D p q r form a square centered at (x y)

                                Digital Image Processing

                                Week 1

                                8

                                2 2 2 2 22 1 1 1 2

                                2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                D

                                The pixels with D8 = 1 are the 8-neighbors of (x y)

                                D4 and D8 distances are independent of any paths that might exist between p and q

                                because these distances involve only the coordinates of the point

                                Digital Image Processing

                                Week 1

                                Array versus Matrix Operations

                                An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                11 12 11 12

                                21 22 21 22

                                a a b ba a b b

                                Array product

                                11 12 11 12 11 11 12 12

                                21 22 21 22 21 21 22 21

                                a a b b a b a ba a b b a b a b

                                Matrix product

                                11 12 11 12 11 11 12 21 11 12 12 21

                                21 22 21 22 21 11 22 21 21 12 22 22

                                a a b b a b a b a b a ba a b b a b a b a b a b

                                We assume array operations unless stated otherwise

                                Digital Image Processing

                                Week 1

                                Linear versus Nonlinear Operations

                                One of the most important classifications of image-processing methods is whether it is

                                linear or nonlinear

                                ( ) ( )H f x y g x y

                                H is said to be a linear operator if

                                images1 2 1 2

                                1 2

                                ( ) ( ) ( ) ( )

                                H a f x y b f x y a H f x y b H f x y

                                a b f f

                                Example of nonlinear operator

                                the maximum value of the pixels of image max ( )H f f x y f

                                1 2

                                0 2 6 5 1 1

                                2 3 4 7f f a b

                                Digital Image Processing

                                Week 1

                                1 2

                                0 2 6 5 6 3max max 1 ( 1) max 2

                                2 3 4 7 2 4a f b f

                                0 2 6 51 max ( 1) max 3 ( 1)7 4

                                2 3 4 7

                                Arithmetic Operations in Image Processing

                                Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                The two random variables are uncorrelated when their covariance is 0

                                Digital Image Processing

                                Week 1

                                Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                used in image enhancement)

                                1

                                1( ) ( )K

                                ii

                                g x y g x yK

                                If the noise satisfies the properties stated above we have

                                2 2( ) ( )

                                1( ) ( ) g x y x yE g x y f x yK

                                ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                and g respectively The standard deviation (square root of the variance) at any point in

                                the average image is

                                ( ) ( )1

                                g x y x yK

                                Digital Image Processing

                                Week 1

                                As K increases the variability (as measured by the variance or the standard deviation) of

                                the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                averaging process increases

                                An important application of image averaging is in the field of astronomy where imaging

                                under very low light levels frequently causes sensor noise to render single images

                                virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                images respectively

                                Digital Image Processing

                                Week 1

                                Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                100 noisy images

                                a b c d e f

                                Digital Image Processing

                                Week 1

                                A frequent application of image subtraction is in the enhancement of differences between

                                images

                                (a) (b) (c)

                                Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                significant bit of each pixel (c) the difference between the two images

                                Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                difference between images (a) and (b)

                                Digital Image Processing

                                Week 1

                                Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                images after injection of the contrast medium

                                In g(x y) we can find the differences between h and f as enhanced detail

                                Images being captured at TV rates we obtain a movie showing how the contrast medium

                                propagates through the various arteries in the area being observed

                                Digital Image Processing

                                Week 1

                                a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                Digital Image Processing

                                Week 1

                                An important application of image multiplication (and division) is shading correction

                                Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                When the shading function is known

                                ( )( )( )

                                g x yf x yh x y

                                h(x y) is unknown but we have access to the imaging system we can obtain an

                                approximation to the shading function by imaging a target of constant intensity When the

                                sensor is not available often the shading pattern can be estimated from the image

                                Digital Image Processing

                                Week 1

                                (a) (b) (c)

                                Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                Digital Image Processing

                                Week 1

                                Another use of image multiplication is in masking also called region of interest (ROI)

                                operations The process consists of multiplying a given image by a mask image that has

                                1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                (a) (b) (c)

                                Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                Digital Image Processing

                                Week 1

                                In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                min( )mf f f

                                0 ( 255)max( )

                                ms

                                m

                                ff K K K

                                f

                                Digital Image Processing

                                Week 1

                                Spatial Operations

                                - are performed directly on the pixels of a given image

                                There are three categories of spatial operations

                                single-pixel operations

                                neighborhood operations

                                geometric spatial transformations

                                Single-pixel operations

                                - change the values of intensity for the individual pixels ( )s T z

                                where z is the intensity of a pixel in the original image and s is the intensity of the

                                corresponding pixel in the processed image

                                Digital Image Processing

                                Week 1

                                Neighborhood operations

                                Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                in an image f Neighborhood processing generates new intensity level at point (x y)

                                based on the values of the intensities of the points in Sxy For example if Sxy is a

                                rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                intensity by computing the average value of the pixels in Sxy

                                ( )

                                1( ) ( )xyr c S

                                g x y f r cm n

                                The net effect is to perform local blurring in the original image This type of process is

                                used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                largest region of an image

                                Digital Image Processing

                                Week 1

                                Geometric spatial transformations and image registration

                                - modify the spatial relationship between pixels in an image

                                - these transformations are often called rubber-sheet transformations (analogous to

                                printing an image on a sheet of rubber and then stretching the sheet according to a

                                predefined set of rules

                                A geometric transformation consists of 2 basic operations

                                1 a spatial transformation of coordinates

                                2 intensity interpolation that assign intensity values to the spatial transformed

                                pixels

                                The coordinate system transformation ( ) [( )]x y T v w

                                (v w) ndash pixel coordinates in the original image

                                (x y) ndash pixel coordinates in the transformed image

                                Digital Image Processing

                                Week 1

                                [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                Affine transform

                                11 1211 21 31

                                21 2212 22 33

                                31 32

                                0[ 1] [ 1] [ 1] 0

                                1

                                t tx t v t w t

                                x y v w T v w t ty t v t w t

                                t t

                                (AT)

                                This transform can scale rotate translate or shear a set of coordinate points depending

                                on the elements of the matrix T If we want to resize an image rotate it and move the

                                result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                scaling rotation and translation matrices from Table 1

                                Digital Image Processing

                                Week 1

                                Affine transformations

                                Digital Image Processing

                                Week 1

                                The preceding transformations relocate pixels on an image to new locations To complete

                                the process we have to assign intensity values to those locations This task is done by

                                using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                In practice we can use equation (AT) in two basic ways

                                forward mapping scan the pixels of the input image (v w) compute the new spatial

                                location (x y) of the corresponding pixel in the new image using (AT) directly

                                Problems

                                - intensity assignment when 2 or more pixels in the original image are transformed to

                                the same location in the output image

                                - some output locations have no correspondent in the original image (no intensity

                                assignment)

                                Digital Image Processing

                                Week 1

                                inverse mapping scans the output pixel locations and at each location (x y)

                                computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                It then interpolates among the nearest input pixels to determine the intensity of the output

                                pixel value

                                Inverse mappings are more efficient to implement than forward mappings and are used in

                                numerous commercial implementations of spatial transformations (MATLAB for ex)

                                Digital Image Processing

                                Week 1

                                Digital Image Processing

                                Week 1

                                Image registration ndash align two or more images of the same scene

                                In image registration we have available the input and output images but the specific

                                transformation that produced the output image from the input is generally unknown

                                The problem is to estimate the transformation function and then use it to register the two

                                images

                                - it may be of interest to align (register) two or more image taken at approximately the

                                same time but using different imaging systems (MRI scanner and a PET scanner)

                                - align images of a given location taken by the same instrument at different moments

                                of time (satellite images)

                                Solving the problem using tie points (also called control points) which are

                                corresponding points whose locations are known precisely in the input and reference

                                image

                                Digital Image Processing

                                Week 1

                                How to select tie points

                                - interactively selecting them

                                - use of algorithms that try to detect these points

                                - some imaging systems have physical artifacts (small metallic objects) embedded in

                                the imaging sensors These objects produce a set of known points (called reseau

                                marks) directly on all images captured by the system which can be used as guides

                                for establishing tie points

                                The problem of estimating the transformation is one of modeling Suppose we have a set

                                of 4 tie points both on the input image and the reference image A simple model based on

                                a bilinear approximation is given by

                                1 2 3 4

                                5 6 7 8

                                x c v c w c v w cy c v c w c v w c

                                (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                Digital Image Processing

                                Week 1

                                When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                frequently is to select a larger number of tie points and using this new set of tie points

                                subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                subregions marked by 4 tie points we applied the transformation model described above

                                The number of tie points and the sophistication of the model required to solve the register

                                problem depend on the severity of the geometrical distortion

                                Digital Image Processing

                                Week 1

                                a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                Digital Image Processing

                                Week 1

                                Probabilistic Methods

                                zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                p(zk) = the probability that the intensity level zk occurs in the given image

                                ( ) kk

                                np zM N

                                nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                pixels in the image) 1

                                0( ) 1

                                L

                                kk

                                p z

                                The mean (average) intensity of an image is given by 1

                                0( )

                                L

                                k kk

                                m z p z

                                Digital Image Processing

                                Week 1

                                The variance of the intensities is 1

                                2 2

                                0( ) ( )

                                L

                                k kk

                                z m p z

                                The variance is a measure of the spread of the values of z about the mean so it is a

                                measure of image contrast Usually for measuring image contrast the standard deviation

                                ( ) is used

                                The n-th moment of a random variable z about the mean is defined as 1

                                0( ) ( ) ( )

                                Ln

                                n k kk

                                z z m p z

                                ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                3( ) 0z the intensities are biased to values higher than the mean

                                ( 3( ) 0z the intensities are biased to values lower than the mean

                                Digital Image Processing

                                Week 1

                                3( ) 0z the intensities are distributed approximately equally on both side of the

                                mean

                                Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                Digital Image Processing

                                Week 1

                                Intensity Transformations and Spatial Filtering

                                ( ) ( )g x y T f x y

                                f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                neighborhood of (x y)

                                - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                and much smaller in size than the image

                                Digital Image Processing

                                Week 1

                                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                called spatial filter (spatial mask kernel template or window)

                                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                ( )s T r

                                s and r are denoting respectively the intensity of g and f at (x y)

                                Figure 2 left - T produces an output image of higher contrast than the original by

                                darkening the intensity levels below k and brightening the levels above k ndash this technique

                                is called contrast stretching

                                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                Digital Image Processing

                                Week 1

                                Figure 2 right - T produces a binary output image A mapping of this form is called

                                thresholding function

                                Some Basic Intensity Transformation Functions

                                Image Negatives

                                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                - equivalent of a photographic negative

                                - technique suited for enhancing white or gray detail embedded in dark regions of an

                                image

                                Digital Image Processing

                                Week 1

                                Original Negative image

                                Digital Image Processing

                                Week 1

                                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                Some basic intensity transformation functions

                                Digital Image Processing

                                Week 1

                                This transformation maps a narrow range of low intensity values in the input into a wider

                                range An operator of this type is used to expand the values of dark pixels in an image

                                while compressing the higher-level values The opposite is true for the inverse log

                                transformation The log functions compress the dynamic range of images with large

                                variations in pixel values

                                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                Digital Image Processing

                                Week 1

                                Power-Law (Gamma) Transformations

                                - positive constants( ) ( ( ) )s T r c r c s c r

                                Plots of gamma transformation for different values of γ (c=1)

                                Digital Image Processing

                                Week 1

                                Power-law curves with 1 map a narrow range of dark input values into a wider range

                                of output values with the opposite being true for higher values of input values The

                                curves with 1 have the opposite effect of those generated with values of 1

                                1c - identity transformation

                                A variety of devices used for image capture printing and display respond according to a

                                power law The process used to correct these power-law response phenomena is called

                                gamma correction

                                Digital Image Processing

                                Week 1

                                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                Digital Image Processing

                                Week 1

                                Piecewise-Linear Transformations Functions

                                Contrast stretching

                                - a process that expands the range of intensity levels in an image so it spans the full

                                intensity range of the recording tool or display device

                                a b c d Fig5

                                Digital Image Processing

                                Week 1

                                11

                                1

                                2 1 1 21 2

                                2 1 2 1

                                22

                                2

                                [0 ]

                                ( ) ( )( ) [ ]( ) ( )

                                ( 1 ) [ 1]( 1 )

                                s r r rrs r r s r rT r r r r

                                r r r rs L r r r L

                                L r

                                Digital Image Processing

                                Week 1

                                1 1 2 2r s r s identity transformation (no change)

                                1 2 1 2 0 1r r s s L thresholding function

                                Figure 5(b) shows an 8-bit image with low contrast

                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                in the image respectively Thus the transformation function stretched the levels linearly

                                from their original range to the full range [0 L-1]

                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                2 2 1r s m L where m is the mean gray level in the image

                                The original image on which these results are based is a scanning electron microscope

                                image of pollen magnified approximately 700 times

                                Digital Image Processing

                                Week 1

                                Intensity-level slicing

                                - highlighting a specific range of intensities in an image

                                There are two approaches for intensity-level slicing

                                1 display in one value (white for example) all the values in the range of interest and in

                                another (say black) all other intensities (Figure 311 (a))

                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                intensities in the image (Figure 311 (b))

                                Digital Image Processing

                                Week 1

                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                the top of the scale of intensities This type of enhancement produces a binary image

                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                Highlights range [A B] and preserves all other intensities

                                Digital Image Processing

                                Week 1

                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                blockageshellip)

                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                image around the mean intensity was set to black the other intensities remain unchanged

                                Fig 6 - Aortic angiogram and intensity sliced versions

                                Digital Image Processing

                                Week 1

                                Bit-plane slicing

                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                This technique highlights the contribution made to the whole image appearances by each

                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                Digital Image Processing

                                Week 1

                                Digital Image Processing

                                Week 1

                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                • DIP 1 2017
                                • DIP 02 (2017)

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Examples of Fields that Use DIP

                                  Images can be classified according to their sources (visual X-ray hellip)

                                  Energy sources for images electromagnetic energy spectrum acoustic ultrasonic electronic computer- generated

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Electromagnetic waves can be thought as propagating sinusoidal

                                  waves of different wavelength or as a stream of massless particles

                                  each moving in a wavelike pattern with the speed of light Each

                                  massless particle contains a certain amount (bundle) of energy Each

                                  bundle of energy is called a photon If spectral bands are grouped

                                  according to energy per photon we obtain the spectrum shown in the

                                  image above ranging from gamma-rays (highest energy) to radio

                                  waves (lowest energy)

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Gamma-Ray Imaging

                                  Nuclear medicine astronomical observations

                                  Nuclear medicine

                                  the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

                                  Images are produced from the emissions collected by gamma-ray detectors

                                  Images of this sort are used to locate sites of bone pathology (infections tumors)

                                  PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Examples of gamma-ray imaging

                                  Bone scan PET image

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  X-ray imaging

                                  Medical diagnosticindustry astronomy

                                  A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                                  The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Angiography = contrast-enhancement radiography

                                  Angiograms = images of blood vessels

                                  A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                                  X-rays are used in CAT (computerized axial tomography)

                                  X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                                  Industrial CAT scans are useful when the parts can be penetreted by X-rays

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Examples of X-ray imaging

                                  Chest X-rayAortic angiogram

                                  Head CT Cygnus LoopCircuit boards

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Imaging in the Ultraviolet Band

                                  Litography industrial inspection microscopy biological imaging astronomical observations

                                  Ultraviolet light is used in fluorescence microscopy

                                  Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                                  other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                                  and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Imaging in the Visible and Infrared Bands

                                  Light microscopy astronomy remote sensing industry law enforcement

                                  LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                                  Weather observations and prediction produce major applications of multispectral image from satellites

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Satellite images of Washington DC area in spectral bands of the Table 1

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Examples of light microscopy

                                  Taxol (anticancer agent)magnified 250X

                                  Cholesterol(40X)

                                  Microprocessor(60X)

                                  Nickel oxidethin film(600X)

                                  Surface of audio CD(1750X)

                                  Organicsuperconductor(450X)

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Automated visual inspection of manufactured goods

                                  a bc de f

                                  a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Imaging in the Microwave Band

                                  The dominant aplication of imaging in the microwave band ndash radar

                                  bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                  bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                  bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                  An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Spaceborne radar image of mountains in southeast Tibet

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Imaging in the Radio Band

                                  medicine astronomy

                                  MRI = Magnetic Resonance Imaging

                                  This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                  Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                  The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  MRI images of a human knee (left) and spine (right)

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Images of the Crab Pulsar covering the electromagnetic spectrum

                                  Gamma X-ray Optical Infrared Radio

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Other Imaging Modalities

                                  acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                  Imaging using sound geological explorations industry medicine

                                  Mineral and oil exploration

                                  For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Biometry - iris

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Biometry - fingerprint

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Face detection and recognition

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Gender identification

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Image morphing

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                                  Fundamental Steps in DIP

                                  methods whose input and output are images

                                  methods whose inputs are images but whose outputs are attributes extracted from those images

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                                  Outputs are images

                                  bull image acquisition

                                  bull image filtering and enhancement

                                  bull image restoration

                                  bull color image processing

                                  bull wavelets and multiresolution processing

                                  bull compression

                                  bull morphological processing

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                                  Outputs are attributes

                                  bull morphological processing

                                  bull segmentation

                                  bull representation and description

                                  bull object recognition

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                                  Image acquisition - may involve preprocessing such as scaling

                                  Image enhancement

                                  bull manipulating an image so that the result is more suitable than the original for a specific operation

                                  bull enhancement is problem oriented

                                  bull there is no general sbquotheoryrsquo of image enhancement

                                  bull enhancement use subjective methods for image emprovement

                                  bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

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                                  Image restoration

                                  bull improving the appearance of an image

                                  bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                  Color image processing

                                  bull fundamental concept in color models

                                  bull basic color processing in a digital domain

                                  Wavelets and multiresolution processing

                                  representing images in various degree of resolution

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                                  Compression

                                  reducing the storage required to save an image or the bandwidth required to transmit it

                                  Morphological processing

                                  bull tools for extracting image components that are useful in the representation and description of shape

                                  bull a transition from processes that output images to processes that outputimage attributes

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                                  Segmentation

                                  bull partitioning an image into its constituents parts or objects

                                  bull autonomous segmentation is one of the most difficult tasks of DIP

                                  bull the more accurate the segmentation the more likley recognition is to succeed

                                  Representation and description (almost always follows segmentation)

                                  bull segmentation produces either the boundary of a region or all the poits in the region itself

                                  bull converting the data produced by segmentation to a form suitable for computer processing

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                                  bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                  bull complete region the focus is on internal properties such as texture or skeletal shape

                                  bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                  Object recognition

                                  the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                  Knowledge database

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                                  Simplified diagramof a cross sectionof the human eye

                                  Digital Image ProcessingDigital Image Processing

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                                  Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                  The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                  Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                  The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                  Digital Image ProcessingDigital Image Processing

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                                  The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                  The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                  Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                  Fovea = the place where the image of the object of interest falls on

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                                  Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                  Blind spot region without receptors

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                                  Image formation in the eye

                                  Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                  Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                  distance between lens and retina along visual axix = 17 mm

                                  range of focal length = 14 mm to 17 mm

                                  Digital Image ProcessingDigital Image Processing

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                                  Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  Optical illusions

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                  Digital Image ProcessingDigital Image Processing

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                                  Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                  gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                  quantities that describe the quality of a chromatic light source radiance

                                  the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                  measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                  Digital Image ProcessingDigital Image Processing

                                  Week 1Week 1

                                  For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                  brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

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                                  the physical meaning is determined by the source of the image

                                  ( )f D f x y

                                  Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                  f(xy) ndash characterized by two components

                                  i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                  r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                  ( ) ( ) ( )

                                  0 ( ) 0 ( ) 1

                                  f x y i x y r x y

                                  i x y r x y

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                                  r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                  i(xy) ndash determined by the illumination source

                                  r(xy) ndash determined by the characteristics of the imaged objects

                                  is called gray (or intensity) scale

                                  In practice

                                  min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                  indoor values without additional illuminationmin max10 1000L L

                                  black whitemin max0 1 0 1 0 1L L L L l l L

                                  min maxL L

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                                  Digital Image Processing

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                                  Image Sampling and Quantization

                                  - the output of the sensors is a continuous voltage waveform related to the sensed

                                  scene

                                  converting a continuous image f to digital form

                                  - digitizing (x y) is called sampling

                                  - digitizing f(x y) is called quantization

                                  Digital Image Processing

                                  Week 1

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                                  Continuous image projected onto a sensor array Result of image sampling and quantization

                                  Digital Image Processing

                                  Week 1

                                  Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                  (00) (01) (0 1)(10) (11) (1 1)

                                  ( )

                                  ( 10) ( 11) ( 1 1)

                                  f f f Nf f f N

                                  f x y

                                  f M f M f M N

                                  image element pixel

                                  00 01 0 1

                                  10 11 1 1

                                  10 11 1 1

                                  ( ) ( )

                                  N

                                  i jN M N

                                  i j

                                  M M M N

                                  a a aa f x i y j f i ja a a

                                  Aa

                                  a a a

                                  f(00) ndash the upper left corner of the image

                                  Digital Image Processing

                                  Week 1

                                  M N ge 0 L=2k

                                  [0 1]i j i ja a L

                                  Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                  Digital Image Processing

                                  Week 1

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                                  Number of bits required to store a digitized image

                                  for 2 b M N k M N b N k

                                  When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                  Digital Image Processing

                                  Week 1

                                  Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                  Measures line pairs per unit distance dots (pixels) per unit distance

                                  Image resolution = the largest number of discernible line pairs per unit distance

                                  (eg 100 line pairs per mm)

                                  Dots per unit distance are commonly used in printing and publishing

                                  In US the measure is expressed in dots per inch (dpi)

                                  (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                  Intensity resolution ndash the smallest discernible change in intensity level

                                  The number of intensity levels (L) is determined by hardware considerations

                                  L=2k ndash most common k = 8

                                  Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                  Digital Image Processing

                                  Week 1

                                  Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                  150 dpi (lower left) 72 dpi (lower right)

                                  Digital Image Processing

                                  Week 1

                                  Reducing the number of gray levels 256 128 64 32

                                  Digital Image Processing

                                  Week 1

                                  Reducing the number of gray levels 16 8 4 2

                                  Digital Image Processing

                                  Week 1

                                  Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                  Shrinking zooming ndash image resizing ndash image resampling methods

                                  Interpolation is the process of using known data to estimate values at unknown locations

                                  Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                  750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                  same spacing as the original and then shrink it so that it fits exactly over the original

                                  image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                  Problem assignment of intensity-level in the new 750 times 750 grid

                                  Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                  intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                  This technique has the tendency to produce undesirable effects like severe distortion of

                                  straight edges

                                  Digital Image Processing

                                  Week 1

                                  Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                  where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                  be written using the 4 nearest neighbors of point (x y)

                                  Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                  modest increase in computational effort

                                  Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                  nearest neighbors of the point 3 3

                                  0 0

                                  ( ) i ji j

                                  i jv x y c x y

                                  The coefficients cij are obtained solving a 16x16 linear system

                                  intensity levels of the 16 nearest neighbors of 3 3

                                  0 0

                                  ( )i ji j

                                  i jc x y x y

                                  Digital Image Processing

                                  Week 1

                                  Generally bicubic interpolation does a better job of preserving fine detail than the

                                  bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                  programs such as Adobe Photoshop and Corel Photopaint

                                  Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                  the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                  then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                  neighbor interpolation was used (both for shrinking and zooming)

                                  Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                  interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                  from 1250 dpi to 150 dpi (instead of 72 dpi)

                                  Digital Image Processing

                                  Week 1

                                  Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                  Digital Image Processing

                                  Week 1

                                  Neighbors of a Pixel

                                  A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                  This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                  The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                  and are denoted ND(p)

                                  The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                  N8 (p)

                                  If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                  fall outside the image

                                  Digital Image Processing

                                  Week 1

                                  Adjacency Connectivity Regions Boundaries

                                  Denote by V the set of intensity levels used to define adjacency

                                  - in a binary image V 01 (V=0 V=1)

                                  - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                  We consider 3 types of adjacency

                                  (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                  (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                  (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                  m-adjacent if

                                  4( )q N p or

                                  ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                  Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                  ambiguities that often arise when 8-adjacency is used Consider the example

                                  Digital Image Processing

                                  Week 1

                                  binary image

                                  0 1 1 0 1 1 0 1 1

                                  1 0 1 0 0 1 0 0 1 0

                                  0 0 1 0 0 1 0 0 1

                                  V

                                  The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                  8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                  m-adjacency

                                  Digital Image Processing

                                  Week 1

                                  A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                  is a sequence of distinct pixels with coordinates

                                  and are adjacent 0 0 1 1

                                  1 1

                                  ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                  n n

                                  i i i i

                                  x y x y x y x y s tx y x y i n

                                  The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                  Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                  Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                  in S if there exists a path between them consisting only of pixels from S

                                  S is a connected set if there is a path in S between any 2 pixels in S

                                  Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                  Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                  that are not adjacent are said to be disjoint When referring to regions only 4- and

                                  8-adjacency are considered

                                  Digital Image Processing

                                  Week 1

                                  Suppose that an image contains K disjoint regions 1 kR k K none of which

                                  touches the image border

                                  the complement of 1

                                  ( )K

                                  cu k u u

                                  k

                                  R R R R

                                  We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                  background of the image

                                  The boundary (border or contour) of a region R is the set of points that are adjacent to

                                  points in the complement of R (R)c The border of an image is the set of pixels in the

                                  region that have at least one background neighbor This definition is referred to as the

                                  inner border to distinguish it from the notion of outer border which is the corresponding

                                  border in the background

                                  Digital Image Processing

                                  Week 1

                                  Distance measures

                                  For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                  function or metric if

                                  (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                  (b) D(p q) = D(q p)

                                  (c) D(p z) le D(p q) + D(q z)

                                  The Euclidean distance between p and q is defined as 1

                                  2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                  The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                  centered at (x y)

                                  Digital Image Processing

                                  Week 1

                                  The D4 distance (also called city-block distance) between p and q is defined as

                                  4( ) | | | |D p q x s y t

                                  The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                  4

                                  22 1 2

                                  2 2 1 0 1 22 1 2

                                  2

                                  D

                                  The pixels with D4 = 1 are the 4-neighbors of (x y)

                                  The D8 distance (called the chessboard distance) between p and q is defined as

                                  8( ) max| | | |D p q x s y t

                                  The pixels q for which 8( )D p q r form a square centered at (x y)

                                  Digital Image Processing

                                  Week 1

                                  8

                                  2 2 2 2 22 1 1 1 2

                                  2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                  D

                                  The pixels with D8 = 1 are the 8-neighbors of (x y)

                                  D4 and D8 distances are independent of any paths that might exist between p and q

                                  because these distances involve only the coordinates of the point

                                  Digital Image Processing

                                  Week 1

                                  Array versus Matrix Operations

                                  An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                  11 12 11 12

                                  21 22 21 22

                                  a a b ba a b b

                                  Array product

                                  11 12 11 12 11 11 12 12

                                  21 22 21 22 21 21 22 21

                                  a a b b a b a ba a b b a b a b

                                  Matrix product

                                  11 12 11 12 11 11 12 21 11 12 12 21

                                  21 22 21 22 21 11 22 21 21 12 22 22

                                  a a b b a b a b a b a ba a b b a b a b a b a b

                                  We assume array operations unless stated otherwise

                                  Digital Image Processing

                                  Week 1

                                  Linear versus Nonlinear Operations

                                  One of the most important classifications of image-processing methods is whether it is

                                  linear or nonlinear

                                  ( ) ( )H f x y g x y

                                  H is said to be a linear operator if

                                  images1 2 1 2

                                  1 2

                                  ( ) ( ) ( ) ( )

                                  H a f x y b f x y a H f x y b H f x y

                                  a b f f

                                  Example of nonlinear operator

                                  the maximum value of the pixels of image max ( )H f f x y f

                                  1 2

                                  0 2 6 5 1 1

                                  2 3 4 7f f a b

                                  Digital Image Processing

                                  Week 1

                                  1 2

                                  0 2 6 5 6 3max max 1 ( 1) max 2

                                  2 3 4 7 2 4a f b f

                                  0 2 6 51 max ( 1) max 3 ( 1)7 4

                                  2 3 4 7

                                  Arithmetic Operations in Image Processing

                                  Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                  f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                  For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                  value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                  The two random variables are uncorrelated when their covariance is 0

                                  Digital Image Processing

                                  Week 1

                                  Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                  used in image enhancement)

                                  1

                                  1( ) ( )K

                                  ii

                                  g x y g x yK

                                  If the noise satisfies the properties stated above we have

                                  2 2( ) ( )

                                  1( ) ( ) g x y x yE g x y f x yK

                                  ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                  and g respectively The standard deviation (square root of the variance) at any point in

                                  the average image is

                                  ( ) ( )1

                                  g x y x yK

                                  Digital Image Processing

                                  Week 1

                                  As K increases the variability (as measured by the variance or the standard deviation) of

                                  the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                  means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                  averaging process increases

                                  An important application of image averaging is in the field of astronomy where imaging

                                  under very low light levels frequently causes sensor noise to render single images

                                  virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                  was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                  64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                  images respectively

                                  Digital Image Processing

                                  Week 1

                                  Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                  100 noisy images

                                  a b c d e f

                                  Digital Image Processing

                                  Week 1

                                  A frequent application of image subtraction is in the enhancement of differences between

                                  images

                                  (a) (b) (c)

                                  Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                  significant bit of each pixel (c) the difference between the two images

                                  Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                  Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                  images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                  difference between images (a) and (b)

                                  Digital Image Processing

                                  Week 1

                                  Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                  h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                  intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                  source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                  bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                  anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                  images after injection of the contrast medium

                                  In g(x y) we can find the differences between h and f as enhanced detail

                                  Images being captured at TV rates we obtain a movie showing how the contrast medium

                                  propagates through the various arteries in the area being observed

                                  Digital Image Processing

                                  Week 1

                                  a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                  Digital Image Processing

                                  Week 1

                                  An important application of image multiplication (and division) is shading correction

                                  Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                  f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                  When the shading function is known

                                  ( )( )( )

                                  g x yf x yh x y

                                  h(x y) is unknown but we have access to the imaging system we can obtain an

                                  approximation to the shading function by imaging a target of constant intensity When the

                                  sensor is not available often the shading pattern can be estimated from the image

                                  Digital Image Processing

                                  Week 1

                                  (a) (b) (c)

                                  Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                  Digital Image Processing

                                  Week 1

                                  Another use of image multiplication is in masking also called region of interest (ROI)

                                  operations The process consists of multiplying a given image by a mask image that has

                                  1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                  image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                  (a) (b) (c)

                                  Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                  Digital Image Processing

                                  Week 1

                                  In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                  min( )mf f f

                                  0 ( 255)max( )

                                  ms

                                  m

                                  ff K K K

                                  f

                                  Digital Image Processing

                                  Week 1

                                  Spatial Operations

                                  - are performed directly on the pixels of a given image

                                  There are three categories of spatial operations

                                  single-pixel operations

                                  neighborhood operations

                                  geometric spatial transformations

                                  Single-pixel operations

                                  - change the values of intensity for the individual pixels ( )s T z

                                  where z is the intensity of a pixel in the original image and s is the intensity of the

                                  corresponding pixel in the processed image

                                  Digital Image Processing

                                  Week 1

                                  Neighborhood operations

                                  Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                  in an image f Neighborhood processing generates new intensity level at point (x y)

                                  based on the values of the intensities of the points in Sxy For example if Sxy is a

                                  rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                  intensity by computing the average value of the pixels in Sxy

                                  ( )

                                  1( ) ( )xyr c S

                                  g x y f r cm n

                                  The net effect is to perform local blurring in the original image This type of process is

                                  used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                  largest region of an image

                                  Digital Image Processing

                                  Week 1

                                  Geometric spatial transformations and image registration

                                  - modify the spatial relationship between pixels in an image

                                  - these transformations are often called rubber-sheet transformations (analogous to

                                  printing an image on a sheet of rubber and then stretching the sheet according to a

                                  predefined set of rules

                                  A geometric transformation consists of 2 basic operations

                                  1 a spatial transformation of coordinates

                                  2 intensity interpolation that assign intensity values to the spatial transformed

                                  pixels

                                  The coordinate system transformation ( ) [( )]x y T v w

                                  (v w) ndash pixel coordinates in the original image

                                  (x y) ndash pixel coordinates in the transformed image

                                  Digital Image Processing

                                  Week 1

                                  [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                  Affine transform

                                  11 1211 21 31

                                  21 2212 22 33

                                  31 32

                                  0[ 1] [ 1] [ 1] 0

                                  1

                                  t tx t v t w t

                                  x y v w T v w t ty t v t w t

                                  t t

                                  (AT)

                                  This transform can scale rotate translate or shear a set of coordinate points depending

                                  on the elements of the matrix T If we want to resize an image rotate it and move the

                                  result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                  scaling rotation and translation matrices from Table 1

                                  Digital Image Processing

                                  Week 1

                                  Affine transformations

                                  Digital Image Processing

                                  Week 1

                                  The preceding transformations relocate pixels on an image to new locations To complete

                                  the process we have to assign intensity values to those locations This task is done by

                                  using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                  In practice we can use equation (AT) in two basic ways

                                  forward mapping scan the pixels of the input image (v w) compute the new spatial

                                  location (x y) of the corresponding pixel in the new image using (AT) directly

                                  Problems

                                  - intensity assignment when 2 or more pixels in the original image are transformed to

                                  the same location in the output image

                                  - some output locations have no correspondent in the original image (no intensity

                                  assignment)

                                  Digital Image Processing

                                  Week 1

                                  inverse mapping scans the output pixel locations and at each location (x y)

                                  computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                  It then interpolates among the nearest input pixels to determine the intensity of the output

                                  pixel value

                                  Inverse mappings are more efficient to implement than forward mappings and are used in

                                  numerous commercial implementations of spatial transformations (MATLAB for ex)

                                  Digital Image Processing

                                  Week 1

                                  Digital Image Processing

                                  Week 1

                                  Image registration ndash align two or more images of the same scene

                                  In image registration we have available the input and output images but the specific

                                  transformation that produced the output image from the input is generally unknown

                                  The problem is to estimate the transformation function and then use it to register the two

                                  images

                                  - it may be of interest to align (register) two or more image taken at approximately the

                                  same time but using different imaging systems (MRI scanner and a PET scanner)

                                  - align images of a given location taken by the same instrument at different moments

                                  of time (satellite images)

                                  Solving the problem using tie points (also called control points) which are

                                  corresponding points whose locations are known precisely in the input and reference

                                  image

                                  Digital Image Processing

                                  Week 1

                                  How to select tie points

                                  - interactively selecting them

                                  - use of algorithms that try to detect these points

                                  - some imaging systems have physical artifacts (small metallic objects) embedded in

                                  the imaging sensors These objects produce a set of known points (called reseau

                                  marks) directly on all images captured by the system which can be used as guides

                                  for establishing tie points

                                  The problem of estimating the transformation is one of modeling Suppose we have a set

                                  of 4 tie points both on the input image and the reference image A simple model based on

                                  a bilinear approximation is given by

                                  1 2 3 4

                                  5 6 7 8

                                  x c v c w c v w cy c v c w c v w c

                                  (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                  Digital Image Processing

                                  Week 1

                                  When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                  frequently is to select a larger number of tie points and using this new set of tie points

                                  subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                  subregions marked by 4 tie points we applied the transformation model described above

                                  The number of tie points and the sophistication of the model required to solve the register

                                  problem depend on the severity of the geometrical distortion

                                  Digital Image Processing

                                  Week 1

                                  a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                  Digital Image Processing

                                  Week 1

                                  Probabilistic Methods

                                  zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                  p(zk) = the probability that the intensity level zk occurs in the given image

                                  ( ) kk

                                  np zM N

                                  nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                  pixels in the image) 1

                                  0( ) 1

                                  L

                                  kk

                                  p z

                                  The mean (average) intensity of an image is given by 1

                                  0( )

                                  L

                                  k kk

                                  m z p z

                                  Digital Image Processing

                                  Week 1

                                  The variance of the intensities is 1

                                  2 2

                                  0( ) ( )

                                  L

                                  k kk

                                  z m p z

                                  The variance is a measure of the spread of the values of z about the mean so it is a

                                  measure of image contrast Usually for measuring image contrast the standard deviation

                                  ( ) is used

                                  The n-th moment of a random variable z about the mean is defined as 1

                                  0( ) ( ) ( )

                                  Ln

                                  n k kk

                                  z z m p z

                                  ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                  3( ) 0z the intensities are biased to values higher than the mean

                                  ( 3( ) 0z the intensities are biased to values lower than the mean

                                  Digital Image Processing

                                  Week 1

                                  3( ) 0z the intensities are distributed approximately equally on both side of the

                                  mean

                                  Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                  Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                  Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                  Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                  Digital Image Processing

                                  Week 1

                                  Intensity Transformations and Spatial Filtering

                                  ( ) ( )g x y T f x y

                                  f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                  neighborhood of (x y)

                                  - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                  and much smaller in size than the image

                                  Digital Image Processing

                                  Week 1

                                  - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                  called spatial filter (spatial mask kernel template or window)

                                  ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                  ( )s T r

                                  s and r are denoting respectively the intensity of g and f at (x y)

                                  Figure 2 left - T produces an output image of higher contrast than the original by

                                  darkening the intensity levels below k and brightening the levels above k ndash this technique

                                  is called contrast stretching

                                  Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                  Digital Image Processing

                                  Week 1

                                  Figure 2 right - T produces a binary output image A mapping of this form is called

                                  thresholding function

                                  Some Basic Intensity Transformation Functions

                                  Image Negatives

                                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                  - equivalent of a photographic negative

                                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                                  image

                                  Digital Image Processing

                                  Week 1

                                  Original Negative image

                                  Digital Image Processing

                                  Week 1

                                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                  Some basic intensity transformation functions

                                  Digital Image Processing

                                  Week 1

                                  This transformation maps a narrow range of low intensity values in the input into a wider

                                  range An operator of this type is used to expand the values of dark pixels in an image

                                  while compressing the higher-level values The opposite is true for the inverse log

                                  transformation The log functions compress the dynamic range of images with large

                                  variations in pixel values

                                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                  Digital Image Processing

                                  Week 1

                                  Power-Law (Gamma) Transformations

                                  - positive constants( ) ( ( ) )s T r c r c s c r

                                  Plots of gamma transformation for different values of γ (c=1)

                                  Digital Image Processing

                                  Week 1

                                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                                  of output values with the opposite being true for higher values of input values The

                                  curves with 1 have the opposite effect of those generated with values of 1

                                  1c - identity transformation

                                  A variety of devices used for image capture printing and display respond according to a

                                  power law The process used to correct these power-law response phenomena is called

                                  gamma correction

                                  Digital Image Processing

                                  Week 1

                                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                  Digital Image Processing

                                  Week 1

                                  Piecewise-Linear Transformations Functions

                                  Contrast stretching

                                  - a process that expands the range of intensity levels in an image so it spans the full

                                  intensity range of the recording tool or display device

                                  a b c d Fig5

                                  Digital Image Processing

                                  Week 1

                                  11

                                  1

                                  2 1 1 21 2

                                  2 1 2 1

                                  22

                                  2

                                  [0 ]

                                  ( ) ( )( ) [ ]( ) ( )

                                  ( 1 ) [ 1]( 1 )

                                  s r r rrs r r s r rT r r r r

                                  r r r rs L r r r L

                                  L r

                                  Digital Image Processing

                                  Week 1

                                  1 1 2 2r s r s identity transformation (no change)

                                  1 2 1 2 0 1r r s s L thresholding function

                                  Figure 5(b) shows an 8-bit image with low contrast

                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                  in the image respectively Thus the transformation function stretched the levels linearly

                                  from their original range to the full range [0 L-1]

                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                  2 2 1r s m L where m is the mean gray level in the image

                                  The original image on which these results are based is a scanning electron microscope

                                  image of pollen magnified approximately 700 times

                                  Digital Image Processing

                                  Week 1

                                  Intensity-level slicing

                                  - highlighting a specific range of intensities in an image

                                  There are two approaches for intensity-level slicing

                                  1 display in one value (white for example) all the values in the range of interest and in

                                  another (say black) all other intensities (Figure 311 (a))

                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                  intensities in the image (Figure 311 (b))

                                  Digital Image Processing

                                  Week 1

                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                  the top of the scale of intensities This type of enhancement produces a binary image

                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                  Highlights range [A B] and preserves all other intensities

                                  Digital Image Processing

                                  Week 1

                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                  blockageshellip)

                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                  image around the mean intensity was set to black the other intensities remain unchanged

                                  Fig 6 - Aortic angiogram and intensity sliced versions

                                  Digital Image Processing

                                  Week 1

                                  Bit-plane slicing

                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                  This technique highlights the contribution made to the whole image appearances by each

                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                  Digital Image Processing

                                  Week 1

                                  Digital Image Processing

                                  Week 1

                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                  • DIP 1 2017
                                  • DIP 02 (2017)

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Electromagnetic waves can be thought as propagating sinusoidal

                                    waves of different wavelength or as a stream of massless particles

                                    each moving in a wavelike pattern with the speed of light Each

                                    massless particle contains a certain amount (bundle) of energy Each

                                    bundle of energy is called a photon If spectral bands are grouped

                                    according to energy per photon we obtain the spectrum shown in the

                                    image above ranging from gamma-rays (highest energy) to radio

                                    waves (lowest energy)

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Gamma-Ray Imaging

                                    Nuclear medicine astronomical observations

                                    Nuclear medicine

                                    the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

                                    Images are produced from the emissions collected by gamma-ray detectors

                                    Images of this sort are used to locate sites of bone pathology (infections tumors)

                                    PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Examples of gamma-ray imaging

                                    Bone scan PET image

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    X-ray imaging

                                    Medical diagnosticindustry astronomy

                                    A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                                    The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Angiography = contrast-enhancement radiography

                                    Angiograms = images of blood vessels

                                    A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                                    X-rays are used in CAT (computerized axial tomography)

                                    X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                                    Industrial CAT scans are useful when the parts can be penetreted by X-rays

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Examples of X-ray imaging

                                    Chest X-rayAortic angiogram

                                    Head CT Cygnus LoopCircuit boards

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Imaging in the Ultraviolet Band

                                    Litography industrial inspection microscopy biological imaging astronomical observations

                                    Ultraviolet light is used in fluorescence microscopy

                                    Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                                    other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                                    and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Imaging in the Visible and Infrared Bands

                                    Light microscopy astronomy remote sensing industry law enforcement

                                    LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                                    Weather observations and prediction produce major applications of multispectral image from satellites

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Satellite images of Washington DC area in spectral bands of the Table 1

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Examples of light microscopy

                                    Taxol (anticancer agent)magnified 250X

                                    Cholesterol(40X)

                                    Microprocessor(60X)

                                    Nickel oxidethin film(600X)

                                    Surface of audio CD(1750X)

                                    Organicsuperconductor(450X)

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Automated visual inspection of manufactured goods

                                    a bc de f

                                    a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Imaging in the Microwave Band

                                    The dominant aplication of imaging in the microwave band ndash radar

                                    bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                    bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                    bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                    An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Spaceborne radar image of mountains in southeast Tibet

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Imaging in the Radio Band

                                    medicine astronomy

                                    MRI = Magnetic Resonance Imaging

                                    This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                    Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                    The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    MRI images of a human knee (left) and spine (right)

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Images of the Crab Pulsar covering the electromagnetic spectrum

                                    Gamma X-ray Optical Infrared Radio

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Other Imaging Modalities

                                    acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                    Imaging using sound geological explorations industry medicine

                                    Mineral and oil exploration

                                    For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Biometry - iris

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Biometry - fingerprint

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Face detection and recognition

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Gender identification

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Image morphing

                                    Digital Image ProcessingDigital Image Processing

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                                    Fundamental Steps in DIP

                                    methods whose input and output are images

                                    methods whose inputs are images but whose outputs are attributes extracted from those images

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Outputs are images

                                    bull image acquisition

                                    bull image filtering and enhancement

                                    bull image restoration

                                    bull color image processing

                                    bull wavelets and multiresolution processing

                                    bull compression

                                    bull morphological processing

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Outputs are attributes

                                    bull morphological processing

                                    bull segmentation

                                    bull representation and description

                                    bull object recognition

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Image acquisition - may involve preprocessing such as scaling

                                    Image enhancement

                                    bull manipulating an image so that the result is more suitable than the original for a specific operation

                                    bull enhancement is problem oriented

                                    bull there is no general sbquotheoryrsquo of image enhancement

                                    bull enhancement use subjective methods for image emprovement

                                    bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Image restoration

                                    bull improving the appearance of an image

                                    bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                    Color image processing

                                    bull fundamental concept in color models

                                    bull basic color processing in a digital domain

                                    Wavelets and multiresolution processing

                                    representing images in various degree of resolution

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Compression

                                    reducing the storage required to save an image or the bandwidth required to transmit it

                                    Morphological processing

                                    bull tools for extracting image components that are useful in the representation and description of shape

                                    bull a transition from processes that output images to processes that outputimage attributes

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Segmentation

                                    bull partitioning an image into its constituents parts or objects

                                    bull autonomous segmentation is one of the most difficult tasks of DIP

                                    bull the more accurate the segmentation the more likley recognition is to succeed

                                    Representation and description (almost always follows segmentation)

                                    bull segmentation produces either the boundary of a region or all the poits in the region itself

                                    bull converting the data produced by segmentation to a form suitable for computer processing

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                    bull complete region the focus is on internal properties such as texture or skeletal shape

                                    bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                    Object recognition

                                    the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                    Knowledge database

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Simplified diagramof a cross sectionof the human eye

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                    The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                    Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                    The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                    The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                    Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                    Fovea = the place where the image of the object of interest falls on

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                    Blind spot region without receptors

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Image formation in the eye

                                    Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                    Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                    distance between lens and retina along visual axix = 17 mm

                                    range of focal length = 14 mm to 17 mm

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Digital Image ProcessingDigital Image Processing

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                                    Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Optical illusions

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                    gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                    quantities that describe the quality of a chromatic light source radiance

                                    the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                    measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                    brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    the physical meaning is determined by the source of the image

                                    ( )f D f x y

                                    Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                    f(xy) ndash characterized by two components

                                    i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                    r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                    ( ) ( ) ( )

                                    0 ( ) 0 ( ) 1

                                    f x y i x y r x y

                                    i x y r x y

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                    i(xy) ndash determined by the illumination source

                                    r(xy) ndash determined by the characteristics of the imaged objects

                                    is called gray (or intensity) scale

                                    In practice

                                    min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                    indoor values without additional illuminationmin max10 1000L L

                                    black whitemin max0 1 0 1 0 1L L L L l l L

                                    min maxL L

                                    Digital Image ProcessingDigital Image Processing

                                    Week 1Week 1

                                    Digital Image Processing

                                    Week 1

                                    Image Sampling and Quantization

                                    - the output of the sensors is a continuous voltage waveform related to the sensed

                                    scene

                                    converting a continuous image f to digital form

                                    - digitizing (x y) is called sampling

                                    - digitizing f(x y) is called quantization

                                    Digital Image Processing

                                    Week 1

                                    Digital Image Processing

                                    Week 1

                                    Continuous image projected onto a sensor array Result of image sampling and quantization

                                    Digital Image Processing

                                    Week 1

                                    Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                    (00) (01) (0 1)(10) (11) (1 1)

                                    ( )

                                    ( 10) ( 11) ( 1 1)

                                    f f f Nf f f N

                                    f x y

                                    f M f M f M N

                                    image element pixel

                                    00 01 0 1

                                    10 11 1 1

                                    10 11 1 1

                                    ( ) ( )

                                    N

                                    i jN M N

                                    i j

                                    M M M N

                                    a a aa f x i y j f i ja a a

                                    Aa

                                    a a a

                                    f(00) ndash the upper left corner of the image

                                    Digital Image Processing

                                    Week 1

                                    M N ge 0 L=2k

                                    [0 1]i j i ja a L

                                    Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                    Digital Image Processing

                                    Week 1

                                    Digital Image Processing

                                    Week 1

                                    Number of bits required to store a digitized image

                                    for 2 b M N k M N b N k

                                    When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                    Digital Image Processing

                                    Week 1

                                    Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                    Measures line pairs per unit distance dots (pixels) per unit distance

                                    Image resolution = the largest number of discernible line pairs per unit distance

                                    (eg 100 line pairs per mm)

                                    Dots per unit distance are commonly used in printing and publishing

                                    In US the measure is expressed in dots per inch (dpi)

                                    (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                    Intensity resolution ndash the smallest discernible change in intensity level

                                    The number of intensity levels (L) is determined by hardware considerations

                                    L=2k ndash most common k = 8

                                    Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                    Digital Image Processing

                                    Week 1

                                    Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                    150 dpi (lower left) 72 dpi (lower right)

                                    Digital Image Processing

                                    Week 1

                                    Reducing the number of gray levels 256 128 64 32

                                    Digital Image Processing

                                    Week 1

                                    Reducing the number of gray levels 16 8 4 2

                                    Digital Image Processing

                                    Week 1

                                    Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                    Shrinking zooming ndash image resizing ndash image resampling methods

                                    Interpolation is the process of using known data to estimate values at unknown locations

                                    Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                    750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                    same spacing as the original and then shrink it so that it fits exactly over the original

                                    image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                    Problem assignment of intensity-level in the new 750 times 750 grid

                                    Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                    intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                    This technique has the tendency to produce undesirable effects like severe distortion of

                                    straight edges

                                    Digital Image Processing

                                    Week 1

                                    Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                    where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                    be written using the 4 nearest neighbors of point (x y)

                                    Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                    modest increase in computational effort

                                    Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                    nearest neighbors of the point 3 3

                                    0 0

                                    ( ) i ji j

                                    i jv x y c x y

                                    The coefficients cij are obtained solving a 16x16 linear system

                                    intensity levels of the 16 nearest neighbors of 3 3

                                    0 0

                                    ( )i ji j

                                    i jc x y x y

                                    Digital Image Processing

                                    Week 1

                                    Generally bicubic interpolation does a better job of preserving fine detail than the

                                    bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                    programs such as Adobe Photoshop and Corel Photopaint

                                    Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                    the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                    then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                    neighbor interpolation was used (both for shrinking and zooming)

                                    Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                    interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                    from 1250 dpi to 150 dpi (instead of 72 dpi)

                                    Digital Image Processing

                                    Week 1

                                    Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                    Digital Image Processing

                                    Week 1

                                    Neighbors of a Pixel

                                    A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                    This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                    The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                    and are denoted ND(p)

                                    The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                    N8 (p)

                                    If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                    fall outside the image

                                    Digital Image Processing

                                    Week 1

                                    Adjacency Connectivity Regions Boundaries

                                    Denote by V the set of intensity levels used to define adjacency

                                    - in a binary image V 01 (V=0 V=1)

                                    - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                    We consider 3 types of adjacency

                                    (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                    (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                    (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                    m-adjacent if

                                    4( )q N p or

                                    ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                    Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                    ambiguities that often arise when 8-adjacency is used Consider the example

                                    Digital Image Processing

                                    Week 1

                                    binary image

                                    0 1 1 0 1 1 0 1 1

                                    1 0 1 0 0 1 0 0 1 0

                                    0 0 1 0 0 1 0 0 1

                                    V

                                    The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                    8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                    m-adjacency

                                    Digital Image Processing

                                    Week 1

                                    A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                    is a sequence of distinct pixels with coordinates

                                    and are adjacent 0 0 1 1

                                    1 1

                                    ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                    n n

                                    i i i i

                                    x y x y x y x y s tx y x y i n

                                    The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                    Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                    Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                    in S if there exists a path between them consisting only of pixels from S

                                    S is a connected set if there is a path in S between any 2 pixels in S

                                    Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                    Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                    that are not adjacent are said to be disjoint When referring to regions only 4- and

                                    8-adjacency are considered

                                    Digital Image Processing

                                    Week 1

                                    Suppose that an image contains K disjoint regions 1 kR k K none of which

                                    touches the image border

                                    the complement of 1

                                    ( )K

                                    cu k u u

                                    k

                                    R R R R

                                    We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                    background of the image

                                    The boundary (border or contour) of a region R is the set of points that are adjacent to

                                    points in the complement of R (R)c The border of an image is the set of pixels in the

                                    region that have at least one background neighbor This definition is referred to as the

                                    inner border to distinguish it from the notion of outer border which is the corresponding

                                    border in the background

                                    Digital Image Processing

                                    Week 1

                                    Distance measures

                                    For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                    function or metric if

                                    (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                    (b) D(p q) = D(q p)

                                    (c) D(p z) le D(p q) + D(q z)

                                    The Euclidean distance between p and q is defined as 1

                                    2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                    The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                    centered at (x y)

                                    Digital Image Processing

                                    Week 1

                                    The D4 distance (also called city-block distance) between p and q is defined as

                                    4( ) | | | |D p q x s y t

                                    The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                    4

                                    22 1 2

                                    2 2 1 0 1 22 1 2

                                    2

                                    D

                                    The pixels with D4 = 1 are the 4-neighbors of (x y)

                                    The D8 distance (called the chessboard distance) between p and q is defined as

                                    8( ) max| | | |D p q x s y t

                                    The pixels q for which 8( )D p q r form a square centered at (x y)

                                    Digital Image Processing

                                    Week 1

                                    8

                                    2 2 2 2 22 1 1 1 2

                                    2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                    D

                                    The pixels with D8 = 1 are the 8-neighbors of (x y)

                                    D4 and D8 distances are independent of any paths that might exist between p and q

                                    because these distances involve only the coordinates of the point

                                    Digital Image Processing

                                    Week 1

                                    Array versus Matrix Operations

                                    An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                    11 12 11 12

                                    21 22 21 22

                                    a a b ba a b b

                                    Array product

                                    11 12 11 12 11 11 12 12

                                    21 22 21 22 21 21 22 21

                                    a a b b a b a ba a b b a b a b

                                    Matrix product

                                    11 12 11 12 11 11 12 21 11 12 12 21

                                    21 22 21 22 21 11 22 21 21 12 22 22

                                    a a b b a b a b a b a ba a b b a b a b a b a b

                                    We assume array operations unless stated otherwise

                                    Digital Image Processing

                                    Week 1

                                    Linear versus Nonlinear Operations

                                    One of the most important classifications of image-processing methods is whether it is

                                    linear or nonlinear

                                    ( ) ( )H f x y g x y

                                    H is said to be a linear operator if

                                    images1 2 1 2

                                    1 2

                                    ( ) ( ) ( ) ( )

                                    H a f x y b f x y a H f x y b H f x y

                                    a b f f

                                    Example of nonlinear operator

                                    the maximum value of the pixels of image max ( )H f f x y f

                                    1 2

                                    0 2 6 5 1 1

                                    2 3 4 7f f a b

                                    Digital Image Processing

                                    Week 1

                                    1 2

                                    0 2 6 5 6 3max max 1 ( 1) max 2

                                    2 3 4 7 2 4a f b f

                                    0 2 6 51 max ( 1) max 3 ( 1)7 4

                                    2 3 4 7

                                    Arithmetic Operations in Image Processing

                                    Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                    f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                    For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                    value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                    The two random variables are uncorrelated when their covariance is 0

                                    Digital Image Processing

                                    Week 1

                                    Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                    used in image enhancement)

                                    1

                                    1( ) ( )K

                                    ii

                                    g x y g x yK

                                    If the noise satisfies the properties stated above we have

                                    2 2( ) ( )

                                    1( ) ( ) g x y x yE g x y f x yK

                                    ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                    and g respectively The standard deviation (square root of the variance) at any point in

                                    the average image is

                                    ( ) ( )1

                                    g x y x yK

                                    Digital Image Processing

                                    Week 1

                                    As K increases the variability (as measured by the variance or the standard deviation) of

                                    the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                    means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                    averaging process increases

                                    An important application of image averaging is in the field of astronomy where imaging

                                    under very low light levels frequently causes sensor noise to render single images

                                    virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                    was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                    64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                    images respectively

                                    Digital Image Processing

                                    Week 1

                                    Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                    100 noisy images

                                    a b c d e f

                                    Digital Image Processing

                                    Week 1

                                    A frequent application of image subtraction is in the enhancement of differences between

                                    images

                                    (a) (b) (c)

                                    Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                    significant bit of each pixel (c) the difference between the two images

                                    Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                    Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                    images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                    difference between images (a) and (b)

                                    Digital Image Processing

                                    Week 1

                                    Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                    h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                    intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                    source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                    bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                    anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                    images after injection of the contrast medium

                                    In g(x y) we can find the differences between h and f as enhanced detail

                                    Images being captured at TV rates we obtain a movie showing how the contrast medium

                                    propagates through the various arteries in the area being observed

                                    Digital Image Processing

                                    Week 1

                                    a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                    Digital Image Processing

                                    Week 1

                                    An important application of image multiplication (and division) is shading correction

                                    Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                    f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                    When the shading function is known

                                    ( )( )( )

                                    g x yf x yh x y

                                    h(x y) is unknown but we have access to the imaging system we can obtain an

                                    approximation to the shading function by imaging a target of constant intensity When the

                                    sensor is not available often the shading pattern can be estimated from the image

                                    Digital Image Processing

                                    Week 1

                                    (a) (b) (c)

                                    Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                    Digital Image Processing

                                    Week 1

                                    Another use of image multiplication is in masking also called region of interest (ROI)

                                    operations The process consists of multiplying a given image by a mask image that has

                                    1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                    image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                    (a) (b) (c)

                                    Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                    Digital Image Processing

                                    Week 1

                                    In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                    min( )mf f f

                                    0 ( 255)max( )

                                    ms

                                    m

                                    ff K K K

                                    f

                                    Digital Image Processing

                                    Week 1

                                    Spatial Operations

                                    - are performed directly on the pixels of a given image

                                    There are three categories of spatial operations

                                    single-pixel operations

                                    neighborhood operations

                                    geometric spatial transformations

                                    Single-pixel operations

                                    - change the values of intensity for the individual pixels ( )s T z

                                    where z is the intensity of a pixel in the original image and s is the intensity of the

                                    corresponding pixel in the processed image

                                    Digital Image Processing

                                    Week 1

                                    Neighborhood operations

                                    Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                    in an image f Neighborhood processing generates new intensity level at point (x y)

                                    based on the values of the intensities of the points in Sxy For example if Sxy is a

                                    rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                    intensity by computing the average value of the pixels in Sxy

                                    ( )

                                    1( ) ( )xyr c S

                                    g x y f r cm n

                                    The net effect is to perform local blurring in the original image This type of process is

                                    used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                    largest region of an image

                                    Digital Image Processing

                                    Week 1

                                    Geometric spatial transformations and image registration

                                    - modify the spatial relationship between pixels in an image

                                    - these transformations are often called rubber-sheet transformations (analogous to

                                    printing an image on a sheet of rubber and then stretching the sheet according to a

                                    predefined set of rules

                                    A geometric transformation consists of 2 basic operations

                                    1 a spatial transformation of coordinates

                                    2 intensity interpolation that assign intensity values to the spatial transformed

                                    pixels

                                    The coordinate system transformation ( ) [( )]x y T v w

                                    (v w) ndash pixel coordinates in the original image

                                    (x y) ndash pixel coordinates in the transformed image

                                    Digital Image Processing

                                    Week 1

                                    [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                    Affine transform

                                    11 1211 21 31

                                    21 2212 22 33

                                    31 32

                                    0[ 1] [ 1] [ 1] 0

                                    1

                                    t tx t v t w t

                                    x y v w T v w t ty t v t w t

                                    t t

                                    (AT)

                                    This transform can scale rotate translate or shear a set of coordinate points depending

                                    on the elements of the matrix T If we want to resize an image rotate it and move the

                                    result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                    scaling rotation and translation matrices from Table 1

                                    Digital Image Processing

                                    Week 1

                                    Affine transformations

                                    Digital Image Processing

                                    Week 1

                                    The preceding transformations relocate pixels on an image to new locations To complete

                                    the process we have to assign intensity values to those locations This task is done by

                                    using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                    In practice we can use equation (AT) in two basic ways

                                    forward mapping scan the pixels of the input image (v w) compute the new spatial

                                    location (x y) of the corresponding pixel in the new image using (AT) directly

                                    Problems

                                    - intensity assignment when 2 or more pixels in the original image are transformed to

                                    the same location in the output image

                                    - some output locations have no correspondent in the original image (no intensity

                                    assignment)

                                    Digital Image Processing

                                    Week 1

                                    inverse mapping scans the output pixel locations and at each location (x y)

                                    computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                    It then interpolates among the nearest input pixels to determine the intensity of the output

                                    pixel value

                                    Inverse mappings are more efficient to implement than forward mappings and are used in

                                    numerous commercial implementations of spatial transformations (MATLAB for ex)

                                    Digital Image Processing

                                    Week 1

                                    Digital Image Processing

                                    Week 1

                                    Image registration ndash align two or more images of the same scene

                                    In image registration we have available the input and output images but the specific

                                    transformation that produced the output image from the input is generally unknown

                                    The problem is to estimate the transformation function and then use it to register the two

                                    images

                                    - it may be of interest to align (register) two or more image taken at approximately the

                                    same time but using different imaging systems (MRI scanner and a PET scanner)

                                    - align images of a given location taken by the same instrument at different moments

                                    of time (satellite images)

                                    Solving the problem using tie points (also called control points) which are

                                    corresponding points whose locations are known precisely in the input and reference

                                    image

                                    Digital Image Processing

                                    Week 1

                                    How to select tie points

                                    - interactively selecting them

                                    - use of algorithms that try to detect these points

                                    - some imaging systems have physical artifacts (small metallic objects) embedded in

                                    the imaging sensors These objects produce a set of known points (called reseau

                                    marks) directly on all images captured by the system which can be used as guides

                                    for establishing tie points

                                    The problem of estimating the transformation is one of modeling Suppose we have a set

                                    of 4 tie points both on the input image and the reference image A simple model based on

                                    a bilinear approximation is given by

                                    1 2 3 4

                                    5 6 7 8

                                    x c v c w c v w cy c v c w c v w c

                                    (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                    Digital Image Processing

                                    Week 1

                                    When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                    frequently is to select a larger number of tie points and using this new set of tie points

                                    subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                    subregions marked by 4 tie points we applied the transformation model described above

                                    The number of tie points and the sophistication of the model required to solve the register

                                    problem depend on the severity of the geometrical distortion

                                    Digital Image Processing

                                    Week 1

                                    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                    Digital Image Processing

                                    Week 1

                                    Probabilistic Methods

                                    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                    p(zk) = the probability that the intensity level zk occurs in the given image

                                    ( ) kk

                                    np zM N

                                    nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                    pixels in the image) 1

                                    0( ) 1

                                    L

                                    kk

                                    p z

                                    The mean (average) intensity of an image is given by 1

                                    0( )

                                    L

                                    k kk

                                    m z p z

                                    Digital Image Processing

                                    Week 1

                                    The variance of the intensities is 1

                                    2 2

                                    0( ) ( )

                                    L

                                    k kk

                                    z m p z

                                    The variance is a measure of the spread of the values of z about the mean so it is a

                                    measure of image contrast Usually for measuring image contrast the standard deviation

                                    ( ) is used

                                    The n-th moment of a random variable z about the mean is defined as 1

                                    0( ) ( ) ( )

                                    Ln

                                    n k kk

                                    z z m p z

                                    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                    3( ) 0z the intensities are biased to values higher than the mean

                                    ( 3( ) 0z the intensities are biased to values lower than the mean

                                    Digital Image Processing

                                    Week 1

                                    3( ) 0z the intensities are distributed approximately equally on both side of the

                                    mean

                                    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                    Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                    Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                    Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                    Digital Image Processing

                                    Week 1

                                    Intensity Transformations and Spatial Filtering

                                    ( ) ( )g x y T f x y

                                    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                    neighborhood of (x y)

                                    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                    and much smaller in size than the image

                                    Digital Image Processing

                                    Week 1

                                    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                    called spatial filter (spatial mask kernel template or window)

                                    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                    ( )s T r

                                    s and r are denoting respectively the intensity of g and f at (x y)

                                    Figure 2 left - T produces an output image of higher contrast than the original by

                                    darkening the intensity levels below k and brightening the levels above k ndash this technique

                                    is called contrast stretching

                                    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                    Digital Image Processing

                                    Week 1

                                    Figure 2 right - T produces a binary output image A mapping of this form is called

                                    thresholding function

                                    Some Basic Intensity Transformation Functions

                                    Image Negatives

                                    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                    - equivalent of a photographic negative

                                    - technique suited for enhancing white or gray detail embedded in dark regions of an

                                    image

                                    Digital Image Processing

                                    Week 1

                                    Original Negative image

                                    Digital Image Processing

                                    Week 1

                                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                    Some basic intensity transformation functions

                                    Digital Image Processing

                                    Week 1

                                    This transformation maps a narrow range of low intensity values in the input into a wider

                                    range An operator of this type is used to expand the values of dark pixels in an image

                                    while compressing the higher-level values The opposite is true for the inverse log

                                    transformation The log functions compress the dynamic range of images with large

                                    variations in pixel values

                                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                    Digital Image Processing

                                    Week 1

                                    Power-Law (Gamma) Transformations

                                    - positive constants( ) ( ( ) )s T r c r c s c r

                                    Plots of gamma transformation for different values of γ (c=1)

                                    Digital Image Processing

                                    Week 1

                                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                                    of output values with the opposite being true for higher values of input values The

                                    curves with 1 have the opposite effect of those generated with values of 1

                                    1c - identity transformation

                                    A variety of devices used for image capture printing and display respond according to a

                                    power law The process used to correct these power-law response phenomena is called

                                    gamma correction

                                    Digital Image Processing

                                    Week 1

                                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                    Digital Image Processing

                                    Week 1

                                    Piecewise-Linear Transformations Functions

                                    Contrast stretching

                                    - a process that expands the range of intensity levels in an image so it spans the full

                                    intensity range of the recording tool or display device

                                    a b c d Fig5

                                    Digital Image Processing

                                    Week 1

                                    11

                                    1

                                    2 1 1 21 2

                                    2 1 2 1

                                    22

                                    2

                                    [0 ]

                                    ( ) ( )( ) [ ]( ) ( )

                                    ( 1 ) [ 1]( 1 )

                                    s r r rrs r r s r rT r r r r

                                    r r r rs L r r r L

                                    L r

                                    Digital Image Processing

                                    Week 1

                                    1 1 2 2r s r s identity transformation (no change)

                                    1 2 1 2 0 1r r s s L thresholding function

                                    Figure 5(b) shows an 8-bit image with low contrast

                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                    in the image respectively Thus the transformation function stretched the levels linearly

                                    from their original range to the full range [0 L-1]

                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                    2 2 1r s m L where m is the mean gray level in the image

                                    The original image on which these results are based is a scanning electron microscope

                                    image of pollen magnified approximately 700 times

                                    Digital Image Processing

                                    Week 1

                                    Intensity-level slicing

                                    - highlighting a specific range of intensities in an image

                                    There are two approaches for intensity-level slicing

                                    1 display in one value (white for example) all the values in the range of interest and in

                                    another (say black) all other intensities (Figure 311 (a))

                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                    intensities in the image (Figure 311 (b))

                                    Digital Image Processing

                                    Week 1

                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                    the top of the scale of intensities This type of enhancement produces a binary image

                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                    Highlights range [A B] and preserves all other intensities

                                    Digital Image Processing

                                    Week 1

                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                    blockageshellip)

                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                    image around the mean intensity was set to black the other intensities remain unchanged

                                    Fig 6 - Aortic angiogram and intensity sliced versions

                                    Digital Image Processing

                                    Week 1

                                    Bit-plane slicing

                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                    This technique highlights the contribution made to the whole image appearances by each

                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                    Digital Image Processing

                                    Week 1

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                                    Week 1

                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                    • DIP 1 2017
                                    • DIP 02 (2017)

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Digital Image ProcessingDigital Image Processing

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                                      Gamma-Ray Imaging

                                      Nuclear medicine astronomical observations

                                      Nuclear medicine

                                      the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

                                      Images are produced from the emissions collected by gamma-ray detectors

                                      Images of this sort are used to locate sites of bone pathology (infections tumors)

                                      PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Examples of gamma-ray imaging

                                      Bone scan PET image

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      X-ray imaging

                                      Medical diagnosticindustry astronomy

                                      A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                                      The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Angiography = contrast-enhancement radiography

                                      Angiograms = images of blood vessels

                                      A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                                      X-rays are used in CAT (computerized axial tomography)

                                      X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                                      Industrial CAT scans are useful when the parts can be penetreted by X-rays

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Examples of X-ray imaging

                                      Chest X-rayAortic angiogram

                                      Head CT Cygnus LoopCircuit boards

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Imaging in the Ultraviolet Band

                                      Litography industrial inspection microscopy biological imaging astronomical observations

                                      Ultraviolet light is used in fluorescence microscopy

                                      Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                                      other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                                      and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Imaging in the Visible and Infrared Bands

                                      Light microscopy astronomy remote sensing industry law enforcement

                                      LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                                      Weather observations and prediction produce major applications of multispectral image from satellites

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Satellite images of Washington DC area in spectral bands of the Table 1

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Examples of light microscopy

                                      Taxol (anticancer agent)magnified 250X

                                      Cholesterol(40X)

                                      Microprocessor(60X)

                                      Nickel oxidethin film(600X)

                                      Surface of audio CD(1750X)

                                      Organicsuperconductor(450X)

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Automated visual inspection of manufactured goods

                                      a bc de f

                                      a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                      Digital Image ProcessingDigital Image Processing

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                                      Imaging in the Microwave Band

                                      The dominant aplication of imaging in the microwave band ndash radar

                                      bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                      bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                      bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                      An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                      Digital Image ProcessingDigital Image Processing

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                                      Spaceborne radar image of mountains in southeast Tibet

                                      Digital Image ProcessingDigital Image Processing

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                                      Imaging in the Radio Band

                                      medicine astronomy

                                      MRI = Magnetic Resonance Imaging

                                      This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                      Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                      The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      MRI images of a human knee (left) and spine (right)

                                      Digital Image ProcessingDigital Image Processing

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                                      Images of the Crab Pulsar covering the electromagnetic spectrum

                                      Gamma X-ray Optical Infrared Radio

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Other Imaging Modalities

                                      acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                      Imaging using sound geological explorations industry medicine

                                      Mineral and oil exploration

                                      For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                      Digital Image ProcessingDigital Image Processing

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                                      Biometry - iris

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                                      Biometry - fingerprint

                                      Digital Image ProcessingDigital Image Processing

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                                      Face detection and recognition

                                      Digital Image ProcessingDigital Image Processing

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                                      Gender identification

                                      Digital Image ProcessingDigital Image Processing

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                                      Image morphing

                                      Digital Image ProcessingDigital Image Processing

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                                      Fundamental Steps in DIP

                                      methods whose input and output are images

                                      methods whose inputs are images but whose outputs are attributes extracted from those images

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Outputs are images

                                      bull image acquisition

                                      bull image filtering and enhancement

                                      bull image restoration

                                      bull color image processing

                                      bull wavelets and multiresolution processing

                                      bull compression

                                      bull morphological processing

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Outputs are attributes

                                      bull morphological processing

                                      bull segmentation

                                      bull representation and description

                                      bull object recognition

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Image acquisition - may involve preprocessing such as scaling

                                      Image enhancement

                                      bull manipulating an image so that the result is more suitable than the original for a specific operation

                                      bull enhancement is problem oriented

                                      bull there is no general sbquotheoryrsquo of image enhancement

                                      bull enhancement use subjective methods for image emprovement

                                      bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Image restoration

                                      bull improving the appearance of an image

                                      bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                      Color image processing

                                      bull fundamental concept in color models

                                      bull basic color processing in a digital domain

                                      Wavelets and multiresolution processing

                                      representing images in various degree of resolution

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Compression

                                      reducing the storage required to save an image or the bandwidth required to transmit it

                                      Morphological processing

                                      bull tools for extracting image components that are useful in the representation and description of shape

                                      bull a transition from processes that output images to processes that outputimage attributes

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                                      Week 1Week 1

                                      Segmentation

                                      bull partitioning an image into its constituents parts or objects

                                      bull autonomous segmentation is one of the most difficult tasks of DIP

                                      bull the more accurate the segmentation the more likley recognition is to succeed

                                      Representation and description (almost always follows segmentation)

                                      bull segmentation produces either the boundary of a region or all the poits in the region itself

                                      bull converting the data produced by segmentation to a form suitable for computer processing

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                      bull complete region the focus is on internal properties such as texture or skeletal shape

                                      bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                      Object recognition

                                      the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                      Knowledge database

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Simplified diagramof a cross sectionof the human eye

                                      Digital Image ProcessingDigital Image Processing

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                                      Week 1Week 1

                                      Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                      The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                      Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                      The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                      The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                      Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                      Fovea = the place where the image of the object of interest falls on

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                      Blind spot region without receptors

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Image formation in the eye

                                      Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                      Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                      distance between lens and retina along visual axix = 17 mm

                                      range of focal length = 14 mm to 17 mm

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

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                                      Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Optical illusions

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                      gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                      quantities that describe the quality of a chromatic light source radiance

                                      the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                      measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                      brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      the physical meaning is determined by the source of the image

                                      ( )f D f x y

                                      Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                      f(xy) ndash characterized by two components

                                      i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                      r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                      ( ) ( ) ( )

                                      0 ( ) 0 ( ) 1

                                      f x y i x y r x y

                                      i x y r x y

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                      i(xy) ndash determined by the illumination source

                                      r(xy) ndash determined by the characteristics of the imaged objects

                                      is called gray (or intensity) scale

                                      In practice

                                      min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                      indoor values without additional illuminationmin max10 1000L L

                                      black whitemin max0 1 0 1 0 1L L L L l l L

                                      min maxL L

                                      Digital Image ProcessingDigital Image Processing

                                      Week 1Week 1

                                      Digital Image Processing

                                      Week 1

                                      Image Sampling and Quantization

                                      - the output of the sensors is a continuous voltage waveform related to the sensed

                                      scene

                                      converting a continuous image f to digital form

                                      - digitizing (x y) is called sampling

                                      - digitizing f(x y) is called quantization

                                      Digital Image Processing

                                      Week 1

                                      Digital Image Processing

                                      Week 1

                                      Continuous image projected onto a sensor array Result of image sampling and quantization

                                      Digital Image Processing

                                      Week 1

                                      Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                      (00) (01) (0 1)(10) (11) (1 1)

                                      ( )

                                      ( 10) ( 11) ( 1 1)

                                      f f f Nf f f N

                                      f x y

                                      f M f M f M N

                                      image element pixel

                                      00 01 0 1

                                      10 11 1 1

                                      10 11 1 1

                                      ( ) ( )

                                      N

                                      i jN M N

                                      i j

                                      M M M N

                                      a a aa f x i y j f i ja a a

                                      Aa

                                      a a a

                                      f(00) ndash the upper left corner of the image

                                      Digital Image Processing

                                      Week 1

                                      M N ge 0 L=2k

                                      [0 1]i j i ja a L

                                      Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                      Digital Image Processing

                                      Week 1

                                      Digital Image Processing

                                      Week 1

                                      Number of bits required to store a digitized image

                                      for 2 b M N k M N b N k

                                      When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                      Digital Image Processing

                                      Week 1

                                      Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                      Measures line pairs per unit distance dots (pixels) per unit distance

                                      Image resolution = the largest number of discernible line pairs per unit distance

                                      (eg 100 line pairs per mm)

                                      Dots per unit distance are commonly used in printing and publishing

                                      In US the measure is expressed in dots per inch (dpi)

                                      (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                      Intensity resolution ndash the smallest discernible change in intensity level

                                      The number of intensity levels (L) is determined by hardware considerations

                                      L=2k ndash most common k = 8

                                      Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                      Digital Image Processing

                                      Week 1

                                      Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                      150 dpi (lower left) 72 dpi (lower right)

                                      Digital Image Processing

                                      Week 1

                                      Reducing the number of gray levels 256 128 64 32

                                      Digital Image Processing

                                      Week 1

                                      Reducing the number of gray levels 16 8 4 2

                                      Digital Image Processing

                                      Week 1

                                      Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                      Shrinking zooming ndash image resizing ndash image resampling methods

                                      Interpolation is the process of using known data to estimate values at unknown locations

                                      Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                      750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                      same spacing as the original and then shrink it so that it fits exactly over the original

                                      image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                      Problem assignment of intensity-level in the new 750 times 750 grid

                                      Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                      intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                      This technique has the tendency to produce undesirable effects like severe distortion of

                                      straight edges

                                      Digital Image Processing

                                      Week 1

                                      Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                      where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                      be written using the 4 nearest neighbors of point (x y)

                                      Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                      modest increase in computational effort

                                      Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                      nearest neighbors of the point 3 3

                                      0 0

                                      ( ) i ji j

                                      i jv x y c x y

                                      The coefficients cij are obtained solving a 16x16 linear system

                                      intensity levels of the 16 nearest neighbors of 3 3

                                      0 0

                                      ( )i ji j

                                      i jc x y x y

                                      Digital Image Processing

                                      Week 1

                                      Generally bicubic interpolation does a better job of preserving fine detail than the

                                      bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                      programs such as Adobe Photoshop and Corel Photopaint

                                      Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                      the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                      then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                      neighbor interpolation was used (both for shrinking and zooming)

                                      Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                      interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                      from 1250 dpi to 150 dpi (instead of 72 dpi)

                                      Digital Image Processing

                                      Week 1

                                      Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                      Digital Image Processing

                                      Week 1

                                      Neighbors of a Pixel

                                      A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                      This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                      The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                      and are denoted ND(p)

                                      The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                      N8 (p)

                                      If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                      fall outside the image

                                      Digital Image Processing

                                      Week 1

                                      Adjacency Connectivity Regions Boundaries

                                      Denote by V the set of intensity levels used to define adjacency

                                      - in a binary image V 01 (V=0 V=1)

                                      - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                      We consider 3 types of adjacency

                                      (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                      (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                      (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                      m-adjacent if

                                      4( )q N p or

                                      ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                      Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                      ambiguities that often arise when 8-adjacency is used Consider the example

                                      Digital Image Processing

                                      Week 1

                                      binary image

                                      0 1 1 0 1 1 0 1 1

                                      1 0 1 0 0 1 0 0 1 0

                                      0 0 1 0 0 1 0 0 1

                                      V

                                      The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                      8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                      m-adjacency

                                      Digital Image Processing

                                      Week 1

                                      A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                      is a sequence of distinct pixels with coordinates

                                      and are adjacent 0 0 1 1

                                      1 1

                                      ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                      n n

                                      i i i i

                                      x y x y x y x y s tx y x y i n

                                      The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                      Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                      Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                      in S if there exists a path between them consisting only of pixels from S

                                      S is a connected set if there is a path in S between any 2 pixels in S

                                      Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                      Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                      that are not adjacent are said to be disjoint When referring to regions only 4- and

                                      8-adjacency are considered

                                      Digital Image Processing

                                      Week 1

                                      Suppose that an image contains K disjoint regions 1 kR k K none of which

                                      touches the image border

                                      the complement of 1

                                      ( )K

                                      cu k u u

                                      k

                                      R R R R

                                      We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                      background of the image

                                      The boundary (border or contour) of a region R is the set of points that are adjacent to

                                      points in the complement of R (R)c The border of an image is the set of pixels in the

                                      region that have at least one background neighbor This definition is referred to as the

                                      inner border to distinguish it from the notion of outer border which is the corresponding

                                      border in the background

                                      Digital Image Processing

                                      Week 1

                                      Distance measures

                                      For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                      function or metric if

                                      (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                      (b) D(p q) = D(q p)

                                      (c) D(p z) le D(p q) + D(q z)

                                      The Euclidean distance between p and q is defined as 1

                                      2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                      The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                      centered at (x y)

                                      Digital Image Processing

                                      Week 1

                                      The D4 distance (also called city-block distance) between p and q is defined as

                                      4( ) | | | |D p q x s y t

                                      The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                      4

                                      22 1 2

                                      2 2 1 0 1 22 1 2

                                      2

                                      D

                                      The pixels with D4 = 1 are the 4-neighbors of (x y)

                                      The D8 distance (called the chessboard distance) between p and q is defined as

                                      8( ) max| | | |D p q x s y t

                                      The pixels q for which 8( )D p q r form a square centered at (x y)

                                      Digital Image Processing

                                      Week 1

                                      8

                                      2 2 2 2 22 1 1 1 2

                                      2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                      D

                                      The pixels with D8 = 1 are the 8-neighbors of (x y)

                                      D4 and D8 distances are independent of any paths that might exist between p and q

                                      because these distances involve only the coordinates of the point

                                      Digital Image Processing

                                      Week 1

                                      Array versus Matrix Operations

                                      An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                      11 12 11 12

                                      21 22 21 22

                                      a a b ba a b b

                                      Array product

                                      11 12 11 12 11 11 12 12

                                      21 22 21 22 21 21 22 21

                                      a a b b a b a ba a b b a b a b

                                      Matrix product

                                      11 12 11 12 11 11 12 21 11 12 12 21

                                      21 22 21 22 21 11 22 21 21 12 22 22

                                      a a b b a b a b a b a ba a b b a b a b a b a b

                                      We assume array operations unless stated otherwise

                                      Digital Image Processing

                                      Week 1

                                      Linear versus Nonlinear Operations

                                      One of the most important classifications of image-processing methods is whether it is

                                      linear or nonlinear

                                      ( ) ( )H f x y g x y

                                      H is said to be a linear operator if

                                      images1 2 1 2

                                      1 2

                                      ( ) ( ) ( ) ( )

                                      H a f x y b f x y a H f x y b H f x y

                                      a b f f

                                      Example of nonlinear operator

                                      the maximum value of the pixels of image max ( )H f f x y f

                                      1 2

                                      0 2 6 5 1 1

                                      2 3 4 7f f a b

                                      Digital Image Processing

                                      Week 1

                                      1 2

                                      0 2 6 5 6 3max max 1 ( 1) max 2

                                      2 3 4 7 2 4a f b f

                                      0 2 6 51 max ( 1) max 3 ( 1)7 4

                                      2 3 4 7

                                      Arithmetic Operations in Image Processing

                                      Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                      f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                      For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                      value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                      The two random variables are uncorrelated when their covariance is 0

                                      Digital Image Processing

                                      Week 1

                                      Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                      used in image enhancement)

                                      1

                                      1( ) ( )K

                                      ii

                                      g x y g x yK

                                      If the noise satisfies the properties stated above we have

                                      2 2( ) ( )

                                      1( ) ( ) g x y x yE g x y f x yK

                                      ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                      and g respectively The standard deviation (square root of the variance) at any point in

                                      the average image is

                                      ( ) ( )1

                                      g x y x yK

                                      Digital Image Processing

                                      Week 1

                                      As K increases the variability (as measured by the variance or the standard deviation) of

                                      the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                      means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                      averaging process increases

                                      An important application of image averaging is in the field of astronomy where imaging

                                      under very low light levels frequently causes sensor noise to render single images

                                      virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                      was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                      64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                      images respectively

                                      Digital Image Processing

                                      Week 1

                                      Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                      100 noisy images

                                      a b c d e f

                                      Digital Image Processing

                                      Week 1

                                      A frequent application of image subtraction is in the enhancement of differences between

                                      images

                                      (a) (b) (c)

                                      Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                      significant bit of each pixel (c) the difference between the two images

                                      Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                      Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                      images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                      difference between images (a) and (b)

                                      Digital Image Processing

                                      Week 1

                                      Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                      h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                      intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                      source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                      bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                      anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                      images after injection of the contrast medium

                                      In g(x y) we can find the differences between h and f as enhanced detail

                                      Images being captured at TV rates we obtain a movie showing how the contrast medium

                                      propagates through the various arteries in the area being observed

                                      Digital Image Processing

                                      Week 1

                                      a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                      Digital Image Processing

                                      Week 1

                                      An important application of image multiplication (and division) is shading correction

                                      Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                      f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                      When the shading function is known

                                      ( )( )( )

                                      g x yf x yh x y

                                      h(x y) is unknown but we have access to the imaging system we can obtain an

                                      approximation to the shading function by imaging a target of constant intensity When the

                                      sensor is not available often the shading pattern can be estimated from the image

                                      Digital Image Processing

                                      Week 1

                                      (a) (b) (c)

                                      Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                      Digital Image Processing

                                      Week 1

                                      Another use of image multiplication is in masking also called region of interest (ROI)

                                      operations The process consists of multiplying a given image by a mask image that has

                                      1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                      image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                      (a) (b) (c)

                                      Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                      Digital Image Processing

                                      Week 1

                                      In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                      min( )mf f f

                                      0 ( 255)max( )

                                      ms

                                      m

                                      ff K K K

                                      f

                                      Digital Image Processing

                                      Week 1

                                      Spatial Operations

                                      - are performed directly on the pixels of a given image

                                      There are three categories of spatial operations

                                      single-pixel operations

                                      neighborhood operations

                                      geometric spatial transformations

                                      Single-pixel operations

                                      - change the values of intensity for the individual pixels ( )s T z

                                      where z is the intensity of a pixel in the original image and s is the intensity of the

                                      corresponding pixel in the processed image

                                      Digital Image Processing

                                      Week 1

                                      Neighborhood operations

                                      Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                      in an image f Neighborhood processing generates new intensity level at point (x y)

                                      based on the values of the intensities of the points in Sxy For example if Sxy is a

                                      rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                      intensity by computing the average value of the pixels in Sxy

                                      ( )

                                      1( ) ( )xyr c S

                                      g x y f r cm n

                                      The net effect is to perform local blurring in the original image This type of process is

                                      used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                      largest region of an image

                                      Digital Image Processing

                                      Week 1

                                      Geometric spatial transformations and image registration

                                      - modify the spatial relationship between pixels in an image

                                      - these transformations are often called rubber-sheet transformations (analogous to

                                      printing an image on a sheet of rubber and then stretching the sheet according to a

                                      predefined set of rules

                                      A geometric transformation consists of 2 basic operations

                                      1 a spatial transformation of coordinates

                                      2 intensity interpolation that assign intensity values to the spatial transformed

                                      pixels

                                      The coordinate system transformation ( ) [( )]x y T v w

                                      (v w) ndash pixel coordinates in the original image

                                      (x y) ndash pixel coordinates in the transformed image

                                      Digital Image Processing

                                      Week 1

                                      [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                      Affine transform

                                      11 1211 21 31

                                      21 2212 22 33

                                      31 32

                                      0[ 1] [ 1] [ 1] 0

                                      1

                                      t tx t v t w t

                                      x y v w T v w t ty t v t w t

                                      t t

                                      (AT)

                                      This transform can scale rotate translate or shear a set of coordinate points depending

                                      on the elements of the matrix T If we want to resize an image rotate it and move the

                                      result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                      scaling rotation and translation matrices from Table 1

                                      Digital Image Processing

                                      Week 1

                                      Affine transformations

                                      Digital Image Processing

                                      Week 1

                                      The preceding transformations relocate pixels on an image to new locations To complete

                                      the process we have to assign intensity values to those locations This task is done by

                                      using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                      In practice we can use equation (AT) in two basic ways

                                      forward mapping scan the pixels of the input image (v w) compute the new spatial

                                      location (x y) of the corresponding pixel in the new image using (AT) directly

                                      Problems

                                      - intensity assignment when 2 or more pixels in the original image are transformed to

                                      the same location in the output image

                                      - some output locations have no correspondent in the original image (no intensity

                                      assignment)

                                      Digital Image Processing

                                      Week 1

                                      inverse mapping scans the output pixel locations and at each location (x y)

                                      computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                      It then interpolates among the nearest input pixels to determine the intensity of the output

                                      pixel value

                                      Inverse mappings are more efficient to implement than forward mappings and are used in

                                      numerous commercial implementations of spatial transformations (MATLAB for ex)

                                      Digital Image Processing

                                      Week 1

                                      Digital Image Processing

                                      Week 1

                                      Image registration ndash align two or more images of the same scene

                                      In image registration we have available the input and output images but the specific

                                      transformation that produced the output image from the input is generally unknown

                                      The problem is to estimate the transformation function and then use it to register the two

                                      images

                                      - it may be of interest to align (register) two or more image taken at approximately the

                                      same time but using different imaging systems (MRI scanner and a PET scanner)

                                      - align images of a given location taken by the same instrument at different moments

                                      of time (satellite images)

                                      Solving the problem using tie points (also called control points) which are

                                      corresponding points whose locations are known precisely in the input and reference

                                      image

                                      Digital Image Processing

                                      Week 1

                                      How to select tie points

                                      - interactively selecting them

                                      - use of algorithms that try to detect these points

                                      - some imaging systems have physical artifacts (small metallic objects) embedded in

                                      the imaging sensors These objects produce a set of known points (called reseau

                                      marks) directly on all images captured by the system which can be used as guides

                                      for establishing tie points

                                      The problem of estimating the transformation is one of modeling Suppose we have a set

                                      of 4 tie points both on the input image and the reference image A simple model based on

                                      a bilinear approximation is given by

                                      1 2 3 4

                                      5 6 7 8

                                      x c v c w c v w cy c v c w c v w c

                                      (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                      Digital Image Processing

                                      Week 1

                                      When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                      frequently is to select a larger number of tie points and using this new set of tie points

                                      subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                      subregions marked by 4 tie points we applied the transformation model described above

                                      The number of tie points and the sophistication of the model required to solve the register

                                      problem depend on the severity of the geometrical distortion

                                      Digital Image Processing

                                      Week 1

                                      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                      Digital Image Processing

                                      Week 1

                                      Probabilistic Methods

                                      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                      p(zk) = the probability that the intensity level zk occurs in the given image

                                      ( ) kk

                                      np zM N

                                      nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                      pixels in the image) 1

                                      0( ) 1

                                      L

                                      kk

                                      p z

                                      The mean (average) intensity of an image is given by 1

                                      0( )

                                      L

                                      k kk

                                      m z p z

                                      Digital Image Processing

                                      Week 1

                                      The variance of the intensities is 1

                                      2 2

                                      0( ) ( )

                                      L

                                      k kk

                                      z m p z

                                      The variance is a measure of the spread of the values of z about the mean so it is a

                                      measure of image contrast Usually for measuring image contrast the standard deviation

                                      ( ) is used

                                      The n-th moment of a random variable z about the mean is defined as 1

                                      0( ) ( ) ( )

                                      Ln

                                      n k kk

                                      z z m p z

                                      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                      3( ) 0z the intensities are biased to values higher than the mean

                                      ( 3( ) 0z the intensities are biased to values lower than the mean

                                      Digital Image Processing

                                      Week 1

                                      3( ) 0z the intensities are distributed approximately equally on both side of the

                                      mean

                                      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                      Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                      Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                      Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                      Digital Image Processing

                                      Week 1

                                      Intensity Transformations and Spatial Filtering

                                      ( ) ( )g x y T f x y

                                      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                      neighborhood of (x y)

                                      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                      and much smaller in size than the image

                                      Digital Image Processing

                                      Week 1

                                      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                      called spatial filter (spatial mask kernel template or window)

                                      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                      ( )s T r

                                      s and r are denoting respectively the intensity of g and f at (x y)

                                      Figure 2 left - T produces an output image of higher contrast than the original by

                                      darkening the intensity levels below k and brightening the levels above k ndash this technique

                                      is called contrast stretching

                                      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                      Digital Image Processing

                                      Week 1

                                      Figure 2 right - T produces a binary output image A mapping of this form is called

                                      thresholding function

                                      Some Basic Intensity Transformation Functions

                                      Image Negatives

                                      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                      - equivalent of a photographic negative

                                      - technique suited for enhancing white or gray detail embedded in dark regions of an

                                      image

                                      Digital Image Processing

                                      Week 1

                                      Original Negative image

                                      Digital Image Processing

                                      Week 1

                                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                      Some basic intensity transformation functions

                                      Digital Image Processing

                                      Week 1

                                      This transformation maps a narrow range of low intensity values in the input into a wider

                                      range An operator of this type is used to expand the values of dark pixels in an image

                                      while compressing the higher-level values The opposite is true for the inverse log

                                      transformation The log functions compress the dynamic range of images with large

                                      variations in pixel values

                                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                      Digital Image Processing

                                      Week 1

                                      Power-Law (Gamma) Transformations

                                      - positive constants( ) ( ( ) )s T r c r c s c r

                                      Plots of gamma transformation for different values of γ (c=1)

                                      Digital Image Processing

                                      Week 1

                                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                                      of output values with the opposite being true for higher values of input values The

                                      curves with 1 have the opposite effect of those generated with values of 1

                                      1c - identity transformation

                                      A variety of devices used for image capture printing and display respond according to a

                                      power law The process used to correct these power-law response phenomena is called

                                      gamma correction

                                      Digital Image Processing

                                      Week 1

                                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                      Digital Image Processing

                                      Week 1

                                      Piecewise-Linear Transformations Functions

                                      Contrast stretching

                                      - a process that expands the range of intensity levels in an image so it spans the full

                                      intensity range of the recording tool or display device

                                      a b c d Fig5

                                      Digital Image Processing

                                      Week 1

                                      11

                                      1

                                      2 1 1 21 2

                                      2 1 2 1

                                      22

                                      2

                                      [0 ]

                                      ( ) ( )( ) [ ]( ) ( )

                                      ( 1 ) [ 1]( 1 )

                                      s r r rrs r r s r rT r r r r

                                      r r r rs L r r r L

                                      L r

                                      Digital Image Processing

                                      Week 1

                                      1 1 2 2r s r s identity transformation (no change)

                                      1 2 1 2 0 1r r s s L thresholding function

                                      Figure 5(b) shows an 8-bit image with low contrast

                                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                      in the image respectively Thus the transformation function stretched the levels linearly

                                      from their original range to the full range [0 L-1]

                                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                      2 2 1r s m L where m is the mean gray level in the image

                                      The original image on which these results are based is a scanning electron microscope

                                      image of pollen magnified approximately 700 times

                                      Digital Image Processing

                                      Week 1

                                      Intensity-level slicing

                                      - highlighting a specific range of intensities in an image

                                      There are two approaches for intensity-level slicing

                                      1 display in one value (white for example) all the values in the range of interest and in

                                      another (say black) all other intensities (Figure 311 (a))

                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                      intensities in the image (Figure 311 (b))

                                      Digital Image Processing

                                      Week 1

                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                      the top of the scale of intensities This type of enhancement produces a binary image

                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                      Highlights range [A B] and preserves all other intensities

                                      Digital Image Processing

                                      Week 1

                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                      blockageshellip)

                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                      image around the mean intensity was set to black the other intensities remain unchanged

                                      Fig 6 - Aortic angiogram and intensity sliced versions

                                      Digital Image Processing

                                      Week 1

                                      Bit-plane slicing

                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                      This technique highlights the contribution made to the whole image appearances by each

                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                      Digital Image Processing

                                      Week 1

                                      Digital Image Processing

                                      Week 1

                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                      • DIP 1 2017
                                      • DIP 02 (2017)

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Gamma-Ray Imaging

                                        Nuclear medicine astronomical observations

                                        Nuclear medicine

                                        the approach is to inject a patient with a radioactive isotope that emits gamma rays as it decays

                                        Images are produced from the emissions collected by gamma-ray detectors

                                        Images of this sort are used to locate sites of bone pathology (infections tumors)

                                        PET (positron emision tomography) ndash the patient is given a radioactive isotope that emits positrons as it decays

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Examples of gamma-ray imaging

                                        Bone scan PET image

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        X-ray imaging

                                        Medical diagnosticindustry astronomy

                                        A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                                        The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Angiography = contrast-enhancement radiography

                                        Angiograms = images of blood vessels

                                        A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                                        X-rays are used in CAT (computerized axial tomography)

                                        X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                                        Industrial CAT scans are useful when the parts can be penetreted by X-rays

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Examples of X-ray imaging

                                        Chest X-rayAortic angiogram

                                        Head CT Cygnus LoopCircuit boards

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Imaging in the Ultraviolet Band

                                        Litography industrial inspection microscopy biological imaging astronomical observations

                                        Ultraviolet light is used in fluorescence microscopy

                                        Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                                        other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                                        and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Imaging in the Visible and Infrared Bands

                                        Light microscopy astronomy remote sensing industry law enforcement

                                        LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                                        Weather observations and prediction produce major applications of multispectral image from satellites

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Satellite images of Washington DC area in spectral bands of the Table 1

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Examples of light microscopy

                                        Taxol (anticancer agent)magnified 250X

                                        Cholesterol(40X)

                                        Microprocessor(60X)

                                        Nickel oxidethin film(600X)

                                        Surface of audio CD(1750X)

                                        Organicsuperconductor(450X)

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Automated visual inspection of manufactured goods

                                        a bc de f

                                        a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Imaging in the Microwave Band

                                        The dominant aplication of imaging in the microwave band ndash radar

                                        bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                        bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                        bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                        An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Spaceborne radar image of mountains in southeast Tibet

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Imaging in the Radio Band

                                        medicine astronomy

                                        MRI = Magnetic Resonance Imaging

                                        This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                        Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                        The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                        Digital Image ProcessingDigital Image Processing

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                                        MRI images of a human knee (left) and spine (right)

                                        Digital Image ProcessingDigital Image Processing

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                                        Images of the Crab Pulsar covering the electromagnetic spectrum

                                        Gamma X-ray Optical Infrared Radio

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                                        Other Imaging Modalities

                                        acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                        Imaging using sound geological explorations industry medicine

                                        Mineral and oil exploration

                                        For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

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                                        Biometry - iris

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                                        Biometry - fingerprint

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                                        Face detection and recognition

                                        Digital Image ProcessingDigital Image Processing

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                                        Gender identification

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                                        Image morphing

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                                        Fundamental Steps in DIP

                                        methods whose input and output are images

                                        methods whose inputs are images but whose outputs are attributes extracted from those images

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                                        Week 1Week 1

                                        Outputs are images

                                        bull image acquisition

                                        bull image filtering and enhancement

                                        bull image restoration

                                        bull color image processing

                                        bull wavelets and multiresolution processing

                                        bull compression

                                        bull morphological processing

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                                        Week 1Week 1

                                        Outputs are attributes

                                        bull morphological processing

                                        bull segmentation

                                        bull representation and description

                                        bull object recognition

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                                        Week 1Week 1

                                        Image acquisition - may involve preprocessing such as scaling

                                        Image enhancement

                                        bull manipulating an image so that the result is more suitable than the original for a specific operation

                                        bull enhancement is problem oriented

                                        bull there is no general sbquotheoryrsquo of image enhancement

                                        bull enhancement use subjective methods for image emprovement

                                        bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

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                                        Week 1Week 1

                                        Image restoration

                                        bull improving the appearance of an image

                                        bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                        Color image processing

                                        bull fundamental concept in color models

                                        bull basic color processing in a digital domain

                                        Wavelets and multiresolution processing

                                        representing images in various degree of resolution

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                                        Week 1Week 1

                                        Compression

                                        reducing the storage required to save an image or the bandwidth required to transmit it

                                        Morphological processing

                                        bull tools for extracting image components that are useful in the representation and description of shape

                                        bull a transition from processes that output images to processes that outputimage attributes

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                                        Week 1Week 1

                                        Segmentation

                                        bull partitioning an image into its constituents parts or objects

                                        bull autonomous segmentation is one of the most difficult tasks of DIP

                                        bull the more accurate the segmentation the more likley recognition is to succeed

                                        Representation and description (almost always follows segmentation)

                                        bull segmentation produces either the boundary of a region or all the poits in the region itself

                                        bull converting the data produced by segmentation to a form suitable for computer processing

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                        bull complete region the focus is on internal properties such as texture or skeletal shape

                                        bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                        Object recognition

                                        the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                        Knowledge database

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                                        Week 1Week 1

                                        Simplified diagramof a cross sectionof the human eye

                                        Digital Image ProcessingDigital Image Processing

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                                        Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                        The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                        Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                        The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                        The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                        Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                        Fovea = the place where the image of the object of interest falls on

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                        Blind spot region without receptors

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Image formation in the eye

                                        Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                        Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                        distance between lens and retina along visual axix = 17 mm

                                        range of focal length = 14 mm to 17 mm

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Digital Image ProcessingDigital Image Processing

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                                        Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Optical illusions

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                        gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                        quantities that describe the quality of a chromatic light source radiance

                                        the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                        measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                        brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        the physical meaning is determined by the source of the image

                                        ( )f D f x y

                                        Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                        f(xy) ndash characterized by two components

                                        i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                        r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                        ( ) ( ) ( )

                                        0 ( ) 0 ( ) 1

                                        f x y i x y r x y

                                        i x y r x y

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                                        Week 1Week 1

                                        r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                        i(xy) ndash determined by the illumination source

                                        r(xy) ndash determined by the characteristics of the imaged objects

                                        is called gray (or intensity) scale

                                        In practice

                                        min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                        indoor values without additional illuminationmin max10 1000L L

                                        black whitemin max0 1 0 1 0 1L L L L l l L

                                        min maxL L

                                        Digital Image ProcessingDigital Image Processing

                                        Week 1Week 1

                                        Digital Image Processing

                                        Week 1

                                        Image Sampling and Quantization

                                        - the output of the sensors is a continuous voltage waveform related to the sensed

                                        scene

                                        converting a continuous image f to digital form

                                        - digitizing (x y) is called sampling

                                        - digitizing f(x y) is called quantization

                                        Digital Image Processing

                                        Week 1

                                        Digital Image Processing

                                        Week 1

                                        Continuous image projected onto a sensor array Result of image sampling and quantization

                                        Digital Image Processing

                                        Week 1

                                        Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                        (00) (01) (0 1)(10) (11) (1 1)

                                        ( )

                                        ( 10) ( 11) ( 1 1)

                                        f f f Nf f f N

                                        f x y

                                        f M f M f M N

                                        image element pixel

                                        00 01 0 1

                                        10 11 1 1

                                        10 11 1 1

                                        ( ) ( )

                                        N

                                        i jN M N

                                        i j

                                        M M M N

                                        a a aa f x i y j f i ja a a

                                        Aa

                                        a a a

                                        f(00) ndash the upper left corner of the image

                                        Digital Image Processing

                                        Week 1

                                        M N ge 0 L=2k

                                        [0 1]i j i ja a L

                                        Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                        Digital Image Processing

                                        Week 1

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                                        Week 1

                                        Number of bits required to store a digitized image

                                        for 2 b M N k M N b N k

                                        When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                        Digital Image Processing

                                        Week 1

                                        Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                        Measures line pairs per unit distance dots (pixels) per unit distance

                                        Image resolution = the largest number of discernible line pairs per unit distance

                                        (eg 100 line pairs per mm)

                                        Dots per unit distance are commonly used in printing and publishing

                                        In US the measure is expressed in dots per inch (dpi)

                                        (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                        Intensity resolution ndash the smallest discernible change in intensity level

                                        The number of intensity levels (L) is determined by hardware considerations

                                        L=2k ndash most common k = 8

                                        Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                        Digital Image Processing

                                        Week 1

                                        Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                        150 dpi (lower left) 72 dpi (lower right)

                                        Digital Image Processing

                                        Week 1

                                        Reducing the number of gray levels 256 128 64 32

                                        Digital Image Processing

                                        Week 1

                                        Reducing the number of gray levels 16 8 4 2

                                        Digital Image Processing

                                        Week 1

                                        Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                        Shrinking zooming ndash image resizing ndash image resampling methods

                                        Interpolation is the process of using known data to estimate values at unknown locations

                                        Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                        750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                        same spacing as the original and then shrink it so that it fits exactly over the original

                                        image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                        Problem assignment of intensity-level in the new 750 times 750 grid

                                        Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                        intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                        This technique has the tendency to produce undesirable effects like severe distortion of

                                        straight edges

                                        Digital Image Processing

                                        Week 1

                                        Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                        where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                        be written using the 4 nearest neighbors of point (x y)

                                        Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                        modest increase in computational effort

                                        Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                        nearest neighbors of the point 3 3

                                        0 0

                                        ( ) i ji j

                                        i jv x y c x y

                                        The coefficients cij are obtained solving a 16x16 linear system

                                        intensity levels of the 16 nearest neighbors of 3 3

                                        0 0

                                        ( )i ji j

                                        i jc x y x y

                                        Digital Image Processing

                                        Week 1

                                        Generally bicubic interpolation does a better job of preserving fine detail than the

                                        bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                        programs such as Adobe Photoshop and Corel Photopaint

                                        Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                        the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                        then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                        neighbor interpolation was used (both for shrinking and zooming)

                                        Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                        interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                        from 1250 dpi to 150 dpi (instead of 72 dpi)

                                        Digital Image Processing

                                        Week 1

                                        Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                        Digital Image Processing

                                        Week 1

                                        Neighbors of a Pixel

                                        A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                        This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                        The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                        and are denoted ND(p)

                                        The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                        N8 (p)

                                        If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                        fall outside the image

                                        Digital Image Processing

                                        Week 1

                                        Adjacency Connectivity Regions Boundaries

                                        Denote by V the set of intensity levels used to define adjacency

                                        - in a binary image V 01 (V=0 V=1)

                                        - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                        We consider 3 types of adjacency

                                        (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                        (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                        (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                        m-adjacent if

                                        4( )q N p or

                                        ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                        Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                        ambiguities that often arise when 8-adjacency is used Consider the example

                                        Digital Image Processing

                                        Week 1

                                        binary image

                                        0 1 1 0 1 1 0 1 1

                                        1 0 1 0 0 1 0 0 1 0

                                        0 0 1 0 0 1 0 0 1

                                        V

                                        The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                        8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                        m-adjacency

                                        Digital Image Processing

                                        Week 1

                                        A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                        is a sequence of distinct pixels with coordinates

                                        and are adjacent 0 0 1 1

                                        1 1

                                        ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                        n n

                                        i i i i

                                        x y x y x y x y s tx y x y i n

                                        The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                        Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                        Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                        in S if there exists a path between them consisting only of pixels from S

                                        S is a connected set if there is a path in S between any 2 pixels in S

                                        Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                        Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                        that are not adjacent are said to be disjoint When referring to regions only 4- and

                                        8-adjacency are considered

                                        Digital Image Processing

                                        Week 1

                                        Suppose that an image contains K disjoint regions 1 kR k K none of which

                                        touches the image border

                                        the complement of 1

                                        ( )K

                                        cu k u u

                                        k

                                        R R R R

                                        We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                        background of the image

                                        The boundary (border or contour) of a region R is the set of points that are adjacent to

                                        points in the complement of R (R)c The border of an image is the set of pixels in the

                                        region that have at least one background neighbor This definition is referred to as the

                                        inner border to distinguish it from the notion of outer border which is the corresponding

                                        border in the background

                                        Digital Image Processing

                                        Week 1

                                        Distance measures

                                        For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                        function or metric if

                                        (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                        (b) D(p q) = D(q p)

                                        (c) D(p z) le D(p q) + D(q z)

                                        The Euclidean distance between p and q is defined as 1

                                        2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                        The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                        centered at (x y)

                                        Digital Image Processing

                                        Week 1

                                        The D4 distance (also called city-block distance) between p and q is defined as

                                        4( ) | | | |D p q x s y t

                                        The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                        4

                                        22 1 2

                                        2 2 1 0 1 22 1 2

                                        2

                                        D

                                        The pixels with D4 = 1 are the 4-neighbors of (x y)

                                        The D8 distance (called the chessboard distance) between p and q is defined as

                                        8( ) max| | | |D p q x s y t

                                        The pixels q for which 8( )D p q r form a square centered at (x y)

                                        Digital Image Processing

                                        Week 1

                                        8

                                        2 2 2 2 22 1 1 1 2

                                        2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                        D

                                        The pixels with D8 = 1 are the 8-neighbors of (x y)

                                        D4 and D8 distances are independent of any paths that might exist between p and q

                                        because these distances involve only the coordinates of the point

                                        Digital Image Processing

                                        Week 1

                                        Array versus Matrix Operations

                                        An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                        11 12 11 12

                                        21 22 21 22

                                        a a b ba a b b

                                        Array product

                                        11 12 11 12 11 11 12 12

                                        21 22 21 22 21 21 22 21

                                        a a b b a b a ba a b b a b a b

                                        Matrix product

                                        11 12 11 12 11 11 12 21 11 12 12 21

                                        21 22 21 22 21 11 22 21 21 12 22 22

                                        a a b b a b a b a b a ba a b b a b a b a b a b

                                        We assume array operations unless stated otherwise

                                        Digital Image Processing

                                        Week 1

                                        Linear versus Nonlinear Operations

                                        One of the most important classifications of image-processing methods is whether it is

                                        linear or nonlinear

                                        ( ) ( )H f x y g x y

                                        H is said to be a linear operator if

                                        images1 2 1 2

                                        1 2

                                        ( ) ( ) ( ) ( )

                                        H a f x y b f x y a H f x y b H f x y

                                        a b f f

                                        Example of nonlinear operator

                                        the maximum value of the pixels of image max ( )H f f x y f

                                        1 2

                                        0 2 6 5 1 1

                                        2 3 4 7f f a b

                                        Digital Image Processing

                                        Week 1

                                        1 2

                                        0 2 6 5 6 3max max 1 ( 1) max 2

                                        2 3 4 7 2 4a f b f

                                        0 2 6 51 max ( 1) max 3 ( 1)7 4

                                        2 3 4 7

                                        Arithmetic Operations in Image Processing

                                        Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                        f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                        For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                        value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                        The two random variables are uncorrelated when their covariance is 0

                                        Digital Image Processing

                                        Week 1

                                        Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                        used in image enhancement)

                                        1

                                        1( ) ( )K

                                        ii

                                        g x y g x yK

                                        If the noise satisfies the properties stated above we have

                                        2 2( ) ( )

                                        1( ) ( ) g x y x yE g x y f x yK

                                        ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                        and g respectively The standard deviation (square root of the variance) at any point in

                                        the average image is

                                        ( ) ( )1

                                        g x y x yK

                                        Digital Image Processing

                                        Week 1

                                        As K increases the variability (as measured by the variance or the standard deviation) of

                                        the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                        means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                        averaging process increases

                                        An important application of image averaging is in the field of astronomy where imaging

                                        under very low light levels frequently causes sensor noise to render single images

                                        virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                        was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                        64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                        images respectively

                                        Digital Image Processing

                                        Week 1

                                        Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                        100 noisy images

                                        a b c d e f

                                        Digital Image Processing

                                        Week 1

                                        A frequent application of image subtraction is in the enhancement of differences between

                                        images

                                        (a) (b) (c)

                                        Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                        significant bit of each pixel (c) the difference between the two images

                                        Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                        Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                        images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                        difference between images (a) and (b)

                                        Digital Image Processing

                                        Week 1

                                        Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                        h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                        intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                        source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                        bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                        anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                        images after injection of the contrast medium

                                        In g(x y) we can find the differences between h and f as enhanced detail

                                        Images being captured at TV rates we obtain a movie showing how the contrast medium

                                        propagates through the various arteries in the area being observed

                                        Digital Image Processing

                                        Week 1

                                        a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                        Digital Image Processing

                                        Week 1

                                        An important application of image multiplication (and division) is shading correction

                                        Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                        f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                        When the shading function is known

                                        ( )( )( )

                                        g x yf x yh x y

                                        h(x y) is unknown but we have access to the imaging system we can obtain an

                                        approximation to the shading function by imaging a target of constant intensity When the

                                        sensor is not available often the shading pattern can be estimated from the image

                                        Digital Image Processing

                                        Week 1

                                        (a) (b) (c)

                                        Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                        Digital Image Processing

                                        Week 1

                                        Another use of image multiplication is in masking also called region of interest (ROI)

                                        operations The process consists of multiplying a given image by a mask image that has

                                        1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                        image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                        (a) (b) (c)

                                        Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                        Digital Image Processing

                                        Week 1

                                        In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                        min( )mf f f

                                        0 ( 255)max( )

                                        ms

                                        m

                                        ff K K K

                                        f

                                        Digital Image Processing

                                        Week 1

                                        Spatial Operations

                                        - are performed directly on the pixels of a given image

                                        There are three categories of spatial operations

                                        single-pixel operations

                                        neighborhood operations

                                        geometric spatial transformations

                                        Single-pixel operations

                                        - change the values of intensity for the individual pixels ( )s T z

                                        where z is the intensity of a pixel in the original image and s is the intensity of the

                                        corresponding pixel in the processed image

                                        Digital Image Processing

                                        Week 1

                                        Neighborhood operations

                                        Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                        in an image f Neighborhood processing generates new intensity level at point (x y)

                                        based on the values of the intensities of the points in Sxy For example if Sxy is a

                                        rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                        intensity by computing the average value of the pixels in Sxy

                                        ( )

                                        1( ) ( )xyr c S

                                        g x y f r cm n

                                        The net effect is to perform local blurring in the original image This type of process is

                                        used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                        largest region of an image

                                        Digital Image Processing

                                        Week 1

                                        Geometric spatial transformations and image registration

                                        - modify the spatial relationship between pixels in an image

                                        - these transformations are often called rubber-sheet transformations (analogous to

                                        printing an image on a sheet of rubber and then stretching the sheet according to a

                                        predefined set of rules

                                        A geometric transformation consists of 2 basic operations

                                        1 a spatial transformation of coordinates

                                        2 intensity interpolation that assign intensity values to the spatial transformed

                                        pixels

                                        The coordinate system transformation ( ) [( )]x y T v w

                                        (v w) ndash pixel coordinates in the original image

                                        (x y) ndash pixel coordinates in the transformed image

                                        Digital Image Processing

                                        Week 1

                                        [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                        Affine transform

                                        11 1211 21 31

                                        21 2212 22 33

                                        31 32

                                        0[ 1] [ 1] [ 1] 0

                                        1

                                        t tx t v t w t

                                        x y v w T v w t ty t v t w t

                                        t t

                                        (AT)

                                        This transform can scale rotate translate or shear a set of coordinate points depending

                                        on the elements of the matrix T If we want to resize an image rotate it and move the

                                        result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                        scaling rotation and translation matrices from Table 1

                                        Digital Image Processing

                                        Week 1

                                        Affine transformations

                                        Digital Image Processing

                                        Week 1

                                        The preceding transformations relocate pixels on an image to new locations To complete

                                        the process we have to assign intensity values to those locations This task is done by

                                        using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                        In practice we can use equation (AT) in two basic ways

                                        forward mapping scan the pixels of the input image (v w) compute the new spatial

                                        location (x y) of the corresponding pixel in the new image using (AT) directly

                                        Problems

                                        - intensity assignment when 2 or more pixels in the original image are transformed to

                                        the same location in the output image

                                        - some output locations have no correspondent in the original image (no intensity

                                        assignment)

                                        Digital Image Processing

                                        Week 1

                                        inverse mapping scans the output pixel locations and at each location (x y)

                                        computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                        It then interpolates among the nearest input pixels to determine the intensity of the output

                                        pixel value

                                        Inverse mappings are more efficient to implement than forward mappings and are used in

                                        numerous commercial implementations of spatial transformations (MATLAB for ex)

                                        Digital Image Processing

                                        Week 1

                                        Digital Image Processing

                                        Week 1

                                        Image registration ndash align two or more images of the same scene

                                        In image registration we have available the input and output images but the specific

                                        transformation that produced the output image from the input is generally unknown

                                        The problem is to estimate the transformation function and then use it to register the two

                                        images

                                        - it may be of interest to align (register) two or more image taken at approximately the

                                        same time but using different imaging systems (MRI scanner and a PET scanner)

                                        - align images of a given location taken by the same instrument at different moments

                                        of time (satellite images)

                                        Solving the problem using tie points (also called control points) which are

                                        corresponding points whose locations are known precisely in the input and reference

                                        image

                                        Digital Image Processing

                                        Week 1

                                        How to select tie points

                                        - interactively selecting them

                                        - use of algorithms that try to detect these points

                                        - some imaging systems have physical artifacts (small metallic objects) embedded in

                                        the imaging sensors These objects produce a set of known points (called reseau

                                        marks) directly on all images captured by the system which can be used as guides

                                        for establishing tie points

                                        The problem of estimating the transformation is one of modeling Suppose we have a set

                                        of 4 tie points both on the input image and the reference image A simple model based on

                                        a bilinear approximation is given by

                                        1 2 3 4

                                        5 6 7 8

                                        x c v c w c v w cy c v c w c v w c

                                        (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                        Digital Image Processing

                                        Week 1

                                        When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                        frequently is to select a larger number of tie points and using this new set of tie points

                                        subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                        subregions marked by 4 tie points we applied the transformation model described above

                                        The number of tie points and the sophistication of the model required to solve the register

                                        problem depend on the severity of the geometrical distortion

                                        Digital Image Processing

                                        Week 1

                                        a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                        Digital Image Processing

                                        Week 1

                                        Probabilistic Methods

                                        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                        p(zk) = the probability that the intensity level zk occurs in the given image

                                        ( ) kk

                                        np zM N

                                        nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                        pixels in the image) 1

                                        0( ) 1

                                        L

                                        kk

                                        p z

                                        The mean (average) intensity of an image is given by 1

                                        0( )

                                        L

                                        k kk

                                        m z p z

                                        Digital Image Processing

                                        Week 1

                                        The variance of the intensities is 1

                                        2 2

                                        0( ) ( )

                                        L

                                        k kk

                                        z m p z

                                        The variance is a measure of the spread of the values of z about the mean so it is a

                                        measure of image contrast Usually for measuring image contrast the standard deviation

                                        ( ) is used

                                        The n-th moment of a random variable z about the mean is defined as 1

                                        0( ) ( ) ( )

                                        Ln

                                        n k kk

                                        z z m p z

                                        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                        3( ) 0z the intensities are biased to values higher than the mean

                                        ( 3( ) 0z the intensities are biased to values lower than the mean

                                        Digital Image Processing

                                        Week 1

                                        3( ) 0z the intensities are distributed approximately equally on both side of the

                                        mean

                                        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                        Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                        Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                        Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                        Digital Image Processing

                                        Week 1

                                        Intensity Transformations and Spatial Filtering

                                        ( ) ( )g x y T f x y

                                        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                        neighborhood of (x y)

                                        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                        and much smaller in size than the image

                                        Digital Image Processing

                                        Week 1

                                        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                        called spatial filter (spatial mask kernel template or window)

                                        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                        ( )s T r

                                        s and r are denoting respectively the intensity of g and f at (x y)

                                        Figure 2 left - T produces an output image of higher contrast than the original by

                                        darkening the intensity levels below k and brightening the levels above k ndash this technique

                                        is called contrast stretching

                                        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                        Digital Image Processing

                                        Week 1

                                        Figure 2 right - T produces a binary output image A mapping of this form is called

                                        thresholding function

                                        Some Basic Intensity Transformation Functions

                                        Image Negatives

                                        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                        - equivalent of a photographic negative

                                        - technique suited for enhancing white or gray detail embedded in dark regions of an

                                        image

                                        Digital Image Processing

                                        Week 1

                                        Original Negative image

                                        Digital Image Processing

                                        Week 1

                                        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                        Some basic intensity transformation functions

                                        Digital Image Processing

                                        Week 1

                                        This transformation maps a narrow range of low intensity values in the input into a wider

                                        range An operator of this type is used to expand the values of dark pixels in an image

                                        while compressing the higher-level values The opposite is true for the inverse log

                                        transformation The log functions compress the dynamic range of images with large

                                        variations in pixel values

                                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                        Digital Image Processing

                                        Week 1

                                        Power-Law (Gamma) Transformations

                                        - positive constants( ) ( ( ) )s T r c r c s c r

                                        Plots of gamma transformation for different values of γ (c=1)

                                        Digital Image Processing

                                        Week 1

                                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                                        of output values with the opposite being true for higher values of input values The

                                        curves with 1 have the opposite effect of those generated with values of 1

                                        1c - identity transformation

                                        A variety of devices used for image capture printing and display respond according to a

                                        power law The process used to correct these power-law response phenomena is called

                                        gamma correction

                                        Digital Image Processing

                                        Week 1

                                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                        Digital Image Processing

                                        Week 1

                                        Piecewise-Linear Transformations Functions

                                        Contrast stretching

                                        - a process that expands the range of intensity levels in an image so it spans the full

                                        intensity range of the recording tool or display device

                                        a b c d Fig5

                                        Digital Image Processing

                                        Week 1

                                        11

                                        1

                                        2 1 1 21 2

                                        2 1 2 1

                                        22

                                        2

                                        [0 ]

                                        ( ) ( )( ) [ ]( ) ( )

                                        ( 1 ) [ 1]( 1 )

                                        s r r rrs r r s r rT r r r r

                                        r r r rs L r r r L

                                        L r

                                        Digital Image Processing

                                        Week 1

                                        1 1 2 2r s r s identity transformation (no change)

                                        1 2 1 2 0 1r r s s L thresholding function

                                        Figure 5(b) shows an 8-bit image with low contrast

                                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                        in the image respectively Thus the transformation function stretched the levels linearly

                                        from their original range to the full range [0 L-1]

                                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                        2 2 1r s m L where m is the mean gray level in the image

                                        The original image on which these results are based is a scanning electron microscope

                                        image of pollen magnified approximately 700 times

                                        Digital Image Processing

                                        Week 1

                                        Intensity-level slicing

                                        - highlighting a specific range of intensities in an image

                                        There are two approaches for intensity-level slicing

                                        1 display in one value (white for example) all the values in the range of interest and in

                                        another (say black) all other intensities (Figure 311 (a))

                                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                        intensities in the image (Figure 311 (b))

                                        Digital Image Processing

                                        Week 1

                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                        the top of the scale of intensities This type of enhancement produces a binary image

                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                        Highlights range [A B] and preserves all other intensities

                                        Digital Image Processing

                                        Week 1

                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                        blockageshellip)

                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                        image around the mean intensity was set to black the other intensities remain unchanged

                                        Fig 6 - Aortic angiogram and intensity sliced versions

                                        Digital Image Processing

                                        Week 1

                                        Bit-plane slicing

                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                        This technique highlights the contribution made to the whole image appearances by each

                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                        Digital Image Processing

                                        Week 1

                                        Digital Image Processing

                                        Week 1

                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                        • DIP 1 2017
                                        • DIP 02 (2017)

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Examples of gamma-ray imaging

                                          Bone scan PET image

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          X-ray imaging

                                          Medical diagnosticindustry astronomy

                                          A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                                          The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Angiography = contrast-enhancement radiography

                                          Angiograms = images of blood vessels

                                          A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                                          X-rays are used in CAT (computerized axial tomography)

                                          X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                                          Industrial CAT scans are useful when the parts can be penetreted by X-rays

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Examples of X-ray imaging

                                          Chest X-rayAortic angiogram

                                          Head CT Cygnus LoopCircuit boards

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Imaging in the Ultraviolet Band

                                          Litography industrial inspection microscopy biological imaging astronomical observations

                                          Ultraviolet light is used in fluorescence microscopy

                                          Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                                          other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                                          and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Imaging in the Visible and Infrared Bands

                                          Light microscopy astronomy remote sensing industry law enforcement

                                          LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                                          Weather observations and prediction produce major applications of multispectral image from satellites

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Satellite images of Washington DC area in spectral bands of the Table 1

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Examples of light microscopy

                                          Taxol (anticancer agent)magnified 250X

                                          Cholesterol(40X)

                                          Microprocessor(60X)

                                          Nickel oxidethin film(600X)

                                          Surface of audio CD(1750X)

                                          Organicsuperconductor(450X)

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Automated visual inspection of manufactured goods

                                          a bc de f

                                          a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Imaging in the Microwave Band

                                          The dominant aplication of imaging in the microwave band ndash radar

                                          bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                          bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                          bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                          An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Spaceborne radar image of mountains in southeast Tibet

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Imaging in the Radio Band

                                          medicine astronomy

                                          MRI = Magnetic Resonance Imaging

                                          This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                          Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                          The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          MRI images of a human knee (left) and spine (right)

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Images of the Crab Pulsar covering the electromagnetic spectrum

                                          Gamma X-ray Optical Infrared Radio

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Other Imaging Modalities

                                          acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                          Imaging using sound geological explorations industry medicine

                                          Mineral and oil exploration

                                          For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Biometry - iris

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Biometry - fingerprint

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Face detection and recognition

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Gender identification

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Image morphing

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Fundamental Steps in DIP

                                          methods whose input and output are images

                                          methods whose inputs are images but whose outputs are attributes extracted from those images

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Outputs are images

                                          bull image acquisition

                                          bull image filtering and enhancement

                                          bull image restoration

                                          bull color image processing

                                          bull wavelets and multiresolution processing

                                          bull compression

                                          bull morphological processing

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Outputs are attributes

                                          bull morphological processing

                                          bull segmentation

                                          bull representation and description

                                          bull object recognition

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Image acquisition - may involve preprocessing such as scaling

                                          Image enhancement

                                          bull manipulating an image so that the result is more suitable than the original for a specific operation

                                          bull enhancement is problem oriented

                                          bull there is no general sbquotheoryrsquo of image enhancement

                                          bull enhancement use subjective methods for image emprovement

                                          bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Image restoration

                                          bull improving the appearance of an image

                                          bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                          Color image processing

                                          bull fundamental concept in color models

                                          bull basic color processing in a digital domain

                                          Wavelets and multiresolution processing

                                          representing images in various degree of resolution

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Compression

                                          reducing the storage required to save an image or the bandwidth required to transmit it

                                          Morphological processing

                                          bull tools for extracting image components that are useful in the representation and description of shape

                                          bull a transition from processes that output images to processes that outputimage attributes

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Segmentation

                                          bull partitioning an image into its constituents parts or objects

                                          bull autonomous segmentation is one of the most difficult tasks of DIP

                                          bull the more accurate the segmentation the more likley recognition is to succeed

                                          Representation and description (almost always follows segmentation)

                                          bull segmentation produces either the boundary of a region or all the poits in the region itself

                                          bull converting the data produced by segmentation to a form suitable for computer processing

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                          bull complete region the focus is on internal properties such as texture or skeletal shape

                                          bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                          Object recognition

                                          the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                          Knowledge database

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Simplified diagramof a cross sectionof the human eye

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                          The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                          Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                          The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                          The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                          Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                          Fovea = the place where the image of the object of interest falls on

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                          Blind spot region without receptors

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Image formation in the eye

                                          Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                          Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                          distance between lens and retina along visual axix = 17 mm

                                          range of focal length = 14 mm to 17 mm

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Optical illusions

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                          gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                          quantities that describe the quality of a chromatic light source radiance

                                          the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                          measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                          brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          the physical meaning is determined by the source of the image

                                          ( )f D f x y

                                          Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                          f(xy) ndash characterized by two components

                                          i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                          r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                          ( ) ( ) ( )

                                          0 ( ) 0 ( ) 1

                                          f x y i x y r x y

                                          i x y r x y

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                          i(xy) ndash determined by the illumination source

                                          r(xy) ndash determined by the characteristics of the imaged objects

                                          is called gray (or intensity) scale

                                          In practice

                                          min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                          indoor values without additional illuminationmin max10 1000L L

                                          black whitemin max0 1 0 1 0 1L L L L l l L

                                          min maxL L

                                          Digital Image ProcessingDigital Image Processing

                                          Week 1Week 1

                                          Digital Image Processing

                                          Week 1

                                          Image Sampling and Quantization

                                          - the output of the sensors is a continuous voltage waveform related to the sensed

                                          scene

                                          converting a continuous image f to digital form

                                          - digitizing (x y) is called sampling

                                          - digitizing f(x y) is called quantization

                                          Digital Image Processing

                                          Week 1

                                          Digital Image Processing

                                          Week 1

                                          Continuous image projected onto a sensor array Result of image sampling and quantization

                                          Digital Image Processing

                                          Week 1

                                          Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                          (00) (01) (0 1)(10) (11) (1 1)

                                          ( )

                                          ( 10) ( 11) ( 1 1)

                                          f f f Nf f f N

                                          f x y

                                          f M f M f M N

                                          image element pixel

                                          00 01 0 1

                                          10 11 1 1

                                          10 11 1 1

                                          ( ) ( )

                                          N

                                          i jN M N

                                          i j

                                          M M M N

                                          a a aa f x i y j f i ja a a

                                          Aa

                                          a a a

                                          f(00) ndash the upper left corner of the image

                                          Digital Image Processing

                                          Week 1

                                          M N ge 0 L=2k

                                          [0 1]i j i ja a L

                                          Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                          Digital Image Processing

                                          Week 1

                                          Digital Image Processing

                                          Week 1

                                          Number of bits required to store a digitized image

                                          for 2 b M N k M N b N k

                                          When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                          Digital Image Processing

                                          Week 1

                                          Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                          Measures line pairs per unit distance dots (pixels) per unit distance

                                          Image resolution = the largest number of discernible line pairs per unit distance

                                          (eg 100 line pairs per mm)

                                          Dots per unit distance are commonly used in printing and publishing

                                          In US the measure is expressed in dots per inch (dpi)

                                          (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                          Intensity resolution ndash the smallest discernible change in intensity level

                                          The number of intensity levels (L) is determined by hardware considerations

                                          L=2k ndash most common k = 8

                                          Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                          Digital Image Processing

                                          Week 1

                                          Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                          150 dpi (lower left) 72 dpi (lower right)

                                          Digital Image Processing

                                          Week 1

                                          Reducing the number of gray levels 256 128 64 32

                                          Digital Image Processing

                                          Week 1

                                          Reducing the number of gray levels 16 8 4 2

                                          Digital Image Processing

                                          Week 1

                                          Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                          Shrinking zooming ndash image resizing ndash image resampling methods

                                          Interpolation is the process of using known data to estimate values at unknown locations

                                          Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                          750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                          same spacing as the original and then shrink it so that it fits exactly over the original

                                          image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                          Problem assignment of intensity-level in the new 750 times 750 grid

                                          Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                          intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                          This technique has the tendency to produce undesirable effects like severe distortion of

                                          straight edges

                                          Digital Image Processing

                                          Week 1

                                          Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                          where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                          be written using the 4 nearest neighbors of point (x y)

                                          Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                          modest increase in computational effort

                                          Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                          nearest neighbors of the point 3 3

                                          0 0

                                          ( ) i ji j

                                          i jv x y c x y

                                          The coefficients cij are obtained solving a 16x16 linear system

                                          intensity levels of the 16 nearest neighbors of 3 3

                                          0 0

                                          ( )i ji j

                                          i jc x y x y

                                          Digital Image Processing

                                          Week 1

                                          Generally bicubic interpolation does a better job of preserving fine detail than the

                                          bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                          programs such as Adobe Photoshop and Corel Photopaint

                                          Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                          the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                          then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                          neighbor interpolation was used (both for shrinking and zooming)

                                          Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                          interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                          from 1250 dpi to 150 dpi (instead of 72 dpi)

                                          Digital Image Processing

                                          Week 1

                                          Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                          Digital Image Processing

                                          Week 1

                                          Neighbors of a Pixel

                                          A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                          This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                          The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                          and are denoted ND(p)

                                          The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                          N8 (p)

                                          If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                          fall outside the image

                                          Digital Image Processing

                                          Week 1

                                          Adjacency Connectivity Regions Boundaries

                                          Denote by V the set of intensity levels used to define adjacency

                                          - in a binary image V 01 (V=0 V=1)

                                          - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                          We consider 3 types of adjacency

                                          (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                          (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                          (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                          m-adjacent if

                                          4( )q N p or

                                          ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                          Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                          ambiguities that often arise when 8-adjacency is used Consider the example

                                          Digital Image Processing

                                          Week 1

                                          binary image

                                          0 1 1 0 1 1 0 1 1

                                          1 0 1 0 0 1 0 0 1 0

                                          0 0 1 0 0 1 0 0 1

                                          V

                                          The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                          8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                          m-adjacency

                                          Digital Image Processing

                                          Week 1

                                          A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                          is a sequence of distinct pixels with coordinates

                                          and are adjacent 0 0 1 1

                                          1 1

                                          ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                          n n

                                          i i i i

                                          x y x y x y x y s tx y x y i n

                                          The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                          Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                          Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                          in S if there exists a path between them consisting only of pixels from S

                                          S is a connected set if there is a path in S between any 2 pixels in S

                                          Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                          Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                          that are not adjacent are said to be disjoint When referring to regions only 4- and

                                          8-adjacency are considered

                                          Digital Image Processing

                                          Week 1

                                          Suppose that an image contains K disjoint regions 1 kR k K none of which

                                          touches the image border

                                          the complement of 1

                                          ( )K

                                          cu k u u

                                          k

                                          R R R R

                                          We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                          background of the image

                                          The boundary (border or contour) of a region R is the set of points that are adjacent to

                                          points in the complement of R (R)c The border of an image is the set of pixels in the

                                          region that have at least one background neighbor This definition is referred to as the

                                          inner border to distinguish it from the notion of outer border which is the corresponding

                                          border in the background

                                          Digital Image Processing

                                          Week 1

                                          Distance measures

                                          For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                          function or metric if

                                          (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                          (b) D(p q) = D(q p)

                                          (c) D(p z) le D(p q) + D(q z)

                                          The Euclidean distance between p and q is defined as 1

                                          2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                          The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                          centered at (x y)

                                          Digital Image Processing

                                          Week 1

                                          The D4 distance (also called city-block distance) between p and q is defined as

                                          4( ) | | | |D p q x s y t

                                          The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                          4

                                          22 1 2

                                          2 2 1 0 1 22 1 2

                                          2

                                          D

                                          The pixels with D4 = 1 are the 4-neighbors of (x y)

                                          The D8 distance (called the chessboard distance) between p and q is defined as

                                          8( ) max| | | |D p q x s y t

                                          The pixels q for which 8( )D p q r form a square centered at (x y)

                                          Digital Image Processing

                                          Week 1

                                          8

                                          2 2 2 2 22 1 1 1 2

                                          2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                          D

                                          The pixels with D8 = 1 are the 8-neighbors of (x y)

                                          D4 and D8 distances are independent of any paths that might exist between p and q

                                          because these distances involve only the coordinates of the point

                                          Digital Image Processing

                                          Week 1

                                          Array versus Matrix Operations

                                          An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                          11 12 11 12

                                          21 22 21 22

                                          a a b ba a b b

                                          Array product

                                          11 12 11 12 11 11 12 12

                                          21 22 21 22 21 21 22 21

                                          a a b b a b a ba a b b a b a b

                                          Matrix product

                                          11 12 11 12 11 11 12 21 11 12 12 21

                                          21 22 21 22 21 11 22 21 21 12 22 22

                                          a a b b a b a b a b a ba a b b a b a b a b a b

                                          We assume array operations unless stated otherwise

                                          Digital Image Processing

                                          Week 1

                                          Linear versus Nonlinear Operations

                                          One of the most important classifications of image-processing methods is whether it is

                                          linear or nonlinear

                                          ( ) ( )H f x y g x y

                                          H is said to be a linear operator if

                                          images1 2 1 2

                                          1 2

                                          ( ) ( ) ( ) ( )

                                          H a f x y b f x y a H f x y b H f x y

                                          a b f f

                                          Example of nonlinear operator

                                          the maximum value of the pixels of image max ( )H f f x y f

                                          1 2

                                          0 2 6 5 1 1

                                          2 3 4 7f f a b

                                          Digital Image Processing

                                          Week 1

                                          1 2

                                          0 2 6 5 6 3max max 1 ( 1) max 2

                                          2 3 4 7 2 4a f b f

                                          0 2 6 51 max ( 1) max 3 ( 1)7 4

                                          2 3 4 7

                                          Arithmetic Operations in Image Processing

                                          Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                          f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                          For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                          value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                          The two random variables are uncorrelated when their covariance is 0

                                          Digital Image Processing

                                          Week 1

                                          Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                          used in image enhancement)

                                          1

                                          1( ) ( )K

                                          ii

                                          g x y g x yK

                                          If the noise satisfies the properties stated above we have

                                          2 2( ) ( )

                                          1( ) ( ) g x y x yE g x y f x yK

                                          ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                          and g respectively The standard deviation (square root of the variance) at any point in

                                          the average image is

                                          ( ) ( )1

                                          g x y x yK

                                          Digital Image Processing

                                          Week 1

                                          As K increases the variability (as measured by the variance or the standard deviation) of

                                          the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                          means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                          averaging process increases

                                          An important application of image averaging is in the field of astronomy where imaging

                                          under very low light levels frequently causes sensor noise to render single images

                                          virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                          was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                          64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                          images respectively

                                          Digital Image Processing

                                          Week 1

                                          Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                          100 noisy images

                                          a b c d e f

                                          Digital Image Processing

                                          Week 1

                                          A frequent application of image subtraction is in the enhancement of differences between

                                          images

                                          (a) (b) (c)

                                          Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                          significant bit of each pixel (c) the difference between the two images

                                          Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                          Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                          images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                          difference between images (a) and (b)

                                          Digital Image Processing

                                          Week 1

                                          Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                          h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                          intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                          source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                          bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                          anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                          images after injection of the contrast medium

                                          In g(x y) we can find the differences between h and f as enhanced detail

                                          Images being captured at TV rates we obtain a movie showing how the contrast medium

                                          propagates through the various arteries in the area being observed

                                          Digital Image Processing

                                          Week 1

                                          a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                          Digital Image Processing

                                          Week 1

                                          An important application of image multiplication (and division) is shading correction

                                          Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                          f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                          When the shading function is known

                                          ( )( )( )

                                          g x yf x yh x y

                                          h(x y) is unknown but we have access to the imaging system we can obtain an

                                          approximation to the shading function by imaging a target of constant intensity When the

                                          sensor is not available often the shading pattern can be estimated from the image

                                          Digital Image Processing

                                          Week 1

                                          (a) (b) (c)

                                          Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                          Digital Image Processing

                                          Week 1

                                          Another use of image multiplication is in masking also called region of interest (ROI)

                                          operations The process consists of multiplying a given image by a mask image that has

                                          1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                          image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                          (a) (b) (c)

                                          Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                          Digital Image Processing

                                          Week 1

                                          In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                          min( )mf f f

                                          0 ( 255)max( )

                                          ms

                                          m

                                          ff K K K

                                          f

                                          Digital Image Processing

                                          Week 1

                                          Spatial Operations

                                          - are performed directly on the pixels of a given image

                                          There are three categories of spatial operations

                                          single-pixel operations

                                          neighborhood operations

                                          geometric spatial transformations

                                          Single-pixel operations

                                          - change the values of intensity for the individual pixels ( )s T z

                                          where z is the intensity of a pixel in the original image and s is the intensity of the

                                          corresponding pixel in the processed image

                                          Digital Image Processing

                                          Week 1

                                          Neighborhood operations

                                          Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                          in an image f Neighborhood processing generates new intensity level at point (x y)

                                          based on the values of the intensities of the points in Sxy For example if Sxy is a

                                          rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                          intensity by computing the average value of the pixels in Sxy

                                          ( )

                                          1( ) ( )xyr c S

                                          g x y f r cm n

                                          The net effect is to perform local blurring in the original image This type of process is

                                          used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                          largest region of an image

                                          Digital Image Processing

                                          Week 1

                                          Geometric spatial transformations and image registration

                                          - modify the spatial relationship between pixels in an image

                                          - these transformations are often called rubber-sheet transformations (analogous to

                                          printing an image on a sheet of rubber and then stretching the sheet according to a

                                          predefined set of rules

                                          A geometric transformation consists of 2 basic operations

                                          1 a spatial transformation of coordinates

                                          2 intensity interpolation that assign intensity values to the spatial transformed

                                          pixels

                                          The coordinate system transformation ( ) [( )]x y T v w

                                          (v w) ndash pixel coordinates in the original image

                                          (x y) ndash pixel coordinates in the transformed image

                                          Digital Image Processing

                                          Week 1

                                          [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                          Affine transform

                                          11 1211 21 31

                                          21 2212 22 33

                                          31 32

                                          0[ 1] [ 1] [ 1] 0

                                          1

                                          t tx t v t w t

                                          x y v w T v w t ty t v t w t

                                          t t

                                          (AT)

                                          This transform can scale rotate translate or shear a set of coordinate points depending

                                          on the elements of the matrix T If we want to resize an image rotate it and move the

                                          result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                          scaling rotation and translation matrices from Table 1

                                          Digital Image Processing

                                          Week 1

                                          Affine transformations

                                          Digital Image Processing

                                          Week 1

                                          The preceding transformations relocate pixels on an image to new locations To complete

                                          the process we have to assign intensity values to those locations This task is done by

                                          using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                          In practice we can use equation (AT) in two basic ways

                                          forward mapping scan the pixels of the input image (v w) compute the new spatial

                                          location (x y) of the corresponding pixel in the new image using (AT) directly

                                          Problems

                                          - intensity assignment when 2 or more pixels in the original image are transformed to

                                          the same location in the output image

                                          - some output locations have no correspondent in the original image (no intensity

                                          assignment)

                                          Digital Image Processing

                                          Week 1

                                          inverse mapping scans the output pixel locations and at each location (x y)

                                          computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                          It then interpolates among the nearest input pixels to determine the intensity of the output

                                          pixel value

                                          Inverse mappings are more efficient to implement than forward mappings and are used in

                                          numerous commercial implementations of spatial transformations (MATLAB for ex)

                                          Digital Image Processing

                                          Week 1

                                          Digital Image Processing

                                          Week 1

                                          Image registration ndash align two or more images of the same scene

                                          In image registration we have available the input and output images but the specific

                                          transformation that produced the output image from the input is generally unknown

                                          The problem is to estimate the transformation function and then use it to register the two

                                          images

                                          - it may be of interest to align (register) two or more image taken at approximately the

                                          same time but using different imaging systems (MRI scanner and a PET scanner)

                                          - align images of a given location taken by the same instrument at different moments

                                          of time (satellite images)

                                          Solving the problem using tie points (also called control points) which are

                                          corresponding points whose locations are known precisely in the input and reference

                                          image

                                          Digital Image Processing

                                          Week 1

                                          How to select tie points

                                          - interactively selecting them

                                          - use of algorithms that try to detect these points

                                          - some imaging systems have physical artifacts (small metallic objects) embedded in

                                          the imaging sensors These objects produce a set of known points (called reseau

                                          marks) directly on all images captured by the system which can be used as guides

                                          for establishing tie points

                                          The problem of estimating the transformation is one of modeling Suppose we have a set

                                          of 4 tie points both on the input image and the reference image A simple model based on

                                          a bilinear approximation is given by

                                          1 2 3 4

                                          5 6 7 8

                                          x c v c w c v w cy c v c w c v w c

                                          (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                          Digital Image Processing

                                          Week 1

                                          When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                          frequently is to select a larger number of tie points and using this new set of tie points

                                          subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                          subregions marked by 4 tie points we applied the transformation model described above

                                          The number of tie points and the sophistication of the model required to solve the register

                                          problem depend on the severity of the geometrical distortion

                                          Digital Image Processing

                                          Week 1

                                          a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                          Digital Image Processing

                                          Week 1

                                          Probabilistic Methods

                                          zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                          p(zk) = the probability that the intensity level zk occurs in the given image

                                          ( ) kk

                                          np zM N

                                          nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                          pixels in the image) 1

                                          0( ) 1

                                          L

                                          kk

                                          p z

                                          The mean (average) intensity of an image is given by 1

                                          0( )

                                          L

                                          k kk

                                          m z p z

                                          Digital Image Processing

                                          Week 1

                                          The variance of the intensities is 1

                                          2 2

                                          0( ) ( )

                                          L

                                          k kk

                                          z m p z

                                          The variance is a measure of the spread of the values of z about the mean so it is a

                                          measure of image contrast Usually for measuring image contrast the standard deviation

                                          ( ) is used

                                          The n-th moment of a random variable z about the mean is defined as 1

                                          0( ) ( ) ( )

                                          Ln

                                          n k kk

                                          z z m p z

                                          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                          3( ) 0z the intensities are biased to values higher than the mean

                                          ( 3( ) 0z the intensities are biased to values lower than the mean

                                          Digital Image Processing

                                          Week 1

                                          3( ) 0z the intensities are distributed approximately equally on both side of the

                                          mean

                                          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                          Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                          Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                          Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                          Digital Image Processing

                                          Week 1

                                          Intensity Transformations and Spatial Filtering

                                          ( ) ( )g x y T f x y

                                          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                          neighborhood of (x y)

                                          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                          and much smaller in size than the image

                                          Digital Image Processing

                                          Week 1

                                          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                          called spatial filter (spatial mask kernel template or window)

                                          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                          ( )s T r

                                          s and r are denoting respectively the intensity of g and f at (x y)

                                          Figure 2 left - T produces an output image of higher contrast than the original by

                                          darkening the intensity levels below k and brightening the levels above k ndash this technique

                                          is called contrast stretching

                                          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                          Digital Image Processing

                                          Week 1

                                          Figure 2 right - T produces a binary output image A mapping of this form is called

                                          thresholding function

                                          Some Basic Intensity Transformation Functions

                                          Image Negatives

                                          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                          - equivalent of a photographic negative

                                          - technique suited for enhancing white or gray detail embedded in dark regions of an

                                          image

                                          Digital Image Processing

                                          Week 1

                                          Original Negative image

                                          Digital Image Processing

                                          Week 1

                                          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                          Some basic intensity transformation functions

                                          Digital Image Processing

                                          Week 1

                                          This transformation maps a narrow range of low intensity values in the input into a wider

                                          range An operator of this type is used to expand the values of dark pixels in an image

                                          while compressing the higher-level values The opposite is true for the inverse log

                                          transformation The log functions compress the dynamic range of images with large

                                          variations in pixel values

                                          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                          Digital Image Processing

                                          Week 1

                                          Power-Law (Gamma) Transformations

                                          - positive constants( ) ( ( ) )s T r c r c s c r

                                          Plots of gamma transformation for different values of γ (c=1)

                                          Digital Image Processing

                                          Week 1

                                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                                          of output values with the opposite being true for higher values of input values The

                                          curves with 1 have the opposite effect of those generated with values of 1

                                          1c - identity transformation

                                          A variety of devices used for image capture printing and display respond according to a

                                          power law The process used to correct these power-law response phenomena is called

                                          gamma correction

                                          Digital Image Processing

                                          Week 1

                                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                          Digital Image Processing

                                          Week 1

                                          Piecewise-Linear Transformations Functions

                                          Contrast stretching

                                          - a process that expands the range of intensity levels in an image so it spans the full

                                          intensity range of the recording tool or display device

                                          a b c d Fig5

                                          Digital Image Processing

                                          Week 1

                                          11

                                          1

                                          2 1 1 21 2

                                          2 1 2 1

                                          22

                                          2

                                          [0 ]

                                          ( ) ( )( ) [ ]( ) ( )

                                          ( 1 ) [ 1]( 1 )

                                          s r r rrs r r s r rT r r r r

                                          r r r rs L r r r L

                                          L r

                                          Digital Image Processing

                                          Week 1

                                          1 1 2 2r s r s identity transformation (no change)

                                          1 2 1 2 0 1r r s s L thresholding function

                                          Figure 5(b) shows an 8-bit image with low contrast

                                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                          in the image respectively Thus the transformation function stretched the levels linearly

                                          from their original range to the full range [0 L-1]

                                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                          2 2 1r s m L where m is the mean gray level in the image

                                          The original image on which these results are based is a scanning electron microscope

                                          image of pollen magnified approximately 700 times

                                          Digital Image Processing

                                          Week 1

                                          Intensity-level slicing

                                          - highlighting a specific range of intensities in an image

                                          There are two approaches for intensity-level slicing

                                          1 display in one value (white for example) all the values in the range of interest and in

                                          another (say black) all other intensities (Figure 311 (a))

                                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                          intensities in the image (Figure 311 (b))

                                          Digital Image Processing

                                          Week 1

                                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                                          the top of the scale of intensities This type of enhancement produces a binary image

                                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                                          Highlights range [A B] and preserves all other intensities

                                          Digital Image Processing

                                          Week 1

                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                          blockageshellip)

                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                          image around the mean intensity was set to black the other intensities remain unchanged

                                          Fig 6 - Aortic angiogram and intensity sliced versions

                                          Digital Image Processing

                                          Week 1

                                          Bit-plane slicing

                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                          This technique highlights the contribution made to the whole image appearances by each

                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                          Digital Image Processing

                                          Week 1

                                          Digital Image Processing

                                          Week 1

                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                          • DIP 1 2017
                                          • DIP 02 (2017)

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            X-ray imaging

                                            Medical diagnosticindustry astronomy

                                            A X-ray tube is a vacuum tube with a cathode and an anode The cathode is heated causing free electrons to be released The electrons flows at high speed to the positively charged anode When the electrons strike a nucleus energy is released in the form of a X-ray radiation The energy (penetreting power) of the X-rays is controlled by a voltage applied across the anode and by a curent applied to the filament in the cathode

                                            The intensity of the X-rays is modified by absorbtion as they pass through the patient and the resulting energy falling develops it much in the same way that light develops photographic film

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Angiography = contrast-enhancement radiography

                                            Angiograms = images of blood vessels

                                            A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                                            X-rays are used in CAT (computerized axial tomography)

                                            X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                                            Industrial CAT scans are useful when the parts can be penetreted by X-rays

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Examples of X-ray imaging

                                            Chest X-rayAortic angiogram

                                            Head CT Cygnus LoopCircuit boards

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Imaging in the Ultraviolet Band

                                            Litography industrial inspection microscopy biological imaging astronomical observations

                                            Ultraviolet light is used in fluorescence microscopy

                                            Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                                            other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                                            and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Imaging in the Visible and Infrared Bands

                                            Light microscopy astronomy remote sensing industry law enforcement

                                            LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                                            Weather observations and prediction produce major applications of multispectral image from satellites

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Satellite images of Washington DC area in spectral bands of the Table 1

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Examples of light microscopy

                                            Taxol (anticancer agent)magnified 250X

                                            Cholesterol(40X)

                                            Microprocessor(60X)

                                            Nickel oxidethin film(600X)

                                            Surface of audio CD(1750X)

                                            Organicsuperconductor(450X)

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Automated visual inspection of manufactured goods

                                            a bc de f

                                            a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Digital Image ProcessingDigital Image Processing

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                                            Imaging in the Microwave Band

                                            The dominant aplication of imaging in the microwave band ndash radar

                                            bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                            bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                            bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                            An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Spaceborne radar image of mountains in southeast Tibet

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Imaging in the Radio Band

                                            medicine astronomy

                                            MRI = Magnetic Resonance Imaging

                                            This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                            Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                            The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            MRI images of a human knee (left) and spine (right)

                                            Digital Image ProcessingDigital Image Processing

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                                            Images of the Crab Pulsar covering the electromagnetic spectrum

                                            Gamma X-ray Optical Infrared Radio

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Other Imaging Modalities

                                            acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                            Imaging using sound geological explorations industry medicine

                                            Mineral and oil exploration

                                            For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                            Digital Image ProcessingDigital Image Processing

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                                            Biometry - iris

                                            Digital Image ProcessingDigital Image Processing

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                                            Biometry - fingerprint

                                            Digital Image ProcessingDigital Image Processing

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                                            Face detection and recognition

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Gender identification

                                            Digital Image ProcessingDigital Image Processing

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                                            Image morphing

                                            Digital Image ProcessingDigital Image Processing

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                                            Fundamental Steps in DIP

                                            methods whose input and output are images

                                            methods whose inputs are images but whose outputs are attributes extracted from those images

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Outputs are images

                                            bull image acquisition

                                            bull image filtering and enhancement

                                            bull image restoration

                                            bull color image processing

                                            bull wavelets and multiresolution processing

                                            bull compression

                                            bull morphological processing

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Outputs are attributes

                                            bull morphological processing

                                            bull segmentation

                                            bull representation and description

                                            bull object recognition

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Image acquisition - may involve preprocessing such as scaling

                                            Image enhancement

                                            bull manipulating an image so that the result is more suitable than the original for a specific operation

                                            bull enhancement is problem oriented

                                            bull there is no general sbquotheoryrsquo of image enhancement

                                            bull enhancement use subjective methods for image emprovement

                                            bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Image restoration

                                            bull improving the appearance of an image

                                            bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                            Color image processing

                                            bull fundamental concept in color models

                                            bull basic color processing in a digital domain

                                            Wavelets and multiresolution processing

                                            representing images in various degree of resolution

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Compression

                                            reducing the storage required to save an image or the bandwidth required to transmit it

                                            Morphological processing

                                            bull tools for extracting image components that are useful in the representation and description of shape

                                            bull a transition from processes that output images to processes that outputimage attributes

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Segmentation

                                            bull partitioning an image into its constituents parts or objects

                                            bull autonomous segmentation is one of the most difficult tasks of DIP

                                            bull the more accurate the segmentation the more likley recognition is to succeed

                                            Representation and description (almost always follows segmentation)

                                            bull segmentation produces either the boundary of a region or all the poits in the region itself

                                            bull converting the data produced by segmentation to a form suitable for computer processing

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                            bull complete region the focus is on internal properties such as texture or skeletal shape

                                            bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                            Object recognition

                                            the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                            Knowledge database

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Simplified diagramof a cross sectionof the human eye

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

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                                            Week 1Week 1

                                            Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                            The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                            Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                            The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                            The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                            Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                            Fovea = the place where the image of the object of interest falls on

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                            Blind spot region without receptors

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Image formation in the eye

                                            Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                            Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                            distance between lens and retina along visual axix = 17 mm

                                            range of focal length = 14 mm to 17 mm

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Digital Image ProcessingDigital Image Processing

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                                            Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Optical illusions

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                            gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                            quantities that describe the quality of a chromatic light source radiance

                                            the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                            measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                            brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            the physical meaning is determined by the source of the image

                                            ( )f D f x y

                                            Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                            f(xy) ndash characterized by two components

                                            i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                            r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                            ( ) ( ) ( )

                                            0 ( ) 0 ( ) 1

                                            f x y i x y r x y

                                            i x y r x y

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                            i(xy) ndash determined by the illumination source

                                            r(xy) ndash determined by the characteristics of the imaged objects

                                            is called gray (or intensity) scale

                                            In practice

                                            min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                            indoor values without additional illuminationmin max10 1000L L

                                            black whitemin max0 1 0 1 0 1L L L L l l L

                                            min maxL L

                                            Digital Image ProcessingDigital Image Processing

                                            Week 1Week 1

                                            Digital Image Processing

                                            Week 1

                                            Image Sampling and Quantization

                                            - the output of the sensors is a continuous voltage waveform related to the sensed

                                            scene

                                            converting a continuous image f to digital form

                                            - digitizing (x y) is called sampling

                                            - digitizing f(x y) is called quantization

                                            Digital Image Processing

                                            Week 1

                                            Digital Image Processing

                                            Week 1

                                            Continuous image projected onto a sensor array Result of image sampling and quantization

                                            Digital Image Processing

                                            Week 1

                                            Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                            (00) (01) (0 1)(10) (11) (1 1)

                                            ( )

                                            ( 10) ( 11) ( 1 1)

                                            f f f Nf f f N

                                            f x y

                                            f M f M f M N

                                            image element pixel

                                            00 01 0 1

                                            10 11 1 1

                                            10 11 1 1

                                            ( ) ( )

                                            N

                                            i jN M N

                                            i j

                                            M M M N

                                            a a aa f x i y j f i ja a a

                                            Aa

                                            a a a

                                            f(00) ndash the upper left corner of the image

                                            Digital Image Processing

                                            Week 1

                                            M N ge 0 L=2k

                                            [0 1]i j i ja a L

                                            Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                            Digital Image Processing

                                            Week 1

                                            Digital Image Processing

                                            Week 1

                                            Number of bits required to store a digitized image

                                            for 2 b M N k M N b N k

                                            When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                            Digital Image Processing

                                            Week 1

                                            Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                            Measures line pairs per unit distance dots (pixels) per unit distance

                                            Image resolution = the largest number of discernible line pairs per unit distance

                                            (eg 100 line pairs per mm)

                                            Dots per unit distance are commonly used in printing and publishing

                                            In US the measure is expressed in dots per inch (dpi)

                                            (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                            Intensity resolution ndash the smallest discernible change in intensity level

                                            The number of intensity levels (L) is determined by hardware considerations

                                            L=2k ndash most common k = 8

                                            Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                            Digital Image Processing

                                            Week 1

                                            Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                            150 dpi (lower left) 72 dpi (lower right)

                                            Digital Image Processing

                                            Week 1

                                            Reducing the number of gray levels 256 128 64 32

                                            Digital Image Processing

                                            Week 1

                                            Reducing the number of gray levels 16 8 4 2

                                            Digital Image Processing

                                            Week 1

                                            Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                            Shrinking zooming ndash image resizing ndash image resampling methods

                                            Interpolation is the process of using known data to estimate values at unknown locations

                                            Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                            750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                            same spacing as the original and then shrink it so that it fits exactly over the original

                                            image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                            Problem assignment of intensity-level in the new 750 times 750 grid

                                            Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                            intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                            This technique has the tendency to produce undesirable effects like severe distortion of

                                            straight edges

                                            Digital Image Processing

                                            Week 1

                                            Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                            where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                            be written using the 4 nearest neighbors of point (x y)

                                            Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                            modest increase in computational effort

                                            Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                            nearest neighbors of the point 3 3

                                            0 0

                                            ( ) i ji j

                                            i jv x y c x y

                                            The coefficients cij are obtained solving a 16x16 linear system

                                            intensity levels of the 16 nearest neighbors of 3 3

                                            0 0

                                            ( )i ji j

                                            i jc x y x y

                                            Digital Image Processing

                                            Week 1

                                            Generally bicubic interpolation does a better job of preserving fine detail than the

                                            bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                            programs such as Adobe Photoshop and Corel Photopaint

                                            Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                            the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                            then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                            neighbor interpolation was used (both for shrinking and zooming)

                                            Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                            interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                            from 1250 dpi to 150 dpi (instead of 72 dpi)

                                            Digital Image Processing

                                            Week 1

                                            Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                            Digital Image Processing

                                            Week 1

                                            Neighbors of a Pixel

                                            A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                            This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                            The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                            and are denoted ND(p)

                                            The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                            N8 (p)

                                            If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                            fall outside the image

                                            Digital Image Processing

                                            Week 1

                                            Adjacency Connectivity Regions Boundaries

                                            Denote by V the set of intensity levels used to define adjacency

                                            - in a binary image V 01 (V=0 V=1)

                                            - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                            We consider 3 types of adjacency

                                            (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                            (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                            (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                            m-adjacent if

                                            4( )q N p or

                                            ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                            Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                            ambiguities that often arise when 8-adjacency is used Consider the example

                                            Digital Image Processing

                                            Week 1

                                            binary image

                                            0 1 1 0 1 1 0 1 1

                                            1 0 1 0 0 1 0 0 1 0

                                            0 0 1 0 0 1 0 0 1

                                            V

                                            The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                            8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                            m-adjacency

                                            Digital Image Processing

                                            Week 1

                                            A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                            is a sequence of distinct pixels with coordinates

                                            and are adjacent 0 0 1 1

                                            1 1

                                            ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                            n n

                                            i i i i

                                            x y x y x y x y s tx y x y i n

                                            The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                            Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                            Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                            in S if there exists a path between them consisting only of pixels from S

                                            S is a connected set if there is a path in S between any 2 pixels in S

                                            Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                            Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                            that are not adjacent are said to be disjoint When referring to regions only 4- and

                                            8-adjacency are considered

                                            Digital Image Processing

                                            Week 1

                                            Suppose that an image contains K disjoint regions 1 kR k K none of which

                                            touches the image border

                                            the complement of 1

                                            ( )K

                                            cu k u u

                                            k

                                            R R R R

                                            We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                            background of the image

                                            The boundary (border or contour) of a region R is the set of points that are adjacent to

                                            points in the complement of R (R)c The border of an image is the set of pixels in the

                                            region that have at least one background neighbor This definition is referred to as the

                                            inner border to distinguish it from the notion of outer border which is the corresponding

                                            border in the background

                                            Digital Image Processing

                                            Week 1

                                            Distance measures

                                            For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                            function or metric if

                                            (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                            (b) D(p q) = D(q p)

                                            (c) D(p z) le D(p q) + D(q z)

                                            The Euclidean distance between p and q is defined as 1

                                            2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                            The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                            centered at (x y)

                                            Digital Image Processing

                                            Week 1

                                            The D4 distance (also called city-block distance) between p and q is defined as

                                            4( ) | | | |D p q x s y t

                                            The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                            4

                                            22 1 2

                                            2 2 1 0 1 22 1 2

                                            2

                                            D

                                            The pixels with D4 = 1 are the 4-neighbors of (x y)

                                            The D8 distance (called the chessboard distance) between p and q is defined as

                                            8( ) max| | | |D p q x s y t

                                            The pixels q for which 8( )D p q r form a square centered at (x y)

                                            Digital Image Processing

                                            Week 1

                                            8

                                            2 2 2 2 22 1 1 1 2

                                            2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                            D

                                            The pixels with D8 = 1 are the 8-neighbors of (x y)

                                            D4 and D8 distances are independent of any paths that might exist between p and q

                                            because these distances involve only the coordinates of the point

                                            Digital Image Processing

                                            Week 1

                                            Array versus Matrix Operations

                                            An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                            11 12 11 12

                                            21 22 21 22

                                            a a b ba a b b

                                            Array product

                                            11 12 11 12 11 11 12 12

                                            21 22 21 22 21 21 22 21

                                            a a b b a b a ba a b b a b a b

                                            Matrix product

                                            11 12 11 12 11 11 12 21 11 12 12 21

                                            21 22 21 22 21 11 22 21 21 12 22 22

                                            a a b b a b a b a b a ba a b b a b a b a b a b

                                            We assume array operations unless stated otherwise

                                            Digital Image Processing

                                            Week 1

                                            Linear versus Nonlinear Operations

                                            One of the most important classifications of image-processing methods is whether it is

                                            linear or nonlinear

                                            ( ) ( )H f x y g x y

                                            H is said to be a linear operator if

                                            images1 2 1 2

                                            1 2

                                            ( ) ( ) ( ) ( )

                                            H a f x y b f x y a H f x y b H f x y

                                            a b f f

                                            Example of nonlinear operator

                                            the maximum value of the pixels of image max ( )H f f x y f

                                            1 2

                                            0 2 6 5 1 1

                                            2 3 4 7f f a b

                                            Digital Image Processing

                                            Week 1

                                            1 2

                                            0 2 6 5 6 3max max 1 ( 1) max 2

                                            2 3 4 7 2 4a f b f

                                            0 2 6 51 max ( 1) max 3 ( 1)7 4

                                            2 3 4 7

                                            Arithmetic Operations in Image Processing

                                            Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                            f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                            For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                            value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                            The two random variables are uncorrelated when their covariance is 0

                                            Digital Image Processing

                                            Week 1

                                            Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                            used in image enhancement)

                                            1

                                            1( ) ( )K

                                            ii

                                            g x y g x yK

                                            If the noise satisfies the properties stated above we have

                                            2 2( ) ( )

                                            1( ) ( ) g x y x yE g x y f x yK

                                            ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                            and g respectively The standard deviation (square root of the variance) at any point in

                                            the average image is

                                            ( ) ( )1

                                            g x y x yK

                                            Digital Image Processing

                                            Week 1

                                            As K increases the variability (as measured by the variance or the standard deviation) of

                                            the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                            means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                            averaging process increases

                                            An important application of image averaging is in the field of astronomy where imaging

                                            under very low light levels frequently causes sensor noise to render single images

                                            virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                            was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                            64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                            images respectively

                                            Digital Image Processing

                                            Week 1

                                            Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                            100 noisy images

                                            a b c d e f

                                            Digital Image Processing

                                            Week 1

                                            A frequent application of image subtraction is in the enhancement of differences between

                                            images

                                            (a) (b) (c)

                                            Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                            significant bit of each pixel (c) the difference between the two images

                                            Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                            Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                            images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                            difference between images (a) and (b)

                                            Digital Image Processing

                                            Week 1

                                            Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                            h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                            intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                            source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                            bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                            anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                            images after injection of the contrast medium

                                            In g(x y) we can find the differences between h and f as enhanced detail

                                            Images being captured at TV rates we obtain a movie showing how the contrast medium

                                            propagates through the various arteries in the area being observed

                                            Digital Image Processing

                                            Week 1

                                            a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                            Digital Image Processing

                                            Week 1

                                            An important application of image multiplication (and division) is shading correction

                                            Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                            f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                            When the shading function is known

                                            ( )( )( )

                                            g x yf x yh x y

                                            h(x y) is unknown but we have access to the imaging system we can obtain an

                                            approximation to the shading function by imaging a target of constant intensity When the

                                            sensor is not available often the shading pattern can be estimated from the image

                                            Digital Image Processing

                                            Week 1

                                            (a) (b) (c)

                                            Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                            Digital Image Processing

                                            Week 1

                                            Another use of image multiplication is in masking also called region of interest (ROI)

                                            operations The process consists of multiplying a given image by a mask image that has

                                            1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                            image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                            (a) (b) (c)

                                            Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                            Digital Image Processing

                                            Week 1

                                            In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                            min( )mf f f

                                            0 ( 255)max( )

                                            ms

                                            m

                                            ff K K K

                                            f

                                            Digital Image Processing

                                            Week 1

                                            Spatial Operations

                                            - are performed directly on the pixels of a given image

                                            There are three categories of spatial operations

                                            single-pixel operations

                                            neighborhood operations

                                            geometric spatial transformations

                                            Single-pixel operations

                                            - change the values of intensity for the individual pixels ( )s T z

                                            where z is the intensity of a pixel in the original image and s is the intensity of the

                                            corresponding pixel in the processed image

                                            Digital Image Processing

                                            Week 1

                                            Neighborhood operations

                                            Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                            in an image f Neighborhood processing generates new intensity level at point (x y)

                                            based on the values of the intensities of the points in Sxy For example if Sxy is a

                                            rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                            intensity by computing the average value of the pixels in Sxy

                                            ( )

                                            1( ) ( )xyr c S

                                            g x y f r cm n

                                            The net effect is to perform local blurring in the original image This type of process is

                                            used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                            largest region of an image

                                            Digital Image Processing

                                            Week 1

                                            Geometric spatial transformations and image registration

                                            - modify the spatial relationship between pixels in an image

                                            - these transformations are often called rubber-sheet transformations (analogous to

                                            printing an image on a sheet of rubber and then stretching the sheet according to a

                                            predefined set of rules

                                            A geometric transformation consists of 2 basic operations

                                            1 a spatial transformation of coordinates

                                            2 intensity interpolation that assign intensity values to the spatial transformed

                                            pixels

                                            The coordinate system transformation ( ) [( )]x y T v w

                                            (v w) ndash pixel coordinates in the original image

                                            (x y) ndash pixel coordinates in the transformed image

                                            Digital Image Processing

                                            Week 1

                                            [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                            Affine transform

                                            11 1211 21 31

                                            21 2212 22 33

                                            31 32

                                            0[ 1] [ 1] [ 1] 0

                                            1

                                            t tx t v t w t

                                            x y v w T v w t ty t v t w t

                                            t t

                                            (AT)

                                            This transform can scale rotate translate or shear a set of coordinate points depending

                                            on the elements of the matrix T If we want to resize an image rotate it and move the

                                            result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                            scaling rotation and translation matrices from Table 1

                                            Digital Image Processing

                                            Week 1

                                            Affine transformations

                                            Digital Image Processing

                                            Week 1

                                            The preceding transformations relocate pixels on an image to new locations To complete

                                            the process we have to assign intensity values to those locations This task is done by

                                            using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                            In practice we can use equation (AT) in two basic ways

                                            forward mapping scan the pixels of the input image (v w) compute the new spatial

                                            location (x y) of the corresponding pixel in the new image using (AT) directly

                                            Problems

                                            - intensity assignment when 2 or more pixels in the original image are transformed to

                                            the same location in the output image

                                            - some output locations have no correspondent in the original image (no intensity

                                            assignment)

                                            Digital Image Processing

                                            Week 1

                                            inverse mapping scans the output pixel locations and at each location (x y)

                                            computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                            It then interpolates among the nearest input pixels to determine the intensity of the output

                                            pixel value

                                            Inverse mappings are more efficient to implement than forward mappings and are used in

                                            numerous commercial implementations of spatial transformations (MATLAB for ex)

                                            Digital Image Processing

                                            Week 1

                                            Digital Image Processing

                                            Week 1

                                            Image registration ndash align two or more images of the same scene

                                            In image registration we have available the input and output images but the specific

                                            transformation that produced the output image from the input is generally unknown

                                            The problem is to estimate the transformation function and then use it to register the two

                                            images

                                            - it may be of interest to align (register) two or more image taken at approximately the

                                            same time but using different imaging systems (MRI scanner and a PET scanner)

                                            - align images of a given location taken by the same instrument at different moments

                                            of time (satellite images)

                                            Solving the problem using tie points (also called control points) which are

                                            corresponding points whose locations are known precisely in the input and reference

                                            image

                                            Digital Image Processing

                                            Week 1

                                            How to select tie points

                                            - interactively selecting them

                                            - use of algorithms that try to detect these points

                                            - some imaging systems have physical artifacts (small metallic objects) embedded in

                                            the imaging sensors These objects produce a set of known points (called reseau

                                            marks) directly on all images captured by the system which can be used as guides

                                            for establishing tie points

                                            The problem of estimating the transformation is one of modeling Suppose we have a set

                                            of 4 tie points both on the input image and the reference image A simple model based on

                                            a bilinear approximation is given by

                                            1 2 3 4

                                            5 6 7 8

                                            x c v c w c v w cy c v c w c v w c

                                            (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                            Digital Image Processing

                                            Week 1

                                            When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                            frequently is to select a larger number of tie points and using this new set of tie points

                                            subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                            subregions marked by 4 tie points we applied the transformation model described above

                                            The number of tie points and the sophistication of the model required to solve the register

                                            problem depend on the severity of the geometrical distortion

                                            Digital Image Processing

                                            Week 1

                                            a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                            Digital Image Processing

                                            Week 1

                                            Probabilistic Methods

                                            zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                            p(zk) = the probability that the intensity level zk occurs in the given image

                                            ( ) kk

                                            np zM N

                                            nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                            pixels in the image) 1

                                            0( ) 1

                                            L

                                            kk

                                            p z

                                            The mean (average) intensity of an image is given by 1

                                            0( )

                                            L

                                            k kk

                                            m z p z

                                            Digital Image Processing

                                            Week 1

                                            The variance of the intensities is 1

                                            2 2

                                            0( ) ( )

                                            L

                                            k kk

                                            z m p z

                                            The variance is a measure of the spread of the values of z about the mean so it is a

                                            measure of image contrast Usually for measuring image contrast the standard deviation

                                            ( ) is used

                                            The n-th moment of a random variable z about the mean is defined as 1

                                            0( ) ( ) ( )

                                            Ln

                                            n k kk

                                            z z m p z

                                            ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                            3( ) 0z the intensities are biased to values higher than the mean

                                            ( 3( ) 0z the intensities are biased to values lower than the mean

                                            Digital Image Processing

                                            Week 1

                                            3( ) 0z the intensities are distributed approximately equally on both side of the

                                            mean

                                            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                            Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                            Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                            Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                            Digital Image Processing

                                            Week 1

                                            Intensity Transformations and Spatial Filtering

                                            ( ) ( )g x y T f x y

                                            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                            neighborhood of (x y)

                                            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                            and much smaller in size than the image

                                            Digital Image Processing

                                            Week 1

                                            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                            called spatial filter (spatial mask kernel template or window)

                                            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                            ( )s T r

                                            s and r are denoting respectively the intensity of g and f at (x y)

                                            Figure 2 left - T produces an output image of higher contrast than the original by

                                            darkening the intensity levels below k and brightening the levels above k ndash this technique

                                            is called contrast stretching

                                            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                            Digital Image Processing

                                            Week 1

                                            Figure 2 right - T produces a binary output image A mapping of this form is called

                                            thresholding function

                                            Some Basic Intensity Transformation Functions

                                            Image Negatives

                                            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                            - equivalent of a photographic negative

                                            - technique suited for enhancing white or gray detail embedded in dark regions of an

                                            image

                                            Digital Image Processing

                                            Week 1

                                            Original Negative image

                                            Digital Image Processing

                                            Week 1

                                            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                            Some basic intensity transformation functions

                                            Digital Image Processing

                                            Week 1

                                            This transformation maps a narrow range of low intensity values in the input into a wider

                                            range An operator of this type is used to expand the values of dark pixels in an image

                                            while compressing the higher-level values The opposite is true for the inverse log

                                            transformation The log functions compress the dynamic range of images with large

                                            variations in pixel values

                                            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                            Digital Image Processing

                                            Week 1

                                            Power-Law (Gamma) Transformations

                                            - positive constants( ) ( ( ) )s T r c r c s c r

                                            Plots of gamma transformation for different values of γ (c=1)

                                            Digital Image Processing

                                            Week 1

                                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                                            of output values with the opposite being true for higher values of input values The

                                            curves with 1 have the opposite effect of those generated with values of 1

                                            1c - identity transformation

                                            A variety of devices used for image capture printing and display respond according to a

                                            power law The process used to correct these power-law response phenomena is called

                                            gamma correction

                                            Digital Image Processing

                                            Week 1

                                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                            Digital Image Processing

                                            Week 1

                                            Piecewise-Linear Transformations Functions

                                            Contrast stretching

                                            - a process that expands the range of intensity levels in an image so it spans the full

                                            intensity range of the recording tool or display device

                                            a b c d Fig5

                                            Digital Image Processing

                                            Week 1

                                            11

                                            1

                                            2 1 1 21 2

                                            2 1 2 1

                                            22

                                            2

                                            [0 ]

                                            ( ) ( )( ) [ ]( ) ( )

                                            ( 1 ) [ 1]( 1 )

                                            s r r rrs r r s r rT r r r r

                                            r r r rs L r r r L

                                            L r

                                            Digital Image Processing

                                            Week 1

                                            1 1 2 2r s r s identity transformation (no change)

                                            1 2 1 2 0 1r r s s L thresholding function

                                            Figure 5(b) shows an 8-bit image with low contrast

                                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                            in the image respectively Thus the transformation function stretched the levels linearly

                                            from their original range to the full range [0 L-1]

                                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                            2 2 1r s m L where m is the mean gray level in the image

                                            The original image on which these results are based is a scanning electron microscope

                                            image of pollen magnified approximately 700 times

                                            Digital Image Processing

                                            Week 1

                                            Intensity-level slicing

                                            - highlighting a specific range of intensities in an image

                                            There are two approaches for intensity-level slicing

                                            1 display in one value (white for example) all the values in the range of interest and in

                                            another (say black) all other intensities (Figure 311 (a))

                                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                            intensities in the image (Figure 311 (b))

                                            Digital Image Processing

                                            Week 1

                                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                                            the top of the scale of intensities This type of enhancement produces a binary image

                                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                                            Highlights range [A B] and preserves all other intensities

                                            Digital Image Processing

                                            Week 1

                                            which is useful for studying the shape of the flow of the contrast substance (to detect

                                            blockageshellip)

                                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                            image around the mean intensity was set to black the other intensities remain unchanged

                                            Fig 6 - Aortic angiogram and intensity sliced versions

                                            Digital Image Processing

                                            Week 1

                                            Bit-plane slicing

                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                            This technique highlights the contribution made to the whole image appearances by each

                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                            Digital Image Processing

                                            Week 1

                                            Digital Image Processing

                                            Week 1

                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                            • DIP 1 2017
                                            • DIP 02 (2017)

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Angiography = contrast-enhancement radiography

                                              Angiograms = images of blood vessels

                                              A catheter is inserted into an artery or vein in the groin The catheter is threaded into the blood vessel and guided to the area to be studied When it reaches the area to be studied a X-ray contrast medium is injected through the catheter This enhances contrast of the blood vessels and enables radiologist to see anyirregularities or blockages

                                              X-rays are used in CAT (computerized axial tomography)

                                              X-rays used in industrial processes (examine circuit boards for flows in manifacturing)

                                              Industrial CAT scans are useful when the parts can be penetreted by X-rays

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Examples of X-ray imaging

                                              Chest X-rayAortic angiogram

                                              Head CT Cygnus LoopCircuit boards

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Imaging in the Ultraviolet Band

                                              Litography industrial inspection microscopy biological imaging astronomical observations

                                              Ultraviolet light is used in fluorescence microscopy

                                              Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                                              other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                                              and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Imaging in the Visible and Infrared Bands

                                              Light microscopy astronomy remote sensing industry law enforcement

                                              LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                                              Weather observations and prediction produce major applications of multispectral image from satellites

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Satellite images of Washington DC area in spectral bands of the Table 1

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Examples of light microscopy

                                              Taxol (anticancer agent)magnified 250X

                                              Cholesterol(40X)

                                              Microprocessor(60X)

                                              Nickel oxidethin film(600X)

                                              Surface of audio CD(1750X)

                                              Organicsuperconductor(450X)

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Automated visual inspection of manufactured goods

                                              a bc de f

                                              a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Imaging in the Microwave Band

                                              The dominant aplication of imaging in the microwave band ndash radar

                                              bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                              bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                              bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                              An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Spaceborne radar image of mountains in southeast Tibet

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Imaging in the Radio Band

                                              medicine astronomy

                                              MRI = Magnetic Resonance Imaging

                                              This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                              Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                              The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              MRI images of a human knee (left) and spine (right)

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Images of the Crab Pulsar covering the electromagnetic spectrum

                                              Gamma X-ray Optical Infrared Radio

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Other Imaging Modalities

                                              acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                              Imaging using sound geological explorations industry medicine

                                              Mineral and oil exploration

                                              For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

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                                              Biometry - iris

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                                              Biometry - fingerprint

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                                              Face detection and recognition

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                                              Gender identification

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                                              Image morphing

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                                              Fundamental Steps in DIP

                                              methods whose input and output are images

                                              methods whose inputs are images but whose outputs are attributes extracted from those images

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Outputs are images

                                              bull image acquisition

                                              bull image filtering and enhancement

                                              bull image restoration

                                              bull color image processing

                                              bull wavelets and multiresolution processing

                                              bull compression

                                              bull morphological processing

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                                              Outputs are attributes

                                              bull morphological processing

                                              bull segmentation

                                              bull representation and description

                                              bull object recognition

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                                              Image acquisition - may involve preprocessing such as scaling

                                              Image enhancement

                                              bull manipulating an image so that the result is more suitable than the original for a specific operation

                                              bull enhancement is problem oriented

                                              bull there is no general sbquotheoryrsquo of image enhancement

                                              bull enhancement use subjective methods for image emprovement

                                              bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

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                                              Week 1Week 1

                                              Image restoration

                                              bull improving the appearance of an image

                                              bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                              Color image processing

                                              bull fundamental concept in color models

                                              bull basic color processing in a digital domain

                                              Wavelets and multiresolution processing

                                              representing images in various degree of resolution

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                                              Week 1Week 1

                                              Compression

                                              reducing the storage required to save an image or the bandwidth required to transmit it

                                              Morphological processing

                                              bull tools for extracting image components that are useful in the representation and description of shape

                                              bull a transition from processes that output images to processes that outputimage attributes

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                                              Week 1Week 1

                                              Segmentation

                                              bull partitioning an image into its constituents parts or objects

                                              bull autonomous segmentation is one of the most difficult tasks of DIP

                                              bull the more accurate the segmentation the more likley recognition is to succeed

                                              Representation and description (almost always follows segmentation)

                                              bull segmentation produces either the boundary of a region or all the poits in the region itself

                                              bull converting the data produced by segmentation to a form suitable for computer processing

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                              bull complete region the focus is on internal properties such as texture or skeletal shape

                                              bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                              Object recognition

                                              the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                              Knowledge database

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                                              Simplified diagramof a cross sectionof the human eye

                                              Digital Image ProcessingDigital Image Processing

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                                              Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                              The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                              Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                              The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                              The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                              Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                              Fovea = the place where the image of the object of interest falls on

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                                              Week 1Week 1

                                              Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                              Blind spot region without receptors

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                                              Week 1Week 1

                                              Image formation in the eye

                                              Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                              Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                              distance between lens and retina along visual axix = 17 mm

                                              range of focal length = 14 mm to 17 mm

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Digital Image ProcessingDigital Image Processing

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                                              Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Optical illusions

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                              gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                              quantities that describe the quality of a chromatic light source radiance

                                              the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                              measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                              brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                              Digital Image ProcessingDigital Image Processing

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                                              the physical meaning is determined by the source of the image

                                              ( )f D f x y

                                              Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                              f(xy) ndash characterized by two components

                                              i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                              r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                              ( ) ( ) ( )

                                              0 ( ) 0 ( ) 1

                                              f x y i x y r x y

                                              i x y r x y

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                                              Week 1Week 1

                                              r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                              i(xy) ndash determined by the illumination source

                                              r(xy) ndash determined by the characteristics of the imaged objects

                                              is called gray (or intensity) scale

                                              In practice

                                              min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                              indoor values without additional illuminationmin max10 1000L L

                                              black whitemin max0 1 0 1 0 1L L L L l l L

                                              min maxL L

                                              Digital Image ProcessingDigital Image Processing

                                              Week 1Week 1

                                              Digital Image Processing

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                                              Image Sampling and Quantization

                                              - the output of the sensors is a continuous voltage waveform related to the sensed

                                              scene

                                              converting a continuous image f to digital form

                                              - digitizing (x y) is called sampling

                                              - digitizing f(x y) is called quantization

                                              Digital Image Processing

                                              Week 1

                                              Digital Image Processing

                                              Week 1

                                              Continuous image projected onto a sensor array Result of image sampling and quantization

                                              Digital Image Processing

                                              Week 1

                                              Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                              (00) (01) (0 1)(10) (11) (1 1)

                                              ( )

                                              ( 10) ( 11) ( 1 1)

                                              f f f Nf f f N

                                              f x y

                                              f M f M f M N

                                              image element pixel

                                              00 01 0 1

                                              10 11 1 1

                                              10 11 1 1

                                              ( ) ( )

                                              N

                                              i jN M N

                                              i j

                                              M M M N

                                              a a aa f x i y j f i ja a a

                                              Aa

                                              a a a

                                              f(00) ndash the upper left corner of the image

                                              Digital Image Processing

                                              Week 1

                                              M N ge 0 L=2k

                                              [0 1]i j i ja a L

                                              Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                              Digital Image Processing

                                              Week 1

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                                              Week 1

                                              Number of bits required to store a digitized image

                                              for 2 b M N k M N b N k

                                              When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                              Digital Image Processing

                                              Week 1

                                              Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                              Measures line pairs per unit distance dots (pixels) per unit distance

                                              Image resolution = the largest number of discernible line pairs per unit distance

                                              (eg 100 line pairs per mm)

                                              Dots per unit distance are commonly used in printing and publishing

                                              In US the measure is expressed in dots per inch (dpi)

                                              (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                              Intensity resolution ndash the smallest discernible change in intensity level

                                              The number of intensity levels (L) is determined by hardware considerations

                                              L=2k ndash most common k = 8

                                              Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                              Digital Image Processing

                                              Week 1

                                              Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                              150 dpi (lower left) 72 dpi (lower right)

                                              Digital Image Processing

                                              Week 1

                                              Reducing the number of gray levels 256 128 64 32

                                              Digital Image Processing

                                              Week 1

                                              Reducing the number of gray levels 16 8 4 2

                                              Digital Image Processing

                                              Week 1

                                              Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                              Shrinking zooming ndash image resizing ndash image resampling methods

                                              Interpolation is the process of using known data to estimate values at unknown locations

                                              Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                              750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                              same spacing as the original and then shrink it so that it fits exactly over the original

                                              image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                              Problem assignment of intensity-level in the new 750 times 750 grid

                                              Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                              intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                              This technique has the tendency to produce undesirable effects like severe distortion of

                                              straight edges

                                              Digital Image Processing

                                              Week 1

                                              Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                              where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                              be written using the 4 nearest neighbors of point (x y)

                                              Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                              modest increase in computational effort

                                              Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                              nearest neighbors of the point 3 3

                                              0 0

                                              ( ) i ji j

                                              i jv x y c x y

                                              The coefficients cij are obtained solving a 16x16 linear system

                                              intensity levels of the 16 nearest neighbors of 3 3

                                              0 0

                                              ( )i ji j

                                              i jc x y x y

                                              Digital Image Processing

                                              Week 1

                                              Generally bicubic interpolation does a better job of preserving fine detail than the

                                              bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                              programs such as Adobe Photoshop and Corel Photopaint

                                              Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                              the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                              then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                              neighbor interpolation was used (both for shrinking and zooming)

                                              Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                              interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                              from 1250 dpi to 150 dpi (instead of 72 dpi)

                                              Digital Image Processing

                                              Week 1

                                              Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                              Digital Image Processing

                                              Week 1

                                              Neighbors of a Pixel

                                              A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                              This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                              The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                              and are denoted ND(p)

                                              The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                              N8 (p)

                                              If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                              fall outside the image

                                              Digital Image Processing

                                              Week 1

                                              Adjacency Connectivity Regions Boundaries

                                              Denote by V the set of intensity levels used to define adjacency

                                              - in a binary image V 01 (V=0 V=1)

                                              - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                              We consider 3 types of adjacency

                                              (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                              (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                              (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                              m-adjacent if

                                              4( )q N p or

                                              ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                              Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                              ambiguities that often arise when 8-adjacency is used Consider the example

                                              Digital Image Processing

                                              Week 1

                                              binary image

                                              0 1 1 0 1 1 0 1 1

                                              1 0 1 0 0 1 0 0 1 0

                                              0 0 1 0 0 1 0 0 1

                                              V

                                              The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                              8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                              m-adjacency

                                              Digital Image Processing

                                              Week 1

                                              A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                              is a sequence of distinct pixels with coordinates

                                              and are adjacent 0 0 1 1

                                              1 1

                                              ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                              n n

                                              i i i i

                                              x y x y x y x y s tx y x y i n

                                              The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                              Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                              Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                              in S if there exists a path between them consisting only of pixels from S

                                              S is a connected set if there is a path in S between any 2 pixels in S

                                              Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                              Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                              that are not adjacent are said to be disjoint When referring to regions only 4- and

                                              8-adjacency are considered

                                              Digital Image Processing

                                              Week 1

                                              Suppose that an image contains K disjoint regions 1 kR k K none of which

                                              touches the image border

                                              the complement of 1

                                              ( )K

                                              cu k u u

                                              k

                                              R R R R

                                              We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                              background of the image

                                              The boundary (border or contour) of a region R is the set of points that are adjacent to

                                              points in the complement of R (R)c The border of an image is the set of pixels in the

                                              region that have at least one background neighbor This definition is referred to as the

                                              inner border to distinguish it from the notion of outer border which is the corresponding

                                              border in the background

                                              Digital Image Processing

                                              Week 1

                                              Distance measures

                                              For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                              function or metric if

                                              (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                              (b) D(p q) = D(q p)

                                              (c) D(p z) le D(p q) + D(q z)

                                              The Euclidean distance between p and q is defined as 1

                                              2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                              The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                              centered at (x y)

                                              Digital Image Processing

                                              Week 1

                                              The D4 distance (also called city-block distance) between p and q is defined as

                                              4( ) | | | |D p q x s y t

                                              The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                              4

                                              22 1 2

                                              2 2 1 0 1 22 1 2

                                              2

                                              D

                                              The pixels with D4 = 1 are the 4-neighbors of (x y)

                                              The D8 distance (called the chessboard distance) between p and q is defined as

                                              8( ) max| | | |D p q x s y t

                                              The pixels q for which 8( )D p q r form a square centered at (x y)

                                              Digital Image Processing

                                              Week 1

                                              8

                                              2 2 2 2 22 1 1 1 2

                                              2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                              D

                                              The pixels with D8 = 1 are the 8-neighbors of (x y)

                                              D4 and D8 distances are independent of any paths that might exist between p and q

                                              because these distances involve only the coordinates of the point

                                              Digital Image Processing

                                              Week 1

                                              Array versus Matrix Operations

                                              An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                              11 12 11 12

                                              21 22 21 22

                                              a a b ba a b b

                                              Array product

                                              11 12 11 12 11 11 12 12

                                              21 22 21 22 21 21 22 21

                                              a a b b a b a ba a b b a b a b

                                              Matrix product

                                              11 12 11 12 11 11 12 21 11 12 12 21

                                              21 22 21 22 21 11 22 21 21 12 22 22

                                              a a b b a b a b a b a ba a b b a b a b a b a b

                                              We assume array operations unless stated otherwise

                                              Digital Image Processing

                                              Week 1

                                              Linear versus Nonlinear Operations

                                              One of the most important classifications of image-processing methods is whether it is

                                              linear or nonlinear

                                              ( ) ( )H f x y g x y

                                              H is said to be a linear operator if

                                              images1 2 1 2

                                              1 2

                                              ( ) ( ) ( ) ( )

                                              H a f x y b f x y a H f x y b H f x y

                                              a b f f

                                              Example of nonlinear operator

                                              the maximum value of the pixels of image max ( )H f f x y f

                                              1 2

                                              0 2 6 5 1 1

                                              2 3 4 7f f a b

                                              Digital Image Processing

                                              Week 1

                                              1 2

                                              0 2 6 5 6 3max max 1 ( 1) max 2

                                              2 3 4 7 2 4a f b f

                                              0 2 6 51 max ( 1) max 3 ( 1)7 4

                                              2 3 4 7

                                              Arithmetic Operations in Image Processing

                                              Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                              f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                              For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                              value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                              The two random variables are uncorrelated when their covariance is 0

                                              Digital Image Processing

                                              Week 1

                                              Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                              used in image enhancement)

                                              1

                                              1( ) ( )K

                                              ii

                                              g x y g x yK

                                              If the noise satisfies the properties stated above we have

                                              2 2( ) ( )

                                              1( ) ( ) g x y x yE g x y f x yK

                                              ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                              and g respectively The standard deviation (square root of the variance) at any point in

                                              the average image is

                                              ( ) ( )1

                                              g x y x yK

                                              Digital Image Processing

                                              Week 1

                                              As K increases the variability (as measured by the variance or the standard deviation) of

                                              the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                              means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                              averaging process increases

                                              An important application of image averaging is in the field of astronomy where imaging

                                              under very low light levels frequently causes sensor noise to render single images

                                              virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                              was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                              64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                              images respectively

                                              Digital Image Processing

                                              Week 1

                                              Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                              100 noisy images

                                              a b c d e f

                                              Digital Image Processing

                                              Week 1

                                              A frequent application of image subtraction is in the enhancement of differences between

                                              images

                                              (a) (b) (c)

                                              Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                              significant bit of each pixel (c) the difference between the two images

                                              Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                              Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                              images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                              difference between images (a) and (b)

                                              Digital Image Processing

                                              Week 1

                                              Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                              h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                              intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                              source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                              bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                              anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                              images after injection of the contrast medium

                                              In g(x y) we can find the differences between h and f as enhanced detail

                                              Images being captured at TV rates we obtain a movie showing how the contrast medium

                                              propagates through the various arteries in the area being observed

                                              Digital Image Processing

                                              Week 1

                                              a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                              Digital Image Processing

                                              Week 1

                                              An important application of image multiplication (and division) is shading correction

                                              Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                              f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                              When the shading function is known

                                              ( )( )( )

                                              g x yf x yh x y

                                              h(x y) is unknown but we have access to the imaging system we can obtain an

                                              approximation to the shading function by imaging a target of constant intensity When the

                                              sensor is not available often the shading pattern can be estimated from the image

                                              Digital Image Processing

                                              Week 1

                                              (a) (b) (c)

                                              Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                              Digital Image Processing

                                              Week 1

                                              Another use of image multiplication is in masking also called region of interest (ROI)

                                              operations The process consists of multiplying a given image by a mask image that has

                                              1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                              image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                              (a) (b) (c)

                                              Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                              Digital Image Processing

                                              Week 1

                                              In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                              min( )mf f f

                                              0 ( 255)max( )

                                              ms

                                              m

                                              ff K K K

                                              f

                                              Digital Image Processing

                                              Week 1

                                              Spatial Operations

                                              - are performed directly on the pixels of a given image

                                              There are three categories of spatial operations

                                              single-pixel operations

                                              neighborhood operations

                                              geometric spatial transformations

                                              Single-pixel operations

                                              - change the values of intensity for the individual pixels ( )s T z

                                              where z is the intensity of a pixel in the original image and s is the intensity of the

                                              corresponding pixel in the processed image

                                              Digital Image Processing

                                              Week 1

                                              Neighborhood operations

                                              Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                              in an image f Neighborhood processing generates new intensity level at point (x y)

                                              based on the values of the intensities of the points in Sxy For example if Sxy is a

                                              rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                              intensity by computing the average value of the pixels in Sxy

                                              ( )

                                              1( ) ( )xyr c S

                                              g x y f r cm n

                                              The net effect is to perform local blurring in the original image This type of process is

                                              used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                              largest region of an image

                                              Digital Image Processing

                                              Week 1

                                              Geometric spatial transformations and image registration

                                              - modify the spatial relationship between pixels in an image

                                              - these transformations are often called rubber-sheet transformations (analogous to

                                              printing an image on a sheet of rubber and then stretching the sheet according to a

                                              predefined set of rules

                                              A geometric transformation consists of 2 basic operations

                                              1 a spatial transformation of coordinates

                                              2 intensity interpolation that assign intensity values to the spatial transformed

                                              pixels

                                              The coordinate system transformation ( ) [( )]x y T v w

                                              (v w) ndash pixel coordinates in the original image

                                              (x y) ndash pixel coordinates in the transformed image

                                              Digital Image Processing

                                              Week 1

                                              [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                              Affine transform

                                              11 1211 21 31

                                              21 2212 22 33

                                              31 32

                                              0[ 1] [ 1] [ 1] 0

                                              1

                                              t tx t v t w t

                                              x y v w T v w t ty t v t w t

                                              t t

                                              (AT)

                                              This transform can scale rotate translate or shear a set of coordinate points depending

                                              on the elements of the matrix T If we want to resize an image rotate it and move the

                                              result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                              scaling rotation and translation matrices from Table 1

                                              Digital Image Processing

                                              Week 1

                                              Affine transformations

                                              Digital Image Processing

                                              Week 1

                                              The preceding transformations relocate pixels on an image to new locations To complete

                                              the process we have to assign intensity values to those locations This task is done by

                                              using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                              In practice we can use equation (AT) in two basic ways

                                              forward mapping scan the pixels of the input image (v w) compute the new spatial

                                              location (x y) of the corresponding pixel in the new image using (AT) directly

                                              Problems

                                              - intensity assignment when 2 or more pixels in the original image are transformed to

                                              the same location in the output image

                                              - some output locations have no correspondent in the original image (no intensity

                                              assignment)

                                              Digital Image Processing

                                              Week 1

                                              inverse mapping scans the output pixel locations and at each location (x y)

                                              computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                              It then interpolates among the nearest input pixels to determine the intensity of the output

                                              pixel value

                                              Inverse mappings are more efficient to implement than forward mappings and are used in

                                              numerous commercial implementations of spatial transformations (MATLAB for ex)

                                              Digital Image Processing

                                              Week 1

                                              Digital Image Processing

                                              Week 1

                                              Image registration ndash align two or more images of the same scene

                                              In image registration we have available the input and output images but the specific

                                              transformation that produced the output image from the input is generally unknown

                                              The problem is to estimate the transformation function and then use it to register the two

                                              images

                                              - it may be of interest to align (register) two or more image taken at approximately the

                                              same time but using different imaging systems (MRI scanner and a PET scanner)

                                              - align images of a given location taken by the same instrument at different moments

                                              of time (satellite images)

                                              Solving the problem using tie points (also called control points) which are

                                              corresponding points whose locations are known precisely in the input and reference

                                              image

                                              Digital Image Processing

                                              Week 1

                                              How to select tie points

                                              - interactively selecting them

                                              - use of algorithms that try to detect these points

                                              - some imaging systems have physical artifacts (small metallic objects) embedded in

                                              the imaging sensors These objects produce a set of known points (called reseau

                                              marks) directly on all images captured by the system which can be used as guides

                                              for establishing tie points

                                              The problem of estimating the transformation is one of modeling Suppose we have a set

                                              of 4 tie points both on the input image and the reference image A simple model based on

                                              a bilinear approximation is given by

                                              1 2 3 4

                                              5 6 7 8

                                              x c v c w c v w cy c v c w c v w c

                                              (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                              Digital Image Processing

                                              Week 1

                                              When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                              frequently is to select a larger number of tie points and using this new set of tie points

                                              subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                              subregions marked by 4 tie points we applied the transformation model described above

                                              The number of tie points and the sophistication of the model required to solve the register

                                              problem depend on the severity of the geometrical distortion

                                              Digital Image Processing

                                              Week 1

                                              a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                              Digital Image Processing

                                              Week 1

                                              Probabilistic Methods

                                              zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                              p(zk) = the probability that the intensity level zk occurs in the given image

                                              ( ) kk

                                              np zM N

                                              nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                              pixels in the image) 1

                                              0( ) 1

                                              L

                                              kk

                                              p z

                                              The mean (average) intensity of an image is given by 1

                                              0( )

                                              L

                                              k kk

                                              m z p z

                                              Digital Image Processing

                                              Week 1

                                              The variance of the intensities is 1

                                              2 2

                                              0( ) ( )

                                              L

                                              k kk

                                              z m p z

                                              The variance is a measure of the spread of the values of z about the mean so it is a

                                              measure of image contrast Usually for measuring image contrast the standard deviation

                                              ( ) is used

                                              The n-th moment of a random variable z about the mean is defined as 1

                                              0( ) ( ) ( )

                                              Ln

                                              n k kk

                                              z z m p z

                                              ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                              3( ) 0z the intensities are biased to values higher than the mean

                                              ( 3( ) 0z the intensities are biased to values lower than the mean

                                              Digital Image Processing

                                              Week 1

                                              3( ) 0z the intensities are distributed approximately equally on both side of the

                                              mean

                                              Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                              Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                              Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                              Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                              Digital Image Processing

                                              Week 1

                                              Intensity Transformations and Spatial Filtering

                                              ( ) ( )g x y T f x y

                                              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                              neighborhood of (x y)

                                              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                              and much smaller in size than the image

                                              Digital Image Processing

                                              Week 1

                                              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                              called spatial filter (spatial mask kernel template or window)

                                              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                              ( )s T r

                                              s and r are denoting respectively the intensity of g and f at (x y)

                                              Figure 2 left - T produces an output image of higher contrast than the original by

                                              darkening the intensity levels below k and brightening the levels above k ndash this technique

                                              is called contrast stretching

                                              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                              Digital Image Processing

                                              Week 1

                                              Figure 2 right - T produces a binary output image A mapping of this form is called

                                              thresholding function

                                              Some Basic Intensity Transformation Functions

                                              Image Negatives

                                              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                              - equivalent of a photographic negative

                                              - technique suited for enhancing white or gray detail embedded in dark regions of an

                                              image

                                              Digital Image Processing

                                              Week 1

                                              Original Negative image

                                              Digital Image Processing

                                              Week 1

                                              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                              Some basic intensity transformation functions

                                              Digital Image Processing

                                              Week 1

                                              This transformation maps a narrow range of low intensity values in the input into a wider

                                              range An operator of this type is used to expand the values of dark pixels in an image

                                              while compressing the higher-level values The opposite is true for the inverse log

                                              transformation The log functions compress the dynamic range of images with large

                                              variations in pixel values

                                              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                              Digital Image Processing

                                              Week 1

                                              Power-Law (Gamma) Transformations

                                              - positive constants( ) ( ( ) )s T r c r c s c r

                                              Plots of gamma transformation for different values of γ (c=1)

                                              Digital Image Processing

                                              Week 1

                                              Power-law curves with 1 map a narrow range of dark input values into a wider range

                                              of output values with the opposite being true for higher values of input values The

                                              curves with 1 have the opposite effect of those generated with values of 1

                                              1c - identity transformation

                                              A variety of devices used for image capture printing and display respond according to a

                                              power law The process used to correct these power-law response phenomena is called

                                              gamma correction

                                              Digital Image Processing

                                              Week 1

                                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                              Digital Image Processing

                                              Week 1

                                              Piecewise-Linear Transformations Functions

                                              Contrast stretching

                                              - a process that expands the range of intensity levels in an image so it spans the full

                                              intensity range of the recording tool or display device

                                              a b c d Fig5

                                              Digital Image Processing

                                              Week 1

                                              11

                                              1

                                              2 1 1 21 2

                                              2 1 2 1

                                              22

                                              2

                                              [0 ]

                                              ( ) ( )( ) [ ]( ) ( )

                                              ( 1 ) [ 1]( 1 )

                                              s r r rrs r r s r rT r r r r

                                              r r r rs L r r r L

                                              L r

                                              Digital Image Processing

                                              Week 1

                                              1 1 2 2r s r s identity transformation (no change)

                                              1 2 1 2 0 1r r s s L thresholding function

                                              Figure 5(b) shows an 8-bit image with low contrast

                                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                              in the image respectively Thus the transformation function stretched the levels linearly

                                              from their original range to the full range [0 L-1]

                                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                              2 2 1r s m L where m is the mean gray level in the image

                                              The original image on which these results are based is a scanning electron microscope

                                              image of pollen magnified approximately 700 times

                                              Digital Image Processing

                                              Week 1

                                              Intensity-level slicing

                                              - highlighting a specific range of intensities in an image

                                              There are two approaches for intensity-level slicing

                                              1 display in one value (white for example) all the values in the range of interest and in

                                              another (say black) all other intensities (Figure 311 (a))

                                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                              intensities in the image (Figure 311 (b))

                                              Digital Image Processing

                                              Week 1

                                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                                              the top of the scale of intensities This type of enhancement produces a binary image

                                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                                              Highlights range [A B] and preserves all other intensities

                                              Digital Image Processing

                                              Week 1

                                              which is useful for studying the shape of the flow of the contrast substance (to detect

                                              blockageshellip)

                                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                              image around the mean intensity was set to black the other intensities remain unchanged

                                              Fig 6 - Aortic angiogram and intensity sliced versions

                                              Digital Image Processing

                                              Week 1

                                              Bit-plane slicing

                                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                              This technique highlights the contribution made to the whole image appearances by each

                                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                              Digital Image Processing

                                              Week 1

                                              Digital Image Processing

                                              Week 1

                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                              • DIP 1 2017
                                              • DIP 02 (2017)

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Examples of X-ray imaging

                                                Chest X-rayAortic angiogram

                                                Head CT Cygnus LoopCircuit boards

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Imaging in the Ultraviolet Band

                                                Litography industrial inspection microscopy biological imaging astronomical observations

                                                Ultraviolet light is used in fluorescence microscopy

                                                Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                                                other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                                                and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Imaging in the Visible and Infrared Bands

                                                Light microscopy astronomy remote sensing industry law enforcement

                                                LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                                                Weather observations and prediction produce major applications of multispectral image from satellites

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Satellite images of Washington DC area in spectral bands of the Table 1

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Examples of light microscopy

                                                Taxol (anticancer agent)magnified 250X

                                                Cholesterol(40X)

                                                Microprocessor(60X)

                                                Nickel oxidethin film(600X)

                                                Surface of audio CD(1750X)

                                                Organicsuperconductor(450X)

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Automated visual inspection of manufactured goods

                                                a bc de f

                                                a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Imaging in the Microwave Band

                                                The dominant aplication of imaging in the microwave band ndash radar

                                                bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                                bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                                bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                                An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Spaceborne radar image of mountains in southeast Tibet

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Imaging in the Radio Band

                                                medicine astronomy

                                                MRI = Magnetic Resonance Imaging

                                                This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                                Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                                The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                MRI images of a human knee (left) and spine (right)

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Images of the Crab Pulsar covering the electromagnetic spectrum

                                                Gamma X-ray Optical Infrared Radio

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Other Imaging Modalities

                                                acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                                Imaging using sound geological explorations industry medicine

                                                Mineral and oil exploration

                                                For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Biometry - iris

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Biometry - fingerprint

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Face detection and recognition

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Gender identification

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Image morphing

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Fundamental Steps in DIP

                                                methods whose input and output are images

                                                methods whose inputs are images but whose outputs are attributes extracted from those images

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Outputs are images

                                                bull image acquisition

                                                bull image filtering and enhancement

                                                bull image restoration

                                                bull color image processing

                                                bull wavelets and multiresolution processing

                                                bull compression

                                                bull morphological processing

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Outputs are attributes

                                                bull morphological processing

                                                bull segmentation

                                                bull representation and description

                                                bull object recognition

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Image acquisition - may involve preprocessing such as scaling

                                                Image enhancement

                                                bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                bull enhancement is problem oriented

                                                bull there is no general sbquotheoryrsquo of image enhancement

                                                bull enhancement use subjective methods for image emprovement

                                                bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Image restoration

                                                bull improving the appearance of an image

                                                bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                Color image processing

                                                bull fundamental concept in color models

                                                bull basic color processing in a digital domain

                                                Wavelets and multiresolution processing

                                                representing images in various degree of resolution

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Compression

                                                reducing the storage required to save an image or the bandwidth required to transmit it

                                                Morphological processing

                                                bull tools for extracting image components that are useful in the representation and description of shape

                                                bull a transition from processes that output images to processes that outputimage attributes

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Segmentation

                                                bull partitioning an image into its constituents parts or objects

                                                bull autonomous segmentation is one of the most difficult tasks of DIP

                                                bull the more accurate the segmentation the more likley recognition is to succeed

                                                Representation and description (almost always follows segmentation)

                                                bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                bull converting the data produced by segmentation to a form suitable for computer processing

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                bull complete region the focus is on internal properties such as texture or skeletal shape

                                                bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                Object recognition

                                                the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                Knowledge database

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Simplified diagramof a cross sectionof the human eye

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                Fovea = the place where the image of the object of interest falls on

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                Blind spot region without receptors

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Image formation in the eye

                                                Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                distance between lens and retina along visual axix = 17 mm

                                                range of focal length = 14 mm to 17 mm

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Optical illusions

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                quantities that describe the quality of a chromatic light source radiance

                                                the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                the physical meaning is determined by the source of the image

                                                ( )f D f x y

                                                Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                f(xy) ndash characterized by two components

                                                i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                ( ) ( ) ( )

                                                0 ( ) 0 ( ) 1

                                                f x y i x y r x y

                                                i x y r x y

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                i(xy) ndash determined by the illumination source

                                                r(xy) ndash determined by the characteristics of the imaged objects

                                                is called gray (or intensity) scale

                                                In practice

                                                min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                indoor values without additional illuminationmin max10 1000L L

                                                black whitemin max0 1 0 1 0 1L L L L l l L

                                                min maxL L

                                                Digital Image ProcessingDigital Image Processing

                                                Week 1Week 1

                                                Digital Image Processing

                                                Week 1

                                                Image Sampling and Quantization

                                                - the output of the sensors is a continuous voltage waveform related to the sensed

                                                scene

                                                converting a continuous image f to digital form

                                                - digitizing (x y) is called sampling

                                                - digitizing f(x y) is called quantization

                                                Digital Image Processing

                                                Week 1

                                                Digital Image Processing

                                                Week 1

                                                Continuous image projected onto a sensor array Result of image sampling and quantization

                                                Digital Image Processing

                                                Week 1

                                                Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                (00) (01) (0 1)(10) (11) (1 1)

                                                ( )

                                                ( 10) ( 11) ( 1 1)

                                                f f f Nf f f N

                                                f x y

                                                f M f M f M N

                                                image element pixel

                                                00 01 0 1

                                                10 11 1 1

                                                10 11 1 1

                                                ( ) ( )

                                                N

                                                i jN M N

                                                i j

                                                M M M N

                                                a a aa f x i y j f i ja a a

                                                Aa

                                                a a a

                                                f(00) ndash the upper left corner of the image

                                                Digital Image Processing

                                                Week 1

                                                M N ge 0 L=2k

                                                [0 1]i j i ja a L

                                                Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                Digital Image Processing

                                                Week 1

                                                Digital Image Processing

                                                Week 1

                                                Number of bits required to store a digitized image

                                                for 2 b M N k M N b N k

                                                When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                Digital Image Processing

                                                Week 1

                                                Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                Measures line pairs per unit distance dots (pixels) per unit distance

                                                Image resolution = the largest number of discernible line pairs per unit distance

                                                (eg 100 line pairs per mm)

                                                Dots per unit distance are commonly used in printing and publishing

                                                In US the measure is expressed in dots per inch (dpi)

                                                (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                Intensity resolution ndash the smallest discernible change in intensity level

                                                The number of intensity levels (L) is determined by hardware considerations

                                                L=2k ndash most common k = 8

                                                Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                Digital Image Processing

                                                Week 1

                                                Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                150 dpi (lower left) 72 dpi (lower right)

                                                Digital Image Processing

                                                Week 1

                                                Reducing the number of gray levels 256 128 64 32

                                                Digital Image Processing

                                                Week 1

                                                Reducing the number of gray levels 16 8 4 2

                                                Digital Image Processing

                                                Week 1

                                                Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                Shrinking zooming ndash image resizing ndash image resampling methods

                                                Interpolation is the process of using known data to estimate values at unknown locations

                                                Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                same spacing as the original and then shrink it so that it fits exactly over the original

                                                image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                Problem assignment of intensity-level in the new 750 times 750 grid

                                                Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                This technique has the tendency to produce undesirable effects like severe distortion of

                                                straight edges

                                                Digital Image Processing

                                                Week 1

                                                Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                be written using the 4 nearest neighbors of point (x y)

                                                Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                modest increase in computational effort

                                                Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                nearest neighbors of the point 3 3

                                                0 0

                                                ( ) i ji j

                                                i jv x y c x y

                                                The coefficients cij are obtained solving a 16x16 linear system

                                                intensity levels of the 16 nearest neighbors of 3 3

                                                0 0

                                                ( )i ji j

                                                i jc x y x y

                                                Digital Image Processing

                                                Week 1

                                                Generally bicubic interpolation does a better job of preserving fine detail than the

                                                bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                programs such as Adobe Photoshop and Corel Photopaint

                                                Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                neighbor interpolation was used (both for shrinking and zooming)

                                                Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                Digital Image Processing

                                                Week 1

                                                Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                Digital Image Processing

                                                Week 1

                                                Neighbors of a Pixel

                                                A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                and are denoted ND(p)

                                                The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                N8 (p)

                                                If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                fall outside the image

                                                Digital Image Processing

                                                Week 1

                                                Adjacency Connectivity Regions Boundaries

                                                Denote by V the set of intensity levels used to define adjacency

                                                - in a binary image V 01 (V=0 V=1)

                                                - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                We consider 3 types of adjacency

                                                (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                m-adjacent if

                                                4( )q N p or

                                                ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                ambiguities that often arise when 8-adjacency is used Consider the example

                                                Digital Image Processing

                                                Week 1

                                                binary image

                                                0 1 1 0 1 1 0 1 1

                                                1 0 1 0 0 1 0 0 1 0

                                                0 0 1 0 0 1 0 0 1

                                                V

                                                The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                m-adjacency

                                                Digital Image Processing

                                                Week 1

                                                A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                is a sequence of distinct pixels with coordinates

                                                and are adjacent 0 0 1 1

                                                1 1

                                                ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                n n

                                                i i i i

                                                x y x y x y x y s tx y x y i n

                                                The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                in S if there exists a path between them consisting only of pixels from S

                                                S is a connected set if there is a path in S between any 2 pixels in S

                                                Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                8-adjacency are considered

                                                Digital Image Processing

                                                Week 1

                                                Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                touches the image border

                                                the complement of 1

                                                ( )K

                                                cu k u u

                                                k

                                                R R R R

                                                We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                background of the image

                                                The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                points in the complement of R (R)c The border of an image is the set of pixels in the

                                                region that have at least one background neighbor This definition is referred to as the

                                                inner border to distinguish it from the notion of outer border which is the corresponding

                                                border in the background

                                                Digital Image Processing

                                                Week 1

                                                Distance measures

                                                For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                function or metric if

                                                (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                (b) D(p q) = D(q p)

                                                (c) D(p z) le D(p q) + D(q z)

                                                The Euclidean distance between p and q is defined as 1

                                                2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                centered at (x y)

                                                Digital Image Processing

                                                Week 1

                                                The D4 distance (also called city-block distance) between p and q is defined as

                                                4( ) | | | |D p q x s y t

                                                The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                4

                                                22 1 2

                                                2 2 1 0 1 22 1 2

                                                2

                                                D

                                                The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                The D8 distance (called the chessboard distance) between p and q is defined as

                                                8( ) max| | | |D p q x s y t

                                                The pixels q for which 8( )D p q r form a square centered at (x y)

                                                Digital Image Processing

                                                Week 1

                                                8

                                                2 2 2 2 22 1 1 1 2

                                                2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                D

                                                The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                D4 and D8 distances are independent of any paths that might exist between p and q

                                                because these distances involve only the coordinates of the point

                                                Digital Image Processing

                                                Week 1

                                                Array versus Matrix Operations

                                                An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                11 12 11 12

                                                21 22 21 22

                                                a a b ba a b b

                                                Array product

                                                11 12 11 12 11 11 12 12

                                                21 22 21 22 21 21 22 21

                                                a a b b a b a ba a b b a b a b

                                                Matrix product

                                                11 12 11 12 11 11 12 21 11 12 12 21

                                                21 22 21 22 21 11 22 21 21 12 22 22

                                                a a b b a b a b a b a ba a b b a b a b a b a b

                                                We assume array operations unless stated otherwise

                                                Digital Image Processing

                                                Week 1

                                                Linear versus Nonlinear Operations

                                                One of the most important classifications of image-processing methods is whether it is

                                                linear or nonlinear

                                                ( ) ( )H f x y g x y

                                                H is said to be a linear operator if

                                                images1 2 1 2

                                                1 2

                                                ( ) ( ) ( ) ( )

                                                H a f x y b f x y a H f x y b H f x y

                                                a b f f

                                                Example of nonlinear operator

                                                the maximum value of the pixels of image max ( )H f f x y f

                                                1 2

                                                0 2 6 5 1 1

                                                2 3 4 7f f a b

                                                Digital Image Processing

                                                Week 1

                                                1 2

                                                0 2 6 5 6 3max max 1 ( 1) max 2

                                                2 3 4 7 2 4a f b f

                                                0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                2 3 4 7

                                                Arithmetic Operations in Image Processing

                                                Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                The two random variables are uncorrelated when their covariance is 0

                                                Digital Image Processing

                                                Week 1

                                                Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                used in image enhancement)

                                                1

                                                1( ) ( )K

                                                ii

                                                g x y g x yK

                                                If the noise satisfies the properties stated above we have

                                                2 2( ) ( )

                                                1( ) ( ) g x y x yE g x y f x yK

                                                ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                and g respectively The standard deviation (square root of the variance) at any point in

                                                the average image is

                                                ( ) ( )1

                                                g x y x yK

                                                Digital Image Processing

                                                Week 1

                                                As K increases the variability (as measured by the variance or the standard deviation) of

                                                the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                averaging process increases

                                                An important application of image averaging is in the field of astronomy where imaging

                                                under very low light levels frequently causes sensor noise to render single images

                                                virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                images respectively

                                                Digital Image Processing

                                                Week 1

                                                Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                100 noisy images

                                                a b c d e f

                                                Digital Image Processing

                                                Week 1

                                                A frequent application of image subtraction is in the enhancement of differences between

                                                images

                                                (a) (b) (c)

                                                Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                significant bit of each pixel (c) the difference between the two images

                                                Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                difference between images (a) and (b)

                                                Digital Image Processing

                                                Week 1

                                                Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                images after injection of the contrast medium

                                                In g(x y) we can find the differences between h and f as enhanced detail

                                                Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                propagates through the various arteries in the area being observed

                                                Digital Image Processing

                                                Week 1

                                                a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                Digital Image Processing

                                                Week 1

                                                An important application of image multiplication (and division) is shading correction

                                                Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                When the shading function is known

                                                ( )( )( )

                                                g x yf x yh x y

                                                h(x y) is unknown but we have access to the imaging system we can obtain an

                                                approximation to the shading function by imaging a target of constant intensity When the

                                                sensor is not available often the shading pattern can be estimated from the image

                                                Digital Image Processing

                                                Week 1

                                                (a) (b) (c)

                                                Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                Digital Image Processing

                                                Week 1

                                                Another use of image multiplication is in masking also called region of interest (ROI)

                                                operations The process consists of multiplying a given image by a mask image that has

                                                1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                (a) (b) (c)

                                                Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                Digital Image Processing

                                                Week 1

                                                In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                min( )mf f f

                                                0 ( 255)max( )

                                                ms

                                                m

                                                ff K K K

                                                f

                                                Digital Image Processing

                                                Week 1

                                                Spatial Operations

                                                - are performed directly on the pixels of a given image

                                                There are three categories of spatial operations

                                                single-pixel operations

                                                neighborhood operations

                                                geometric spatial transformations

                                                Single-pixel operations

                                                - change the values of intensity for the individual pixels ( )s T z

                                                where z is the intensity of a pixel in the original image and s is the intensity of the

                                                corresponding pixel in the processed image

                                                Digital Image Processing

                                                Week 1

                                                Neighborhood operations

                                                Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                in an image f Neighborhood processing generates new intensity level at point (x y)

                                                based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                intensity by computing the average value of the pixels in Sxy

                                                ( )

                                                1( ) ( )xyr c S

                                                g x y f r cm n

                                                The net effect is to perform local blurring in the original image This type of process is

                                                used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                largest region of an image

                                                Digital Image Processing

                                                Week 1

                                                Geometric spatial transformations and image registration

                                                - modify the spatial relationship between pixels in an image

                                                - these transformations are often called rubber-sheet transformations (analogous to

                                                printing an image on a sheet of rubber and then stretching the sheet according to a

                                                predefined set of rules

                                                A geometric transformation consists of 2 basic operations

                                                1 a spatial transformation of coordinates

                                                2 intensity interpolation that assign intensity values to the spatial transformed

                                                pixels

                                                The coordinate system transformation ( ) [( )]x y T v w

                                                (v w) ndash pixel coordinates in the original image

                                                (x y) ndash pixel coordinates in the transformed image

                                                Digital Image Processing

                                                Week 1

                                                [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                Affine transform

                                                11 1211 21 31

                                                21 2212 22 33

                                                31 32

                                                0[ 1] [ 1] [ 1] 0

                                                1

                                                t tx t v t w t

                                                x y v w T v w t ty t v t w t

                                                t t

                                                (AT)

                                                This transform can scale rotate translate or shear a set of coordinate points depending

                                                on the elements of the matrix T If we want to resize an image rotate it and move the

                                                result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                scaling rotation and translation matrices from Table 1

                                                Digital Image Processing

                                                Week 1

                                                Affine transformations

                                                Digital Image Processing

                                                Week 1

                                                The preceding transformations relocate pixels on an image to new locations To complete

                                                the process we have to assign intensity values to those locations This task is done by

                                                using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                In practice we can use equation (AT) in two basic ways

                                                forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                location (x y) of the corresponding pixel in the new image using (AT) directly

                                                Problems

                                                - intensity assignment when 2 or more pixels in the original image are transformed to

                                                the same location in the output image

                                                - some output locations have no correspondent in the original image (no intensity

                                                assignment)

                                                Digital Image Processing

                                                Week 1

                                                inverse mapping scans the output pixel locations and at each location (x y)

                                                computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                It then interpolates among the nearest input pixels to determine the intensity of the output

                                                pixel value

                                                Inverse mappings are more efficient to implement than forward mappings and are used in

                                                numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                Digital Image Processing

                                                Week 1

                                                Digital Image Processing

                                                Week 1

                                                Image registration ndash align two or more images of the same scene

                                                In image registration we have available the input and output images but the specific

                                                transformation that produced the output image from the input is generally unknown

                                                The problem is to estimate the transformation function and then use it to register the two

                                                images

                                                - it may be of interest to align (register) two or more image taken at approximately the

                                                same time but using different imaging systems (MRI scanner and a PET scanner)

                                                - align images of a given location taken by the same instrument at different moments

                                                of time (satellite images)

                                                Solving the problem using tie points (also called control points) which are

                                                corresponding points whose locations are known precisely in the input and reference

                                                image

                                                Digital Image Processing

                                                Week 1

                                                How to select tie points

                                                - interactively selecting them

                                                - use of algorithms that try to detect these points

                                                - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                the imaging sensors These objects produce a set of known points (called reseau

                                                marks) directly on all images captured by the system which can be used as guides

                                                for establishing tie points

                                                The problem of estimating the transformation is one of modeling Suppose we have a set

                                                of 4 tie points both on the input image and the reference image A simple model based on

                                                a bilinear approximation is given by

                                                1 2 3 4

                                                5 6 7 8

                                                x c v c w c v w cy c v c w c v w c

                                                (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                Digital Image Processing

                                                Week 1

                                                When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                frequently is to select a larger number of tie points and using this new set of tie points

                                                subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                subregions marked by 4 tie points we applied the transformation model described above

                                                The number of tie points and the sophistication of the model required to solve the register

                                                problem depend on the severity of the geometrical distortion

                                                Digital Image Processing

                                                Week 1

                                                a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                Digital Image Processing

                                                Week 1

                                                Probabilistic Methods

                                                zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                p(zk) = the probability that the intensity level zk occurs in the given image

                                                ( ) kk

                                                np zM N

                                                nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                pixels in the image) 1

                                                0( ) 1

                                                L

                                                kk

                                                p z

                                                The mean (average) intensity of an image is given by 1

                                                0( )

                                                L

                                                k kk

                                                m z p z

                                                Digital Image Processing

                                                Week 1

                                                The variance of the intensities is 1

                                                2 2

                                                0( ) ( )

                                                L

                                                k kk

                                                z m p z

                                                The variance is a measure of the spread of the values of z about the mean so it is a

                                                measure of image contrast Usually for measuring image contrast the standard deviation

                                                ( ) is used

                                                The n-th moment of a random variable z about the mean is defined as 1

                                                0( ) ( ) ( )

                                                Ln

                                                n k kk

                                                z z m p z

                                                ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                3( ) 0z the intensities are biased to values higher than the mean

                                                ( 3( ) 0z the intensities are biased to values lower than the mean

                                                Digital Image Processing

                                                Week 1

                                                3( ) 0z the intensities are distributed approximately equally on both side of the

                                                mean

                                                Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                Digital Image Processing

                                                Week 1

                                                Intensity Transformations and Spatial Filtering

                                                ( ) ( )g x y T f x y

                                                f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                neighborhood of (x y)

                                                - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                and much smaller in size than the image

                                                Digital Image Processing

                                                Week 1

                                                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                called spatial filter (spatial mask kernel template or window)

                                                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                ( )s T r

                                                s and r are denoting respectively the intensity of g and f at (x y)

                                                Figure 2 left - T produces an output image of higher contrast than the original by

                                                darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                is called contrast stretching

                                                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                Digital Image Processing

                                                Week 1

                                                Figure 2 right - T produces a binary output image A mapping of this form is called

                                                thresholding function

                                                Some Basic Intensity Transformation Functions

                                                Image Negatives

                                                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                - equivalent of a photographic negative

                                                - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                image

                                                Digital Image Processing

                                                Week 1

                                                Original Negative image

                                                Digital Image Processing

                                                Week 1

                                                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                Some basic intensity transformation functions

                                                Digital Image Processing

                                                Week 1

                                                This transformation maps a narrow range of low intensity values in the input into a wider

                                                range An operator of this type is used to expand the values of dark pixels in an image

                                                while compressing the higher-level values The opposite is true for the inverse log

                                                transformation The log functions compress the dynamic range of images with large

                                                variations in pixel values

                                                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                Digital Image Processing

                                                Week 1

                                                Power-Law (Gamma) Transformations

                                                - positive constants( ) ( ( ) )s T r c r c s c r

                                                Plots of gamma transformation for different values of γ (c=1)

                                                Digital Image Processing

                                                Week 1

                                                Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                of output values with the opposite being true for higher values of input values The

                                                curves with 1 have the opposite effect of those generated with values of 1

                                                1c - identity transformation

                                                A variety of devices used for image capture printing and display respond according to a

                                                power law The process used to correct these power-law response phenomena is called

                                                gamma correction

                                                Digital Image Processing

                                                Week 1

                                                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                Digital Image Processing

                                                Week 1

                                                Piecewise-Linear Transformations Functions

                                                Contrast stretching

                                                - a process that expands the range of intensity levels in an image so it spans the full

                                                intensity range of the recording tool or display device

                                                a b c d Fig5

                                                Digital Image Processing

                                                Week 1

                                                11

                                                1

                                                2 1 1 21 2

                                                2 1 2 1

                                                22

                                                2

                                                [0 ]

                                                ( ) ( )( ) [ ]( ) ( )

                                                ( 1 ) [ 1]( 1 )

                                                s r r rrs r r s r rT r r r r

                                                r r r rs L r r r L

                                                L r

                                                Digital Image Processing

                                                Week 1

                                                1 1 2 2r s r s identity transformation (no change)

                                                1 2 1 2 0 1r r s s L thresholding function

                                                Figure 5(b) shows an 8-bit image with low contrast

                                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                in the image respectively Thus the transformation function stretched the levels linearly

                                                from their original range to the full range [0 L-1]

                                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                2 2 1r s m L where m is the mean gray level in the image

                                                The original image on which these results are based is a scanning electron microscope

                                                image of pollen magnified approximately 700 times

                                                Digital Image Processing

                                                Week 1

                                                Intensity-level slicing

                                                - highlighting a specific range of intensities in an image

                                                There are two approaches for intensity-level slicing

                                                1 display in one value (white for example) all the values in the range of interest and in

                                                another (say black) all other intensities (Figure 311 (a))

                                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                intensities in the image (Figure 311 (b))

                                                Digital Image Processing

                                                Week 1

                                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                the top of the scale of intensities This type of enhancement produces a binary image

                                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                Highlights range [A B] and preserves all other intensities

                                                Digital Image Processing

                                                Week 1

                                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                                blockageshellip)

                                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                image around the mean intensity was set to black the other intensities remain unchanged

                                                Fig 6 - Aortic angiogram and intensity sliced versions

                                                Digital Image Processing

                                                Week 1

                                                Bit-plane slicing

                                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                This technique highlights the contribution made to the whole image appearances by each

                                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                Digital Image Processing

                                                Week 1

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                                                Week 1

                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                • DIP 1 2017
                                                • DIP 02 (2017)

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Imaging in the Ultraviolet Band

                                                  Litography industrial inspection microscopy biological imaging astronomical observations

                                                  Ultraviolet light is used in fluorescence microscopy

                                                  Ultraviolet light is not visible to human eye but when a photon of ultraviolet radiation collides with an electron in an atom of a fluorescent material it elevates the electron to a higher energy level After that the electron relaxes to a lower level and emits light in the form of a lower-energy photon in the visible (red) light regionFluorescence = emission of light by a substance that has absorbed light or

                                                  other electromagentic radiation of a different wavelengthFlourescence microscope = uses an excitation light to irradiate a prepared specimen

                                                  and then it separates the much weaker radiating fluorescent light from the brighter excitation light

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Imaging in the Visible and Infrared Bands

                                                  Light microscopy astronomy remote sensing industry law enforcement

                                                  LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                                                  Weather observations and prediction produce major applications of multispectral image from satellites

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Satellite images of Washington DC area in spectral bands of the Table 1

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Examples of light microscopy

                                                  Taxol (anticancer agent)magnified 250X

                                                  Cholesterol(40X)

                                                  Microprocessor(60X)

                                                  Nickel oxidethin film(600X)

                                                  Surface of audio CD(1750X)

                                                  Organicsuperconductor(450X)

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Automated visual inspection of manufactured goods

                                                  a bc de f

                                                  a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Imaging in the Microwave Band

                                                  The dominant aplication of imaging in the microwave band ndash radar

                                                  bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                                  bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                                  bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                                  An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Spaceborne radar image of mountains in southeast Tibet

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Imaging in the Radio Band

                                                  medicine astronomy

                                                  MRI = Magnetic Resonance Imaging

                                                  This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                                  Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                                  The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  MRI images of a human knee (left) and spine (right)

                                                  Digital Image ProcessingDigital Image Processing

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                                                  Images of the Crab Pulsar covering the electromagnetic spectrum

                                                  Gamma X-ray Optical Infrared Radio

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Other Imaging Modalities

                                                  acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                                  Imaging using sound geological explorations industry medicine

                                                  Mineral and oil exploration

                                                  For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

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                                                  Biometry - iris

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                                                  Biometry - fingerprint

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Face detection and recognition

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Gender identification

                                                  Digital Image ProcessingDigital Image Processing

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                                                  Image morphing

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Fundamental Steps in DIP

                                                  methods whose input and output are images

                                                  methods whose inputs are images but whose outputs are attributes extracted from those images

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Outputs are images

                                                  bull image acquisition

                                                  bull image filtering and enhancement

                                                  bull image restoration

                                                  bull color image processing

                                                  bull wavelets and multiresolution processing

                                                  bull compression

                                                  bull morphological processing

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Outputs are attributes

                                                  bull morphological processing

                                                  bull segmentation

                                                  bull representation and description

                                                  bull object recognition

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Image acquisition - may involve preprocessing such as scaling

                                                  Image enhancement

                                                  bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                  bull enhancement is problem oriented

                                                  bull there is no general sbquotheoryrsquo of image enhancement

                                                  bull enhancement use subjective methods for image emprovement

                                                  bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Image restoration

                                                  bull improving the appearance of an image

                                                  bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                  Color image processing

                                                  bull fundamental concept in color models

                                                  bull basic color processing in a digital domain

                                                  Wavelets and multiresolution processing

                                                  representing images in various degree of resolution

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                                                  Week 1Week 1

                                                  Compression

                                                  reducing the storage required to save an image or the bandwidth required to transmit it

                                                  Morphological processing

                                                  bull tools for extracting image components that are useful in the representation and description of shape

                                                  bull a transition from processes that output images to processes that outputimage attributes

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Segmentation

                                                  bull partitioning an image into its constituents parts or objects

                                                  bull autonomous segmentation is one of the most difficult tasks of DIP

                                                  bull the more accurate the segmentation the more likley recognition is to succeed

                                                  Representation and description (almost always follows segmentation)

                                                  bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                  bull converting the data produced by segmentation to a form suitable for computer processing

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                  bull complete region the focus is on internal properties such as texture or skeletal shape

                                                  bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                  Object recognition

                                                  the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                  Knowledge database

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                                                  Week 1Week 1

                                                  Simplified diagramof a cross sectionof the human eye

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

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                                                  Week 1Week 1

                                                  Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                  The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                  Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                  The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                  The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                  Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                  Fovea = the place where the image of the object of interest falls on

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                  Blind spot region without receptors

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Image formation in the eye

                                                  Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                  Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                  distance between lens and retina along visual axix = 17 mm

                                                  range of focal length = 14 mm to 17 mm

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

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                                                  Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Optical illusions

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                  gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                  quantities that describe the quality of a chromatic light source radiance

                                                  the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                  measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                  brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  the physical meaning is determined by the source of the image

                                                  ( )f D f x y

                                                  Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                  f(xy) ndash characterized by two components

                                                  i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                  r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                  ( ) ( ) ( )

                                                  0 ( ) 0 ( ) 1

                                                  f x y i x y r x y

                                                  i x y r x y

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                  i(xy) ndash determined by the illumination source

                                                  r(xy) ndash determined by the characteristics of the imaged objects

                                                  is called gray (or intensity) scale

                                                  In practice

                                                  min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                  indoor values without additional illuminationmin max10 1000L L

                                                  black whitemin max0 1 0 1 0 1L L L L l l L

                                                  min maxL L

                                                  Digital Image ProcessingDigital Image Processing

                                                  Week 1Week 1

                                                  Digital Image Processing

                                                  Week 1

                                                  Image Sampling and Quantization

                                                  - the output of the sensors is a continuous voltage waveform related to the sensed

                                                  scene

                                                  converting a continuous image f to digital form

                                                  - digitizing (x y) is called sampling

                                                  - digitizing f(x y) is called quantization

                                                  Digital Image Processing

                                                  Week 1

                                                  Digital Image Processing

                                                  Week 1

                                                  Continuous image projected onto a sensor array Result of image sampling and quantization

                                                  Digital Image Processing

                                                  Week 1

                                                  Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                  (00) (01) (0 1)(10) (11) (1 1)

                                                  ( )

                                                  ( 10) ( 11) ( 1 1)

                                                  f f f Nf f f N

                                                  f x y

                                                  f M f M f M N

                                                  image element pixel

                                                  00 01 0 1

                                                  10 11 1 1

                                                  10 11 1 1

                                                  ( ) ( )

                                                  N

                                                  i jN M N

                                                  i j

                                                  M M M N

                                                  a a aa f x i y j f i ja a a

                                                  Aa

                                                  a a a

                                                  f(00) ndash the upper left corner of the image

                                                  Digital Image Processing

                                                  Week 1

                                                  M N ge 0 L=2k

                                                  [0 1]i j i ja a L

                                                  Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                  Digital Image Processing

                                                  Week 1

                                                  Digital Image Processing

                                                  Week 1

                                                  Number of bits required to store a digitized image

                                                  for 2 b M N k M N b N k

                                                  When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                  Digital Image Processing

                                                  Week 1

                                                  Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                  Measures line pairs per unit distance dots (pixels) per unit distance

                                                  Image resolution = the largest number of discernible line pairs per unit distance

                                                  (eg 100 line pairs per mm)

                                                  Dots per unit distance are commonly used in printing and publishing

                                                  In US the measure is expressed in dots per inch (dpi)

                                                  (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                  Intensity resolution ndash the smallest discernible change in intensity level

                                                  The number of intensity levels (L) is determined by hardware considerations

                                                  L=2k ndash most common k = 8

                                                  Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                  Digital Image Processing

                                                  Week 1

                                                  Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                  150 dpi (lower left) 72 dpi (lower right)

                                                  Digital Image Processing

                                                  Week 1

                                                  Reducing the number of gray levels 256 128 64 32

                                                  Digital Image Processing

                                                  Week 1

                                                  Reducing the number of gray levels 16 8 4 2

                                                  Digital Image Processing

                                                  Week 1

                                                  Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                  Shrinking zooming ndash image resizing ndash image resampling methods

                                                  Interpolation is the process of using known data to estimate values at unknown locations

                                                  Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                  750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                  same spacing as the original and then shrink it so that it fits exactly over the original

                                                  image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                  Problem assignment of intensity-level in the new 750 times 750 grid

                                                  Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                  intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                  This technique has the tendency to produce undesirable effects like severe distortion of

                                                  straight edges

                                                  Digital Image Processing

                                                  Week 1

                                                  Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                  where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                  be written using the 4 nearest neighbors of point (x y)

                                                  Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                  modest increase in computational effort

                                                  Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                  nearest neighbors of the point 3 3

                                                  0 0

                                                  ( ) i ji j

                                                  i jv x y c x y

                                                  The coefficients cij are obtained solving a 16x16 linear system

                                                  intensity levels of the 16 nearest neighbors of 3 3

                                                  0 0

                                                  ( )i ji j

                                                  i jc x y x y

                                                  Digital Image Processing

                                                  Week 1

                                                  Generally bicubic interpolation does a better job of preserving fine detail than the

                                                  bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                  programs such as Adobe Photoshop and Corel Photopaint

                                                  Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                  the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                  then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                  neighbor interpolation was used (both for shrinking and zooming)

                                                  Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                  interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                  from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                  Digital Image Processing

                                                  Week 1

                                                  Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                  Digital Image Processing

                                                  Week 1

                                                  Neighbors of a Pixel

                                                  A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                  This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                  The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                  and are denoted ND(p)

                                                  The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                  N8 (p)

                                                  If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                  fall outside the image

                                                  Digital Image Processing

                                                  Week 1

                                                  Adjacency Connectivity Regions Boundaries

                                                  Denote by V the set of intensity levels used to define adjacency

                                                  - in a binary image V 01 (V=0 V=1)

                                                  - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                  We consider 3 types of adjacency

                                                  (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                  (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                  (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                  m-adjacent if

                                                  4( )q N p or

                                                  ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                  Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                  ambiguities that often arise when 8-adjacency is used Consider the example

                                                  Digital Image Processing

                                                  Week 1

                                                  binary image

                                                  0 1 1 0 1 1 0 1 1

                                                  1 0 1 0 0 1 0 0 1 0

                                                  0 0 1 0 0 1 0 0 1

                                                  V

                                                  The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                  8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                  m-adjacency

                                                  Digital Image Processing

                                                  Week 1

                                                  A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                  is a sequence of distinct pixels with coordinates

                                                  and are adjacent 0 0 1 1

                                                  1 1

                                                  ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                  n n

                                                  i i i i

                                                  x y x y x y x y s tx y x y i n

                                                  The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                  Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                  Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                  in S if there exists a path between them consisting only of pixels from S

                                                  S is a connected set if there is a path in S between any 2 pixels in S

                                                  Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                  Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                  that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                  8-adjacency are considered

                                                  Digital Image Processing

                                                  Week 1

                                                  Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                  touches the image border

                                                  the complement of 1

                                                  ( )K

                                                  cu k u u

                                                  k

                                                  R R R R

                                                  We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                  background of the image

                                                  The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                  points in the complement of R (R)c The border of an image is the set of pixels in the

                                                  region that have at least one background neighbor This definition is referred to as the

                                                  inner border to distinguish it from the notion of outer border which is the corresponding

                                                  border in the background

                                                  Digital Image Processing

                                                  Week 1

                                                  Distance measures

                                                  For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                  function or metric if

                                                  (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                  (b) D(p q) = D(q p)

                                                  (c) D(p z) le D(p q) + D(q z)

                                                  The Euclidean distance between p and q is defined as 1

                                                  2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                  The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                  centered at (x y)

                                                  Digital Image Processing

                                                  Week 1

                                                  The D4 distance (also called city-block distance) between p and q is defined as

                                                  4( ) | | | |D p q x s y t

                                                  The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                  4

                                                  22 1 2

                                                  2 2 1 0 1 22 1 2

                                                  2

                                                  D

                                                  The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                  The D8 distance (called the chessboard distance) between p and q is defined as

                                                  8( ) max| | | |D p q x s y t

                                                  The pixels q for which 8( )D p q r form a square centered at (x y)

                                                  Digital Image Processing

                                                  Week 1

                                                  8

                                                  2 2 2 2 22 1 1 1 2

                                                  2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                  D

                                                  The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                  D4 and D8 distances are independent of any paths that might exist between p and q

                                                  because these distances involve only the coordinates of the point

                                                  Digital Image Processing

                                                  Week 1

                                                  Array versus Matrix Operations

                                                  An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                  11 12 11 12

                                                  21 22 21 22

                                                  a a b ba a b b

                                                  Array product

                                                  11 12 11 12 11 11 12 12

                                                  21 22 21 22 21 21 22 21

                                                  a a b b a b a ba a b b a b a b

                                                  Matrix product

                                                  11 12 11 12 11 11 12 21 11 12 12 21

                                                  21 22 21 22 21 11 22 21 21 12 22 22

                                                  a a b b a b a b a b a ba a b b a b a b a b a b

                                                  We assume array operations unless stated otherwise

                                                  Digital Image Processing

                                                  Week 1

                                                  Linear versus Nonlinear Operations

                                                  One of the most important classifications of image-processing methods is whether it is

                                                  linear or nonlinear

                                                  ( ) ( )H f x y g x y

                                                  H is said to be a linear operator if

                                                  images1 2 1 2

                                                  1 2

                                                  ( ) ( ) ( ) ( )

                                                  H a f x y b f x y a H f x y b H f x y

                                                  a b f f

                                                  Example of nonlinear operator

                                                  the maximum value of the pixels of image max ( )H f f x y f

                                                  1 2

                                                  0 2 6 5 1 1

                                                  2 3 4 7f f a b

                                                  Digital Image Processing

                                                  Week 1

                                                  1 2

                                                  0 2 6 5 6 3max max 1 ( 1) max 2

                                                  2 3 4 7 2 4a f b f

                                                  0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                  2 3 4 7

                                                  Arithmetic Operations in Image Processing

                                                  Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                  f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                  For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                  value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                  The two random variables are uncorrelated when their covariance is 0

                                                  Digital Image Processing

                                                  Week 1

                                                  Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                  used in image enhancement)

                                                  1

                                                  1( ) ( )K

                                                  ii

                                                  g x y g x yK

                                                  If the noise satisfies the properties stated above we have

                                                  2 2( ) ( )

                                                  1( ) ( ) g x y x yE g x y f x yK

                                                  ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                  and g respectively The standard deviation (square root of the variance) at any point in

                                                  the average image is

                                                  ( ) ( )1

                                                  g x y x yK

                                                  Digital Image Processing

                                                  Week 1

                                                  As K increases the variability (as measured by the variance or the standard deviation) of

                                                  the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                  means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                  averaging process increases

                                                  An important application of image averaging is in the field of astronomy where imaging

                                                  under very low light levels frequently causes sensor noise to render single images

                                                  virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                  was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                  64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                  images respectively

                                                  Digital Image Processing

                                                  Week 1

                                                  Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                  100 noisy images

                                                  a b c d e f

                                                  Digital Image Processing

                                                  Week 1

                                                  A frequent application of image subtraction is in the enhancement of differences between

                                                  images

                                                  (a) (b) (c)

                                                  Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                  significant bit of each pixel (c) the difference between the two images

                                                  Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                  Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                  images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                  difference between images (a) and (b)

                                                  Digital Image Processing

                                                  Week 1

                                                  Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                  h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                  intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                  source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                  bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                  anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                  images after injection of the contrast medium

                                                  In g(x y) we can find the differences between h and f as enhanced detail

                                                  Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                  propagates through the various arteries in the area being observed

                                                  Digital Image Processing

                                                  Week 1

                                                  a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                  Digital Image Processing

                                                  Week 1

                                                  An important application of image multiplication (and division) is shading correction

                                                  Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                  f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                  When the shading function is known

                                                  ( )( )( )

                                                  g x yf x yh x y

                                                  h(x y) is unknown but we have access to the imaging system we can obtain an

                                                  approximation to the shading function by imaging a target of constant intensity When the

                                                  sensor is not available often the shading pattern can be estimated from the image

                                                  Digital Image Processing

                                                  Week 1

                                                  (a) (b) (c)

                                                  Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                  Digital Image Processing

                                                  Week 1

                                                  Another use of image multiplication is in masking also called region of interest (ROI)

                                                  operations The process consists of multiplying a given image by a mask image that has

                                                  1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                  image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                  (a) (b) (c)

                                                  Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                  Digital Image Processing

                                                  Week 1

                                                  In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                  min( )mf f f

                                                  0 ( 255)max( )

                                                  ms

                                                  m

                                                  ff K K K

                                                  f

                                                  Digital Image Processing

                                                  Week 1

                                                  Spatial Operations

                                                  - are performed directly on the pixels of a given image

                                                  There are three categories of spatial operations

                                                  single-pixel operations

                                                  neighborhood operations

                                                  geometric spatial transformations

                                                  Single-pixel operations

                                                  - change the values of intensity for the individual pixels ( )s T z

                                                  where z is the intensity of a pixel in the original image and s is the intensity of the

                                                  corresponding pixel in the processed image

                                                  Digital Image Processing

                                                  Week 1

                                                  Neighborhood operations

                                                  Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                  in an image f Neighborhood processing generates new intensity level at point (x y)

                                                  based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                  rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                  intensity by computing the average value of the pixels in Sxy

                                                  ( )

                                                  1( ) ( )xyr c S

                                                  g x y f r cm n

                                                  The net effect is to perform local blurring in the original image This type of process is

                                                  used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                  largest region of an image

                                                  Digital Image Processing

                                                  Week 1

                                                  Geometric spatial transformations and image registration

                                                  - modify the spatial relationship between pixels in an image

                                                  - these transformations are often called rubber-sheet transformations (analogous to

                                                  printing an image on a sheet of rubber and then stretching the sheet according to a

                                                  predefined set of rules

                                                  A geometric transformation consists of 2 basic operations

                                                  1 a spatial transformation of coordinates

                                                  2 intensity interpolation that assign intensity values to the spatial transformed

                                                  pixels

                                                  The coordinate system transformation ( ) [( )]x y T v w

                                                  (v w) ndash pixel coordinates in the original image

                                                  (x y) ndash pixel coordinates in the transformed image

                                                  Digital Image Processing

                                                  Week 1

                                                  [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                  Affine transform

                                                  11 1211 21 31

                                                  21 2212 22 33

                                                  31 32

                                                  0[ 1] [ 1] [ 1] 0

                                                  1

                                                  t tx t v t w t

                                                  x y v w T v w t ty t v t w t

                                                  t t

                                                  (AT)

                                                  This transform can scale rotate translate or shear a set of coordinate points depending

                                                  on the elements of the matrix T If we want to resize an image rotate it and move the

                                                  result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                  scaling rotation and translation matrices from Table 1

                                                  Digital Image Processing

                                                  Week 1

                                                  Affine transformations

                                                  Digital Image Processing

                                                  Week 1

                                                  The preceding transformations relocate pixels on an image to new locations To complete

                                                  the process we have to assign intensity values to those locations This task is done by

                                                  using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                  In practice we can use equation (AT) in two basic ways

                                                  forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                  location (x y) of the corresponding pixel in the new image using (AT) directly

                                                  Problems

                                                  - intensity assignment when 2 or more pixels in the original image are transformed to

                                                  the same location in the output image

                                                  - some output locations have no correspondent in the original image (no intensity

                                                  assignment)

                                                  Digital Image Processing

                                                  Week 1

                                                  inverse mapping scans the output pixel locations and at each location (x y)

                                                  computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                  It then interpolates among the nearest input pixels to determine the intensity of the output

                                                  pixel value

                                                  Inverse mappings are more efficient to implement than forward mappings and are used in

                                                  numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                  Digital Image Processing

                                                  Week 1

                                                  Digital Image Processing

                                                  Week 1

                                                  Image registration ndash align two or more images of the same scene

                                                  In image registration we have available the input and output images but the specific

                                                  transformation that produced the output image from the input is generally unknown

                                                  The problem is to estimate the transformation function and then use it to register the two

                                                  images

                                                  - it may be of interest to align (register) two or more image taken at approximately the

                                                  same time but using different imaging systems (MRI scanner and a PET scanner)

                                                  - align images of a given location taken by the same instrument at different moments

                                                  of time (satellite images)

                                                  Solving the problem using tie points (also called control points) which are

                                                  corresponding points whose locations are known precisely in the input and reference

                                                  image

                                                  Digital Image Processing

                                                  Week 1

                                                  How to select tie points

                                                  - interactively selecting them

                                                  - use of algorithms that try to detect these points

                                                  - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                  the imaging sensors These objects produce a set of known points (called reseau

                                                  marks) directly on all images captured by the system which can be used as guides

                                                  for establishing tie points

                                                  The problem of estimating the transformation is one of modeling Suppose we have a set

                                                  of 4 tie points both on the input image and the reference image A simple model based on

                                                  a bilinear approximation is given by

                                                  1 2 3 4

                                                  5 6 7 8

                                                  x c v c w c v w cy c v c w c v w c

                                                  (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                  Digital Image Processing

                                                  Week 1

                                                  When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                  frequently is to select a larger number of tie points and using this new set of tie points

                                                  subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                  subregions marked by 4 tie points we applied the transformation model described above

                                                  The number of tie points and the sophistication of the model required to solve the register

                                                  problem depend on the severity of the geometrical distortion

                                                  Digital Image Processing

                                                  Week 1

                                                  a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                  Digital Image Processing

                                                  Week 1

                                                  Probabilistic Methods

                                                  zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                  p(zk) = the probability that the intensity level zk occurs in the given image

                                                  ( ) kk

                                                  np zM N

                                                  nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                  pixels in the image) 1

                                                  0( ) 1

                                                  L

                                                  kk

                                                  p z

                                                  The mean (average) intensity of an image is given by 1

                                                  0( )

                                                  L

                                                  k kk

                                                  m z p z

                                                  Digital Image Processing

                                                  Week 1

                                                  The variance of the intensities is 1

                                                  2 2

                                                  0( ) ( )

                                                  L

                                                  k kk

                                                  z m p z

                                                  The variance is a measure of the spread of the values of z about the mean so it is a

                                                  measure of image contrast Usually for measuring image contrast the standard deviation

                                                  ( ) is used

                                                  The n-th moment of a random variable z about the mean is defined as 1

                                                  0( ) ( ) ( )

                                                  Ln

                                                  n k kk

                                                  z z m p z

                                                  ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                  3( ) 0z the intensities are biased to values higher than the mean

                                                  ( 3( ) 0z the intensities are biased to values lower than the mean

                                                  Digital Image Processing

                                                  Week 1

                                                  3( ) 0z the intensities are distributed approximately equally on both side of the

                                                  mean

                                                  Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                  Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                  Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                  Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                  Digital Image Processing

                                                  Week 1

                                                  Intensity Transformations and Spatial Filtering

                                                  ( ) ( )g x y T f x y

                                                  f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                  neighborhood of (x y)

                                                  - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                  and much smaller in size than the image

                                                  Digital Image Processing

                                                  Week 1

                                                  - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                  called spatial filter (spatial mask kernel template or window)

                                                  ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                  ( )s T r

                                                  s and r are denoting respectively the intensity of g and f at (x y)

                                                  Figure 2 left - T produces an output image of higher contrast than the original by

                                                  darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                  is called contrast stretching

                                                  Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                  Digital Image Processing

                                                  Week 1

                                                  Figure 2 right - T produces a binary output image A mapping of this form is called

                                                  thresholding function

                                                  Some Basic Intensity Transformation Functions

                                                  Image Negatives

                                                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                  - equivalent of a photographic negative

                                                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                  image

                                                  Digital Image Processing

                                                  Week 1

                                                  Original Negative image

                                                  Digital Image Processing

                                                  Week 1

                                                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                  Some basic intensity transformation functions

                                                  Digital Image Processing

                                                  Week 1

                                                  This transformation maps a narrow range of low intensity values in the input into a wider

                                                  range An operator of this type is used to expand the values of dark pixels in an image

                                                  while compressing the higher-level values The opposite is true for the inverse log

                                                  transformation The log functions compress the dynamic range of images with large

                                                  variations in pixel values

                                                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                  Digital Image Processing

                                                  Week 1

                                                  Power-Law (Gamma) Transformations

                                                  - positive constants( ) ( ( ) )s T r c r c s c r

                                                  Plots of gamma transformation for different values of γ (c=1)

                                                  Digital Image Processing

                                                  Week 1

                                                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                  of output values with the opposite being true for higher values of input values The

                                                  curves with 1 have the opposite effect of those generated with values of 1

                                                  1c - identity transformation

                                                  A variety of devices used for image capture printing and display respond according to a

                                                  power law The process used to correct these power-law response phenomena is called

                                                  gamma correction

                                                  Digital Image Processing

                                                  Week 1

                                                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                  Digital Image Processing

                                                  Week 1

                                                  Piecewise-Linear Transformations Functions

                                                  Contrast stretching

                                                  - a process that expands the range of intensity levels in an image so it spans the full

                                                  intensity range of the recording tool or display device

                                                  a b c d Fig5

                                                  Digital Image Processing

                                                  Week 1

                                                  11

                                                  1

                                                  2 1 1 21 2

                                                  2 1 2 1

                                                  22

                                                  2

                                                  [0 ]

                                                  ( ) ( )( ) [ ]( ) ( )

                                                  ( 1 ) [ 1]( 1 )

                                                  s r r rrs r r s r rT r r r r

                                                  r r r rs L r r r L

                                                  L r

                                                  Digital Image Processing

                                                  Week 1

                                                  1 1 2 2r s r s identity transformation (no change)

                                                  1 2 1 2 0 1r r s s L thresholding function

                                                  Figure 5(b) shows an 8-bit image with low contrast

                                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                  in the image respectively Thus the transformation function stretched the levels linearly

                                                  from their original range to the full range [0 L-1]

                                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                  2 2 1r s m L where m is the mean gray level in the image

                                                  The original image on which these results are based is a scanning electron microscope

                                                  image of pollen magnified approximately 700 times

                                                  Digital Image Processing

                                                  Week 1

                                                  Intensity-level slicing

                                                  - highlighting a specific range of intensities in an image

                                                  There are two approaches for intensity-level slicing

                                                  1 display in one value (white for example) all the values in the range of interest and in

                                                  another (say black) all other intensities (Figure 311 (a))

                                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                  intensities in the image (Figure 311 (b))

                                                  Digital Image Processing

                                                  Week 1

                                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                  the top of the scale of intensities This type of enhancement produces a binary image

                                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                  Highlights range [A B] and preserves all other intensities

                                                  Digital Image Processing

                                                  Week 1

                                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                                  blockageshellip)

                                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                  image around the mean intensity was set to black the other intensities remain unchanged

                                                  Fig 6 - Aortic angiogram and intensity sliced versions

                                                  Digital Image Processing

                                                  Week 1

                                                  Bit-plane slicing

                                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                  This technique highlights the contribution made to the whole image appearances by each

                                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                  Digital Image Processing

                                                  Week 1

                                                  Digital Image Processing

                                                  Week 1

                                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                  • DIP 1 2017
                                                  • DIP 02 (2017)

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Imaging in the Visible and Infrared Bands

                                                    Light microscopy astronomy remote sensing industry law enforcement

                                                    LANDSAT satellite ndash obtained and transmitted images of the Earth from space for purpose of monitoring environmental conditions on the planet

                                                    Weather observations and prediction produce major applications of multispectral image from satellites

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Satellite images of Washington DC area in spectral bands of the Table 1

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Examples of light microscopy

                                                    Taxol (anticancer agent)magnified 250X

                                                    Cholesterol(40X)

                                                    Microprocessor(60X)

                                                    Nickel oxidethin film(600X)

                                                    Surface of audio CD(1750X)

                                                    Organicsuperconductor(450X)

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Automated visual inspection of manufactured goods

                                                    a bc de f

                                                    a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Imaging in the Microwave Band

                                                    The dominant aplication of imaging in the microwave band ndash radar

                                                    bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                                    bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                                    bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                                    An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Spaceborne radar image of mountains in southeast Tibet

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Imaging in the Radio Band

                                                    medicine astronomy

                                                    MRI = Magnetic Resonance Imaging

                                                    This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                                    Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                                    The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    MRI images of a human knee (left) and spine (right)

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Images of the Crab Pulsar covering the electromagnetic spectrum

                                                    Gamma X-ray Optical Infrared Radio

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Other Imaging Modalities

                                                    acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                                    Imaging using sound geological explorations industry medicine

                                                    Mineral and oil exploration

                                                    For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Biometry - iris

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Biometry - fingerprint

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Face detection and recognition

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Gender identification

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Image morphing

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Fundamental Steps in DIP

                                                    methods whose input and output are images

                                                    methods whose inputs are images but whose outputs are attributes extracted from those images

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Outputs are images

                                                    bull image acquisition

                                                    bull image filtering and enhancement

                                                    bull image restoration

                                                    bull color image processing

                                                    bull wavelets and multiresolution processing

                                                    bull compression

                                                    bull morphological processing

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Outputs are attributes

                                                    bull morphological processing

                                                    bull segmentation

                                                    bull representation and description

                                                    bull object recognition

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Image acquisition - may involve preprocessing such as scaling

                                                    Image enhancement

                                                    bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                    bull enhancement is problem oriented

                                                    bull there is no general sbquotheoryrsquo of image enhancement

                                                    bull enhancement use subjective methods for image emprovement

                                                    bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Image restoration

                                                    bull improving the appearance of an image

                                                    bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                    Color image processing

                                                    bull fundamental concept in color models

                                                    bull basic color processing in a digital domain

                                                    Wavelets and multiresolution processing

                                                    representing images in various degree of resolution

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Compression

                                                    reducing the storage required to save an image or the bandwidth required to transmit it

                                                    Morphological processing

                                                    bull tools for extracting image components that are useful in the representation and description of shape

                                                    bull a transition from processes that output images to processes that outputimage attributes

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                                                    Week 1Week 1

                                                    Segmentation

                                                    bull partitioning an image into its constituents parts or objects

                                                    bull autonomous segmentation is one of the most difficult tasks of DIP

                                                    bull the more accurate the segmentation the more likley recognition is to succeed

                                                    Representation and description (almost always follows segmentation)

                                                    bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                    bull converting the data produced by segmentation to a form suitable for computer processing

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                    bull complete region the focus is on internal properties such as texture or skeletal shape

                                                    bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                    Object recognition

                                                    the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                    Knowledge database

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Simplified diagramof a cross sectionof the human eye

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                    The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                    Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                    The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                    The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                    Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                    Fovea = the place where the image of the object of interest falls on

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                    Blind spot region without receptors

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Image formation in the eye

                                                    Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                    Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                    distance between lens and retina along visual axix = 17 mm

                                                    range of focal length = 14 mm to 17 mm

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

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                                                    Week 1Week 1

                                                    Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Optical illusions

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                    gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                    quantities that describe the quality of a chromatic light source radiance

                                                    the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                    measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                    brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    the physical meaning is determined by the source of the image

                                                    ( )f D f x y

                                                    Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                    f(xy) ndash characterized by two components

                                                    i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                    r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                    ( ) ( ) ( )

                                                    0 ( ) 0 ( ) 1

                                                    f x y i x y r x y

                                                    i x y r x y

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                    i(xy) ndash determined by the illumination source

                                                    r(xy) ndash determined by the characteristics of the imaged objects

                                                    is called gray (or intensity) scale

                                                    In practice

                                                    min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                    indoor values without additional illuminationmin max10 1000L L

                                                    black whitemin max0 1 0 1 0 1L L L L l l L

                                                    min maxL L

                                                    Digital Image ProcessingDigital Image Processing

                                                    Week 1Week 1

                                                    Digital Image Processing

                                                    Week 1

                                                    Image Sampling and Quantization

                                                    - the output of the sensors is a continuous voltage waveform related to the sensed

                                                    scene

                                                    converting a continuous image f to digital form

                                                    - digitizing (x y) is called sampling

                                                    - digitizing f(x y) is called quantization

                                                    Digital Image Processing

                                                    Week 1

                                                    Digital Image Processing

                                                    Week 1

                                                    Continuous image projected onto a sensor array Result of image sampling and quantization

                                                    Digital Image Processing

                                                    Week 1

                                                    Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                    (00) (01) (0 1)(10) (11) (1 1)

                                                    ( )

                                                    ( 10) ( 11) ( 1 1)

                                                    f f f Nf f f N

                                                    f x y

                                                    f M f M f M N

                                                    image element pixel

                                                    00 01 0 1

                                                    10 11 1 1

                                                    10 11 1 1

                                                    ( ) ( )

                                                    N

                                                    i jN M N

                                                    i j

                                                    M M M N

                                                    a a aa f x i y j f i ja a a

                                                    Aa

                                                    a a a

                                                    f(00) ndash the upper left corner of the image

                                                    Digital Image Processing

                                                    Week 1

                                                    M N ge 0 L=2k

                                                    [0 1]i j i ja a L

                                                    Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                    Digital Image Processing

                                                    Week 1

                                                    Digital Image Processing

                                                    Week 1

                                                    Number of bits required to store a digitized image

                                                    for 2 b M N k M N b N k

                                                    When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                    Digital Image Processing

                                                    Week 1

                                                    Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                    Measures line pairs per unit distance dots (pixels) per unit distance

                                                    Image resolution = the largest number of discernible line pairs per unit distance

                                                    (eg 100 line pairs per mm)

                                                    Dots per unit distance are commonly used in printing and publishing

                                                    In US the measure is expressed in dots per inch (dpi)

                                                    (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                    Intensity resolution ndash the smallest discernible change in intensity level

                                                    The number of intensity levels (L) is determined by hardware considerations

                                                    L=2k ndash most common k = 8

                                                    Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                    Digital Image Processing

                                                    Week 1

                                                    Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                    150 dpi (lower left) 72 dpi (lower right)

                                                    Digital Image Processing

                                                    Week 1

                                                    Reducing the number of gray levels 256 128 64 32

                                                    Digital Image Processing

                                                    Week 1

                                                    Reducing the number of gray levels 16 8 4 2

                                                    Digital Image Processing

                                                    Week 1

                                                    Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                    Shrinking zooming ndash image resizing ndash image resampling methods

                                                    Interpolation is the process of using known data to estimate values at unknown locations

                                                    Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                    750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                    same spacing as the original and then shrink it so that it fits exactly over the original

                                                    image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                    Problem assignment of intensity-level in the new 750 times 750 grid

                                                    Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                    intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                    This technique has the tendency to produce undesirable effects like severe distortion of

                                                    straight edges

                                                    Digital Image Processing

                                                    Week 1

                                                    Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                    where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                    be written using the 4 nearest neighbors of point (x y)

                                                    Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                    modest increase in computational effort

                                                    Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                    nearest neighbors of the point 3 3

                                                    0 0

                                                    ( ) i ji j

                                                    i jv x y c x y

                                                    The coefficients cij are obtained solving a 16x16 linear system

                                                    intensity levels of the 16 nearest neighbors of 3 3

                                                    0 0

                                                    ( )i ji j

                                                    i jc x y x y

                                                    Digital Image Processing

                                                    Week 1

                                                    Generally bicubic interpolation does a better job of preserving fine detail than the

                                                    bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                    programs such as Adobe Photoshop and Corel Photopaint

                                                    Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                    the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                    then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                    neighbor interpolation was used (both for shrinking and zooming)

                                                    Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                    interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                    from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                    Digital Image Processing

                                                    Week 1

                                                    Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                    Digital Image Processing

                                                    Week 1

                                                    Neighbors of a Pixel

                                                    A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                    This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                    The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                    and are denoted ND(p)

                                                    The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                    N8 (p)

                                                    If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                    fall outside the image

                                                    Digital Image Processing

                                                    Week 1

                                                    Adjacency Connectivity Regions Boundaries

                                                    Denote by V the set of intensity levels used to define adjacency

                                                    - in a binary image V 01 (V=0 V=1)

                                                    - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                    We consider 3 types of adjacency

                                                    (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                    (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                    (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                    m-adjacent if

                                                    4( )q N p or

                                                    ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                    Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                    ambiguities that often arise when 8-adjacency is used Consider the example

                                                    Digital Image Processing

                                                    Week 1

                                                    binary image

                                                    0 1 1 0 1 1 0 1 1

                                                    1 0 1 0 0 1 0 0 1 0

                                                    0 0 1 0 0 1 0 0 1

                                                    V

                                                    The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                    8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                    m-adjacency

                                                    Digital Image Processing

                                                    Week 1

                                                    A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                    is a sequence of distinct pixels with coordinates

                                                    and are adjacent 0 0 1 1

                                                    1 1

                                                    ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                    n n

                                                    i i i i

                                                    x y x y x y x y s tx y x y i n

                                                    The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                    Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                    Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                    in S if there exists a path between them consisting only of pixels from S

                                                    S is a connected set if there is a path in S between any 2 pixels in S

                                                    Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                    Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                    that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                    8-adjacency are considered

                                                    Digital Image Processing

                                                    Week 1

                                                    Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                    touches the image border

                                                    the complement of 1

                                                    ( )K

                                                    cu k u u

                                                    k

                                                    R R R R

                                                    We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                    background of the image

                                                    The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                    points in the complement of R (R)c The border of an image is the set of pixels in the

                                                    region that have at least one background neighbor This definition is referred to as the

                                                    inner border to distinguish it from the notion of outer border which is the corresponding

                                                    border in the background

                                                    Digital Image Processing

                                                    Week 1

                                                    Distance measures

                                                    For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                    function or metric if

                                                    (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                    (b) D(p q) = D(q p)

                                                    (c) D(p z) le D(p q) + D(q z)

                                                    The Euclidean distance between p and q is defined as 1

                                                    2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                    The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                    centered at (x y)

                                                    Digital Image Processing

                                                    Week 1

                                                    The D4 distance (also called city-block distance) between p and q is defined as

                                                    4( ) | | | |D p q x s y t

                                                    The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                    4

                                                    22 1 2

                                                    2 2 1 0 1 22 1 2

                                                    2

                                                    D

                                                    The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                    The D8 distance (called the chessboard distance) between p and q is defined as

                                                    8( ) max| | | |D p q x s y t

                                                    The pixels q for which 8( )D p q r form a square centered at (x y)

                                                    Digital Image Processing

                                                    Week 1

                                                    8

                                                    2 2 2 2 22 1 1 1 2

                                                    2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                    D

                                                    The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                    D4 and D8 distances are independent of any paths that might exist between p and q

                                                    because these distances involve only the coordinates of the point

                                                    Digital Image Processing

                                                    Week 1

                                                    Array versus Matrix Operations

                                                    An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                    11 12 11 12

                                                    21 22 21 22

                                                    a a b ba a b b

                                                    Array product

                                                    11 12 11 12 11 11 12 12

                                                    21 22 21 22 21 21 22 21

                                                    a a b b a b a ba a b b a b a b

                                                    Matrix product

                                                    11 12 11 12 11 11 12 21 11 12 12 21

                                                    21 22 21 22 21 11 22 21 21 12 22 22

                                                    a a b b a b a b a b a ba a b b a b a b a b a b

                                                    We assume array operations unless stated otherwise

                                                    Digital Image Processing

                                                    Week 1

                                                    Linear versus Nonlinear Operations

                                                    One of the most important classifications of image-processing methods is whether it is

                                                    linear or nonlinear

                                                    ( ) ( )H f x y g x y

                                                    H is said to be a linear operator if

                                                    images1 2 1 2

                                                    1 2

                                                    ( ) ( ) ( ) ( )

                                                    H a f x y b f x y a H f x y b H f x y

                                                    a b f f

                                                    Example of nonlinear operator

                                                    the maximum value of the pixels of image max ( )H f f x y f

                                                    1 2

                                                    0 2 6 5 1 1

                                                    2 3 4 7f f a b

                                                    Digital Image Processing

                                                    Week 1

                                                    1 2

                                                    0 2 6 5 6 3max max 1 ( 1) max 2

                                                    2 3 4 7 2 4a f b f

                                                    0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                    2 3 4 7

                                                    Arithmetic Operations in Image Processing

                                                    Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                    f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                    For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                    value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                    The two random variables are uncorrelated when their covariance is 0

                                                    Digital Image Processing

                                                    Week 1

                                                    Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                    used in image enhancement)

                                                    1

                                                    1( ) ( )K

                                                    ii

                                                    g x y g x yK

                                                    If the noise satisfies the properties stated above we have

                                                    2 2( ) ( )

                                                    1( ) ( ) g x y x yE g x y f x yK

                                                    ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                    and g respectively The standard deviation (square root of the variance) at any point in

                                                    the average image is

                                                    ( ) ( )1

                                                    g x y x yK

                                                    Digital Image Processing

                                                    Week 1

                                                    As K increases the variability (as measured by the variance or the standard deviation) of

                                                    the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                    means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                    averaging process increases

                                                    An important application of image averaging is in the field of astronomy where imaging

                                                    under very low light levels frequently causes sensor noise to render single images

                                                    virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                    was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                    64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                    images respectively

                                                    Digital Image Processing

                                                    Week 1

                                                    Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                    100 noisy images

                                                    a b c d e f

                                                    Digital Image Processing

                                                    Week 1

                                                    A frequent application of image subtraction is in the enhancement of differences between

                                                    images

                                                    (a) (b) (c)

                                                    Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                    significant bit of each pixel (c) the difference between the two images

                                                    Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                    Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                    images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                    difference between images (a) and (b)

                                                    Digital Image Processing

                                                    Week 1

                                                    Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                    h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                    intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                    source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                    bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                    anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                    images after injection of the contrast medium

                                                    In g(x y) we can find the differences between h and f as enhanced detail

                                                    Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                    propagates through the various arteries in the area being observed

                                                    Digital Image Processing

                                                    Week 1

                                                    a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                    Digital Image Processing

                                                    Week 1

                                                    An important application of image multiplication (and division) is shading correction

                                                    Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                    f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                    When the shading function is known

                                                    ( )( )( )

                                                    g x yf x yh x y

                                                    h(x y) is unknown but we have access to the imaging system we can obtain an

                                                    approximation to the shading function by imaging a target of constant intensity When the

                                                    sensor is not available often the shading pattern can be estimated from the image

                                                    Digital Image Processing

                                                    Week 1

                                                    (a) (b) (c)

                                                    Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                    Digital Image Processing

                                                    Week 1

                                                    Another use of image multiplication is in masking also called region of interest (ROI)

                                                    operations The process consists of multiplying a given image by a mask image that has

                                                    1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                    image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                    (a) (b) (c)

                                                    Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                    Digital Image Processing

                                                    Week 1

                                                    In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                    min( )mf f f

                                                    0 ( 255)max( )

                                                    ms

                                                    m

                                                    ff K K K

                                                    f

                                                    Digital Image Processing

                                                    Week 1

                                                    Spatial Operations

                                                    - are performed directly on the pixels of a given image

                                                    There are three categories of spatial operations

                                                    single-pixel operations

                                                    neighborhood operations

                                                    geometric spatial transformations

                                                    Single-pixel operations

                                                    - change the values of intensity for the individual pixels ( )s T z

                                                    where z is the intensity of a pixel in the original image and s is the intensity of the

                                                    corresponding pixel in the processed image

                                                    Digital Image Processing

                                                    Week 1

                                                    Neighborhood operations

                                                    Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                    in an image f Neighborhood processing generates new intensity level at point (x y)

                                                    based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                    rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                    intensity by computing the average value of the pixels in Sxy

                                                    ( )

                                                    1( ) ( )xyr c S

                                                    g x y f r cm n

                                                    The net effect is to perform local blurring in the original image This type of process is

                                                    used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                    largest region of an image

                                                    Digital Image Processing

                                                    Week 1

                                                    Geometric spatial transformations and image registration

                                                    - modify the spatial relationship between pixels in an image

                                                    - these transformations are often called rubber-sheet transformations (analogous to

                                                    printing an image on a sheet of rubber and then stretching the sheet according to a

                                                    predefined set of rules

                                                    A geometric transformation consists of 2 basic operations

                                                    1 a spatial transformation of coordinates

                                                    2 intensity interpolation that assign intensity values to the spatial transformed

                                                    pixels

                                                    The coordinate system transformation ( ) [( )]x y T v w

                                                    (v w) ndash pixel coordinates in the original image

                                                    (x y) ndash pixel coordinates in the transformed image

                                                    Digital Image Processing

                                                    Week 1

                                                    [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                    Affine transform

                                                    11 1211 21 31

                                                    21 2212 22 33

                                                    31 32

                                                    0[ 1] [ 1] [ 1] 0

                                                    1

                                                    t tx t v t w t

                                                    x y v w T v w t ty t v t w t

                                                    t t

                                                    (AT)

                                                    This transform can scale rotate translate or shear a set of coordinate points depending

                                                    on the elements of the matrix T If we want to resize an image rotate it and move the

                                                    result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                    scaling rotation and translation matrices from Table 1

                                                    Digital Image Processing

                                                    Week 1

                                                    Affine transformations

                                                    Digital Image Processing

                                                    Week 1

                                                    The preceding transformations relocate pixels on an image to new locations To complete

                                                    the process we have to assign intensity values to those locations This task is done by

                                                    using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                    In practice we can use equation (AT) in two basic ways

                                                    forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                    location (x y) of the corresponding pixel in the new image using (AT) directly

                                                    Problems

                                                    - intensity assignment when 2 or more pixels in the original image are transformed to

                                                    the same location in the output image

                                                    - some output locations have no correspondent in the original image (no intensity

                                                    assignment)

                                                    Digital Image Processing

                                                    Week 1

                                                    inverse mapping scans the output pixel locations and at each location (x y)

                                                    computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                    It then interpolates among the nearest input pixels to determine the intensity of the output

                                                    pixel value

                                                    Inverse mappings are more efficient to implement than forward mappings and are used in

                                                    numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                    Digital Image Processing

                                                    Week 1

                                                    Digital Image Processing

                                                    Week 1

                                                    Image registration ndash align two or more images of the same scene

                                                    In image registration we have available the input and output images but the specific

                                                    transformation that produced the output image from the input is generally unknown

                                                    The problem is to estimate the transformation function and then use it to register the two

                                                    images

                                                    - it may be of interest to align (register) two or more image taken at approximately the

                                                    same time but using different imaging systems (MRI scanner and a PET scanner)

                                                    - align images of a given location taken by the same instrument at different moments

                                                    of time (satellite images)

                                                    Solving the problem using tie points (also called control points) which are

                                                    corresponding points whose locations are known precisely in the input and reference

                                                    image

                                                    Digital Image Processing

                                                    Week 1

                                                    How to select tie points

                                                    - interactively selecting them

                                                    - use of algorithms that try to detect these points

                                                    - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                    the imaging sensors These objects produce a set of known points (called reseau

                                                    marks) directly on all images captured by the system which can be used as guides

                                                    for establishing tie points

                                                    The problem of estimating the transformation is one of modeling Suppose we have a set

                                                    of 4 tie points both on the input image and the reference image A simple model based on

                                                    a bilinear approximation is given by

                                                    1 2 3 4

                                                    5 6 7 8

                                                    x c v c w c v w cy c v c w c v w c

                                                    (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                    Digital Image Processing

                                                    Week 1

                                                    When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                    frequently is to select a larger number of tie points and using this new set of tie points

                                                    subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                    subregions marked by 4 tie points we applied the transformation model described above

                                                    The number of tie points and the sophistication of the model required to solve the register

                                                    problem depend on the severity of the geometrical distortion

                                                    Digital Image Processing

                                                    Week 1

                                                    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                    Digital Image Processing

                                                    Week 1

                                                    Probabilistic Methods

                                                    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                    p(zk) = the probability that the intensity level zk occurs in the given image

                                                    ( ) kk

                                                    np zM N

                                                    nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                    pixels in the image) 1

                                                    0( ) 1

                                                    L

                                                    kk

                                                    p z

                                                    The mean (average) intensity of an image is given by 1

                                                    0( )

                                                    L

                                                    k kk

                                                    m z p z

                                                    Digital Image Processing

                                                    Week 1

                                                    The variance of the intensities is 1

                                                    2 2

                                                    0( ) ( )

                                                    L

                                                    k kk

                                                    z m p z

                                                    The variance is a measure of the spread of the values of z about the mean so it is a

                                                    measure of image contrast Usually for measuring image contrast the standard deviation

                                                    ( ) is used

                                                    The n-th moment of a random variable z about the mean is defined as 1

                                                    0( ) ( ) ( )

                                                    Ln

                                                    n k kk

                                                    z z m p z

                                                    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                    3( ) 0z the intensities are biased to values higher than the mean

                                                    ( 3( ) 0z the intensities are biased to values lower than the mean

                                                    Digital Image Processing

                                                    Week 1

                                                    3( ) 0z the intensities are distributed approximately equally on both side of the

                                                    mean

                                                    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                    Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                    Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                    Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                    Digital Image Processing

                                                    Week 1

                                                    Intensity Transformations and Spatial Filtering

                                                    ( ) ( )g x y T f x y

                                                    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                    neighborhood of (x y)

                                                    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                    and much smaller in size than the image

                                                    Digital Image Processing

                                                    Week 1

                                                    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                    called spatial filter (spatial mask kernel template or window)

                                                    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                    ( )s T r

                                                    s and r are denoting respectively the intensity of g and f at (x y)

                                                    Figure 2 left - T produces an output image of higher contrast than the original by

                                                    darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                    is called contrast stretching

                                                    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                    Digital Image Processing

                                                    Week 1

                                                    Figure 2 right - T produces a binary output image A mapping of this form is called

                                                    thresholding function

                                                    Some Basic Intensity Transformation Functions

                                                    Image Negatives

                                                    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                    - equivalent of a photographic negative

                                                    - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                    image

                                                    Digital Image Processing

                                                    Week 1

                                                    Original Negative image

                                                    Digital Image Processing

                                                    Week 1

                                                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                    Some basic intensity transformation functions

                                                    Digital Image Processing

                                                    Week 1

                                                    This transformation maps a narrow range of low intensity values in the input into a wider

                                                    range An operator of this type is used to expand the values of dark pixels in an image

                                                    while compressing the higher-level values The opposite is true for the inverse log

                                                    transformation The log functions compress the dynamic range of images with large

                                                    variations in pixel values

                                                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                    Digital Image Processing

                                                    Week 1

                                                    Power-Law (Gamma) Transformations

                                                    - positive constants( ) ( ( ) )s T r c r c s c r

                                                    Plots of gamma transformation for different values of γ (c=1)

                                                    Digital Image Processing

                                                    Week 1

                                                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                    of output values with the opposite being true for higher values of input values The

                                                    curves with 1 have the opposite effect of those generated with values of 1

                                                    1c - identity transformation

                                                    A variety of devices used for image capture printing and display respond according to a

                                                    power law The process used to correct these power-law response phenomena is called

                                                    gamma correction

                                                    Digital Image Processing

                                                    Week 1

                                                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                    Digital Image Processing

                                                    Week 1

                                                    Piecewise-Linear Transformations Functions

                                                    Contrast stretching

                                                    - a process that expands the range of intensity levels in an image so it spans the full

                                                    intensity range of the recording tool or display device

                                                    a b c d Fig5

                                                    Digital Image Processing

                                                    Week 1

                                                    11

                                                    1

                                                    2 1 1 21 2

                                                    2 1 2 1

                                                    22

                                                    2

                                                    [0 ]

                                                    ( ) ( )( ) [ ]( ) ( )

                                                    ( 1 ) [ 1]( 1 )

                                                    s r r rrs r r s r rT r r r r

                                                    r r r rs L r r r L

                                                    L r

                                                    Digital Image Processing

                                                    Week 1

                                                    1 1 2 2r s r s identity transformation (no change)

                                                    1 2 1 2 0 1r r s s L thresholding function

                                                    Figure 5(b) shows an 8-bit image with low contrast

                                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                    in the image respectively Thus the transformation function stretched the levels linearly

                                                    from their original range to the full range [0 L-1]

                                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                    2 2 1r s m L where m is the mean gray level in the image

                                                    The original image on which these results are based is a scanning electron microscope

                                                    image of pollen magnified approximately 700 times

                                                    Digital Image Processing

                                                    Week 1

                                                    Intensity-level slicing

                                                    - highlighting a specific range of intensities in an image

                                                    There are two approaches for intensity-level slicing

                                                    1 display in one value (white for example) all the values in the range of interest and in

                                                    another (say black) all other intensities (Figure 311 (a))

                                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                    intensities in the image (Figure 311 (b))

                                                    Digital Image Processing

                                                    Week 1

                                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                    the top of the scale of intensities This type of enhancement produces a binary image

                                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                    Highlights range [A B] and preserves all other intensities

                                                    Digital Image Processing

                                                    Week 1

                                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                                    blockageshellip)

                                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                    image around the mean intensity was set to black the other intensities remain unchanged

                                                    Fig 6 - Aortic angiogram and intensity sliced versions

                                                    Digital Image Processing

                                                    Week 1

                                                    Bit-plane slicing

                                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                    This technique highlights the contribution made to the whole image appearances by each

                                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                    Digital Image Processing

                                                    Week 1

                                                    Digital Image Processing

                                                    Week 1

                                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                    • DIP 1 2017
                                                    • DIP 02 (2017)

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Table 1 ndash Thematic bands in NASArsquos LANDSAT satellite

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Satellite images of Washington DC area in spectral bands of the Table 1

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Examples of light microscopy

                                                      Taxol (anticancer agent)magnified 250X

                                                      Cholesterol(40X)

                                                      Microprocessor(60X)

                                                      Nickel oxidethin film(600X)

                                                      Surface of audio CD(1750X)

                                                      Organicsuperconductor(450X)

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Automated visual inspection of manufactured goods

                                                      a bc de f

                                                      a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Imaging in the Microwave Band

                                                      The dominant aplication of imaging in the microwave band ndash radar

                                                      bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                                      bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                                      bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                                      An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Spaceborne radar image of mountains in southeast Tibet

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Imaging in the Radio Band

                                                      medicine astronomy

                                                      MRI = Magnetic Resonance Imaging

                                                      This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                                      Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                                      The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      MRI images of a human knee (left) and spine (right)

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Images of the Crab Pulsar covering the electromagnetic spectrum

                                                      Gamma X-ray Optical Infrared Radio

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Other Imaging Modalities

                                                      acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                                      Imaging using sound geological explorations industry medicine

                                                      Mineral and oil exploration

                                                      For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Biometry - iris

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Biometry - fingerprint

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Face detection and recognition

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Gender identification

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Image morphing

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Fundamental Steps in DIP

                                                      methods whose input and output are images

                                                      methods whose inputs are images but whose outputs are attributes extracted from those images

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Outputs are images

                                                      bull image acquisition

                                                      bull image filtering and enhancement

                                                      bull image restoration

                                                      bull color image processing

                                                      bull wavelets and multiresolution processing

                                                      bull compression

                                                      bull morphological processing

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Outputs are attributes

                                                      bull morphological processing

                                                      bull segmentation

                                                      bull representation and description

                                                      bull object recognition

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Image acquisition - may involve preprocessing such as scaling

                                                      Image enhancement

                                                      bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                      bull enhancement is problem oriented

                                                      bull there is no general sbquotheoryrsquo of image enhancement

                                                      bull enhancement use subjective methods for image emprovement

                                                      bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Image restoration

                                                      bull improving the appearance of an image

                                                      bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                      Color image processing

                                                      bull fundamental concept in color models

                                                      bull basic color processing in a digital domain

                                                      Wavelets and multiresolution processing

                                                      representing images in various degree of resolution

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Compression

                                                      reducing the storage required to save an image or the bandwidth required to transmit it

                                                      Morphological processing

                                                      bull tools for extracting image components that are useful in the representation and description of shape

                                                      bull a transition from processes that output images to processes that outputimage attributes

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Segmentation

                                                      bull partitioning an image into its constituents parts or objects

                                                      bull autonomous segmentation is one of the most difficult tasks of DIP

                                                      bull the more accurate the segmentation the more likley recognition is to succeed

                                                      Representation and description (almost always follows segmentation)

                                                      bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                      bull converting the data produced by segmentation to a form suitable for computer processing

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                      bull complete region the focus is on internal properties such as texture or skeletal shape

                                                      bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                      Object recognition

                                                      the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                      Knowledge database

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Simplified diagramof a cross sectionof the human eye

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                      The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                      Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                      The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                      The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                      Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                      Fovea = the place where the image of the object of interest falls on

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                      Blind spot region without receptors

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Image formation in the eye

                                                      Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                      Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                      distance between lens and retina along visual axix = 17 mm

                                                      range of focal length = 14 mm to 17 mm

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Optical illusions

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                      gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                      quantities that describe the quality of a chromatic light source radiance

                                                      the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                      measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                      brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      the physical meaning is determined by the source of the image

                                                      ( )f D f x y

                                                      Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                      f(xy) ndash characterized by two components

                                                      i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                      r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                      ( ) ( ) ( )

                                                      0 ( ) 0 ( ) 1

                                                      f x y i x y r x y

                                                      i x y r x y

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                      i(xy) ndash determined by the illumination source

                                                      r(xy) ndash determined by the characteristics of the imaged objects

                                                      is called gray (or intensity) scale

                                                      In practice

                                                      min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                      indoor values without additional illuminationmin max10 1000L L

                                                      black whitemin max0 1 0 1 0 1L L L L l l L

                                                      min maxL L

                                                      Digital Image ProcessingDigital Image Processing

                                                      Week 1Week 1

                                                      Digital Image Processing

                                                      Week 1

                                                      Image Sampling and Quantization

                                                      - the output of the sensors is a continuous voltage waveform related to the sensed

                                                      scene

                                                      converting a continuous image f to digital form

                                                      - digitizing (x y) is called sampling

                                                      - digitizing f(x y) is called quantization

                                                      Digital Image Processing

                                                      Week 1

                                                      Digital Image Processing

                                                      Week 1

                                                      Continuous image projected onto a sensor array Result of image sampling and quantization

                                                      Digital Image Processing

                                                      Week 1

                                                      Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                      (00) (01) (0 1)(10) (11) (1 1)

                                                      ( )

                                                      ( 10) ( 11) ( 1 1)

                                                      f f f Nf f f N

                                                      f x y

                                                      f M f M f M N

                                                      image element pixel

                                                      00 01 0 1

                                                      10 11 1 1

                                                      10 11 1 1

                                                      ( ) ( )

                                                      N

                                                      i jN M N

                                                      i j

                                                      M M M N

                                                      a a aa f x i y j f i ja a a

                                                      Aa

                                                      a a a

                                                      f(00) ndash the upper left corner of the image

                                                      Digital Image Processing

                                                      Week 1

                                                      M N ge 0 L=2k

                                                      [0 1]i j i ja a L

                                                      Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                      Digital Image Processing

                                                      Week 1

                                                      Digital Image Processing

                                                      Week 1

                                                      Number of bits required to store a digitized image

                                                      for 2 b M N k M N b N k

                                                      When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                      Digital Image Processing

                                                      Week 1

                                                      Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                      Measures line pairs per unit distance dots (pixels) per unit distance

                                                      Image resolution = the largest number of discernible line pairs per unit distance

                                                      (eg 100 line pairs per mm)

                                                      Dots per unit distance are commonly used in printing and publishing

                                                      In US the measure is expressed in dots per inch (dpi)

                                                      (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                      Intensity resolution ndash the smallest discernible change in intensity level

                                                      The number of intensity levels (L) is determined by hardware considerations

                                                      L=2k ndash most common k = 8

                                                      Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                      Digital Image Processing

                                                      Week 1

                                                      Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                      150 dpi (lower left) 72 dpi (lower right)

                                                      Digital Image Processing

                                                      Week 1

                                                      Reducing the number of gray levels 256 128 64 32

                                                      Digital Image Processing

                                                      Week 1

                                                      Reducing the number of gray levels 16 8 4 2

                                                      Digital Image Processing

                                                      Week 1

                                                      Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                      Shrinking zooming ndash image resizing ndash image resampling methods

                                                      Interpolation is the process of using known data to estimate values at unknown locations

                                                      Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                      750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                      same spacing as the original and then shrink it so that it fits exactly over the original

                                                      image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                      Problem assignment of intensity-level in the new 750 times 750 grid

                                                      Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                      intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                      This technique has the tendency to produce undesirable effects like severe distortion of

                                                      straight edges

                                                      Digital Image Processing

                                                      Week 1

                                                      Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                      where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                      be written using the 4 nearest neighbors of point (x y)

                                                      Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                      modest increase in computational effort

                                                      Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                      nearest neighbors of the point 3 3

                                                      0 0

                                                      ( ) i ji j

                                                      i jv x y c x y

                                                      The coefficients cij are obtained solving a 16x16 linear system

                                                      intensity levels of the 16 nearest neighbors of 3 3

                                                      0 0

                                                      ( )i ji j

                                                      i jc x y x y

                                                      Digital Image Processing

                                                      Week 1

                                                      Generally bicubic interpolation does a better job of preserving fine detail than the

                                                      bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                      programs such as Adobe Photoshop and Corel Photopaint

                                                      Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                      the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                      then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                      neighbor interpolation was used (both for shrinking and zooming)

                                                      Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                      interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                      from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                      Digital Image Processing

                                                      Week 1

                                                      Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                      Digital Image Processing

                                                      Week 1

                                                      Neighbors of a Pixel

                                                      A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                      This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                      The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                      and are denoted ND(p)

                                                      The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                      N8 (p)

                                                      If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                      fall outside the image

                                                      Digital Image Processing

                                                      Week 1

                                                      Adjacency Connectivity Regions Boundaries

                                                      Denote by V the set of intensity levels used to define adjacency

                                                      - in a binary image V 01 (V=0 V=1)

                                                      - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                      We consider 3 types of adjacency

                                                      (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                      (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                      (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                      m-adjacent if

                                                      4( )q N p or

                                                      ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                      Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                      ambiguities that often arise when 8-adjacency is used Consider the example

                                                      Digital Image Processing

                                                      Week 1

                                                      binary image

                                                      0 1 1 0 1 1 0 1 1

                                                      1 0 1 0 0 1 0 0 1 0

                                                      0 0 1 0 0 1 0 0 1

                                                      V

                                                      The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                      8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                      m-adjacency

                                                      Digital Image Processing

                                                      Week 1

                                                      A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                      is a sequence of distinct pixels with coordinates

                                                      and are adjacent 0 0 1 1

                                                      1 1

                                                      ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                      n n

                                                      i i i i

                                                      x y x y x y x y s tx y x y i n

                                                      The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                      Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                      Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                      in S if there exists a path between them consisting only of pixels from S

                                                      S is a connected set if there is a path in S between any 2 pixels in S

                                                      Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                      Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                      that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                      8-adjacency are considered

                                                      Digital Image Processing

                                                      Week 1

                                                      Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                      touches the image border

                                                      the complement of 1

                                                      ( )K

                                                      cu k u u

                                                      k

                                                      R R R R

                                                      We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                      background of the image

                                                      The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                      points in the complement of R (R)c The border of an image is the set of pixels in the

                                                      region that have at least one background neighbor This definition is referred to as the

                                                      inner border to distinguish it from the notion of outer border which is the corresponding

                                                      border in the background

                                                      Digital Image Processing

                                                      Week 1

                                                      Distance measures

                                                      For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                      function or metric if

                                                      (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                      (b) D(p q) = D(q p)

                                                      (c) D(p z) le D(p q) + D(q z)

                                                      The Euclidean distance between p and q is defined as 1

                                                      2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                      The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                      centered at (x y)

                                                      Digital Image Processing

                                                      Week 1

                                                      The D4 distance (also called city-block distance) between p and q is defined as

                                                      4( ) | | | |D p q x s y t

                                                      The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                      4

                                                      22 1 2

                                                      2 2 1 0 1 22 1 2

                                                      2

                                                      D

                                                      The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                      The D8 distance (called the chessboard distance) between p and q is defined as

                                                      8( ) max| | | |D p q x s y t

                                                      The pixels q for which 8( )D p q r form a square centered at (x y)

                                                      Digital Image Processing

                                                      Week 1

                                                      8

                                                      2 2 2 2 22 1 1 1 2

                                                      2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                      D

                                                      The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                      D4 and D8 distances are independent of any paths that might exist between p and q

                                                      because these distances involve only the coordinates of the point

                                                      Digital Image Processing

                                                      Week 1

                                                      Array versus Matrix Operations

                                                      An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                      11 12 11 12

                                                      21 22 21 22

                                                      a a b ba a b b

                                                      Array product

                                                      11 12 11 12 11 11 12 12

                                                      21 22 21 22 21 21 22 21

                                                      a a b b a b a ba a b b a b a b

                                                      Matrix product

                                                      11 12 11 12 11 11 12 21 11 12 12 21

                                                      21 22 21 22 21 11 22 21 21 12 22 22

                                                      a a b b a b a b a b a ba a b b a b a b a b a b

                                                      We assume array operations unless stated otherwise

                                                      Digital Image Processing

                                                      Week 1

                                                      Linear versus Nonlinear Operations

                                                      One of the most important classifications of image-processing methods is whether it is

                                                      linear or nonlinear

                                                      ( ) ( )H f x y g x y

                                                      H is said to be a linear operator if

                                                      images1 2 1 2

                                                      1 2

                                                      ( ) ( ) ( ) ( )

                                                      H a f x y b f x y a H f x y b H f x y

                                                      a b f f

                                                      Example of nonlinear operator

                                                      the maximum value of the pixels of image max ( )H f f x y f

                                                      1 2

                                                      0 2 6 5 1 1

                                                      2 3 4 7f f a b

                                                      Digital Image Processing

                                                      Week 1

                                                      1 2

                                                      0 2 6 5 6 3max max 1 ( 1) max 2

                                                      2 3 4 7 2 4a f b f

                                                      0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                      2 3 4 7

                                                      Arithmetic Operations in Image Processing

                                                      Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                      f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                      For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                      value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                      The two random variables are uncorrelated when their covariance is 0

                                                      Digital Image Processing

                                                      Week 1

                                                      Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                      used in image enhancement)

                                                      1

                                                      1( ) ( )K

                                                      ii

                                                      g x y g x yK

                                                      If the noise satisfies the properties stated above we have

                                                      2 2( ) ( )

                                                      1( ) ( ) g x y x yE g x y f x yK

                                                      ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                      and g respectively The standard deviation (square root of the variance) at any point in

                                                      the average image is

                                                      ( ) ( )1

                                                      g x y x yK

                                                      Digital Image Processing

                                                      Week 1

                                                      As K increases the variability (as measured by the variance or the standard deviation) of

                                                      the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                      means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                      averaging process increases

                                                      An important application of image averaging is in the field of astronomy where imaging

                                                      under very low light levels frequently causes sensor noise to render single images

                                                      virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                      was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                      64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                      images respectively

                                                      Digital Image Processing

                                                      Week 1

                                                      Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                      100 noisy images

                                                      a b c d e f

                                                      Digital Image Processing

                                                      Week 1

                                                      A frequent application of image subtraction is in the enhancement of differences between

                                                      images

                                                      (a) (b) (c)

                                                      Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                      significant bit of each pixel (c) the difference between the two images

                                                      Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                      Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                      images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                      difference between images (a) and (b)

                                                      Digital Image Processing

                                                      Week 1

                                                      Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                      h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                      intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                      source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                      bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                      anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                      images after injection of the contrast medium

                                                      In g(x y) we can find the differences between h and f as enhanced detail

                                                      Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                      propagates through the various arteries in the area being observed

                                                      Digital Image Processing

                                                      Week 1

                                                      a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                      Digital Image Processing

                                                      Week 1

                                                      An important application of image multiplication (and division) is shading correction

                                                      Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                      f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                      When the shading function is known

                                                      ( )( )( )

                                                      g x yf x yh x y

                                                      h(x y) is unknown but we have access to the imaging system we can obtain an

                                                      approximation to the shading function by imaging a target of constant intensity When the

                                                      sensor is not available often the shading pattern can be estimated from the image

                                                      Digital Image Processing

                                                      Week 1

                                                      (a) (b) (c)

                                                      Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                      Digital Image Processing

                                                      Week 1

                                                      Another use of image multiplication is in masking also called region of interest (ROI)

                                                      operations The process consists of multiplying a given image by a mask image that has

                                                      1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                      image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                      (a) (b) (c)

                                                      Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                      Digital Image Processing

                                                      Week 1

                                                      In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                      min( )mf f f

                                                      0 ( 255)max( )

                                                      ms

                                                      m

                                                      ff K K K

                                                      f

                                                      Digital Image Processing

                                                      Week 1

                                                      Spatial Operations

                                                      - are performed directly on the pixels of a given image

                                                      There are three categories of spatial operations

                                                      single-pixel operations

                                                      neighborhood operations

                                                      geometric spatial transformations

                                                      Single-pixel operations

                                                      - change the values of intensity for the individual pixels ( )s T z

                                                      where z is the intensity of a pixel in the original image and s is the intensity of the

                                                      corresponding pixel in the processed image

                                                      Digital Image Processing

                                                      Week 1

                                                      Neighborhood operations

                                                      Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                      in an image f Neighborhood processing generates new intensity level at point (x y)

                                                      based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                      rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                      intensity by computing the average value of the pixels in Sxy

                                                      ( )

                                                      1( ) ( )xyr c S

                                                      g x y f r cm n

                                                      The net effect is to perform local blurring in the original image This type of process is

                                                      used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                      largest region of an image

                                                      Digital Image Processing

                                                      Week 1

                                                      Geometric spatial transformations and image registration

                                                      - modify the spatial relationship between pixels in an image

                                                      - these transformations are often called rubber-sheet transformations (analogous to

                                                      printing an image on a sheet of rubber and then stretching the sheet according to a

                                                      predefined set of rules

                                                      A geometric transformation consists of 2 basic operations

                                                      1 a spatial transformation of coordinates

                                                      2 intensity interpolation that assign intensity values to the spatial transformed

                                                      pixels

                                                      The coordinate system transformation ( ) [( )]x y T v w

                                                      (v w) ndash pixel coordinates in the original image

                                                      (x y) ndash pixel coordinates in the transformed image

                                                      Digital Image Processing

                                                      Week 1

                                                      [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                      Affine transform

                                                      11 1211 21 31

                                                      21 2212 22 33

                                                      31 32

                                                      0[ 1] [ 1] [ 1] 0

                                                      1

                                                      t tx t v t w t

                                                      x y v w T v w t ty t v t w t

                                                      t t

                                                      (AT)

                                                      This transform can scale rotate translate or shear a set of coordinate points depending

                                                      on the elements of the matrix T If we want to resize an image rotate it and move the

                                                      result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                      scaling rotation and translation matrices from Table 1

                                                      Digital Image Processing

                                                      Week 1

                                                      Affine transformations

                                                      Digital Image Processing

                                                      Week 1

                                                      The preceding transformations relocate pixels on an image to new locations To complete

                                                      the process we have to assign intensity values to those locations This task is done by

                                                      using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                      In practice we can use equation (AT) in two basic ways

                                                      forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                      location (x y) of the corresponding pixel in the new image using (AT) directly

                                                      Problems

                                                      - intensity assignment when 2 or more pixels in the original image are transformed to

                                                      the same location in the output image

                                                      - some output locations have no correspondent in the original image (no intensity

                                                      assignment)

                                                      Digital Image Processing

                                                      Week 1

                                                      inverse mapping scans the output pixel locations and at each location (x y)

                                                      computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                      It then interpolates among the nearest input pixels to determine the intensity of the output

                                                      pixel value

                                                      Inverse mappings are more efficient to implement than forward mappings and are used in

                                                      numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                      Digital Image Processing

                                                      Week 1

                                                      Digital Image Processing

                                                      Week 1

                                                      Image registration ndash align two or more images of the same scene

                                                      In image registration we have available the input and output images but the specific

                                                      transformation that produced the output image from the input is generally unknown

                                                      The problem is to estimate the transformation function and then use it to register the two

                                                      images

                                                      - it may be of interest to align (register) two or more image taken at approximately the

                                                      same time but using different imaging systems (MRI scanner and a PET scanner)

                                                      - align images of a given location taken by the same instrument at different moments

                                                      of time (satellite images)

                                                      Solving the problem using tie points (also called control points) which are

                                                      corresponding points whose locations are known precisely in the input and reference

                                                      image

                                                      Digital Image Processing

                                                      Week 1

                                                      How to select tie points

                                                      - interactively selecting them

                                                      - use of algorithms that try to detect these points

                                                      - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                      the imaging sensors These objects produce a set of known points (called reseau

                                                      marks) directly on all images captured by the system which can be used as guides

                                                      for establishing tie points

                                                      The problem of estimating the transformation is one of modeling Suppose we have a set

                                                      of 4 tie points both on the input image and the reference image A simple model based on

                                                      a bilinear approximation is given by

                                                      1 2 3 4

                                                      5 6 7 8

                                                      x c v c w c v w cy c v c w c v w c

                                                      (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                      Digital Image Processing

                                                      Week 1

                                                      When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                      frequently is to select a larger number of tie points and using this new set of tie points

                                                      subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                      subregions marked by 4 tie points we applied the transformation model described above

                                                      The number of tie points and the sophistication of the model required to solve the register

                                                      problem depend on the severity of the geometrical distortion

                                                      Digital Image Processing

                                                      Week 1

                                                      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                      Digital Image Processing

                                                      Week 1

                                                      Probabilistic Methods

                                                      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                      p(zk) = the probability that the intensity level zk occurs in the given image

                                                      ( ) kk

                                                      np zM N

                                                      nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                      pixels in the image) 1

                                                      0( ) 1

                                                      L

                                                      kk

                                                      p z

                                                      The mean (average) intensity of an image is given by 1

                                                      0( )

                                                      L

                                                      k kk

                                                      m z p z

                                                      Digital Image Processing

                                                      Week 1

                                                      The variance of the intensities is 1

                                                      2 2

                                                      0( ) ( )

                                                      L

                                                      k kk

                                                      z m p z

                                                      The variance is a measure of the spread of the values of z about the mean so it is a

                                                      measure of image contrast Usually for measuring image contrast the standard deviation

                                                      ( ) is used

                                                      The n-th moment of a random variable z about the mean is defined as 1

                                                      0( ) ( ) ( )

                                                      Ln

                                                      n k kk

                                                      z z m p z

                                                      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                      3( ) 0z the intensities are biased to values higher than the mean

                                                      ( 3( ) 0z the intensities are biased to values lower than the mean

                                                      Digital Image Processing

                                                      Week 1

                                                      3( ) 0z the intensities are distributed approximately equally on both side of the

                                                      mean

                                                      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                      Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                      Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                      Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                      Digital Image Processing

                                                      Week 1

                                                      Intensity Transformations and Spatial Filtering

                                                      ( ) ( )g x y T f x y

                                                      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                      neighborhood of (x y)

                                                      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                      and much smaller in size than the image

                                                      Digital Image Processing

                                                      Week 1

                                                      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                      called spatial filter (spatial mask kernel template or window)

                                                      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                      ( )s T r

                                                      s and r are denoting respectively the intensity of g and f at (x y)

                                                      Figure 2 left - T produces an output image of higher contrast than the original by

                                                      darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                      is called contrast stretching

                                                      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                      Digital Image Processing

                                                      Week 1

                                                      Figure 2 right - T produces a binary output image A mapping of this form is called

                                                      thresholding function

                                                      Some Basic Intensity Transformation Functions

                                                      Image Negatives

                                                      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                      - equivalent of a photographic negative

                                                      - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                      image

                                                      Digital Image Processing

                                                      Week 1

                                                      Original Negative image

                                                      Digital Image Processing

                                                      Week 1

                                                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                      Some basic intensity transformation functions

                                                      Digital Image Processing

                                                      Week 1

                                                      This transformation maps a narrow range of low intensity values in the input into a wider

                                                      range An operator of this type is used to expand the values of dark pixels in an image

                                                      while compressing the higher-level values The opposite is true for the inverse log

                                                      transformation The log functions compress the dynamic range of images with large

                                                      variations in pixel values

                                                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                      Digital Image Processing

                                                      Week 1

                                                      Power-Law (Gamma) Transformations

                                                      - positive constants( ) ( ( ) )s T r c r c s c r

                                                      Plots of gamma transformation for different values of γ (c=1)

                                                      Digital Image Processing

                                                      Week 1

                                                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                      of output values with the opposite being true for higher values of input values The

                                                      curves with 1 have the opposite effect of those generated with values of 1

                                                      1c - identity transformation

                                                      A variety of devices used for image capture printing and display respond according to a

                                                      power law The process used to correct these power-law response phenomena is called

                                                      gamma correction

                                                      Digital Image Processing

                                                      Week 1

                                                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                      Digital Image Processing

                                                      Week 1

                                                      Piecewise-Linear Transformations Functions

                                                      Contrast stretching

                                                      - a process that expands the range of intensity levels in an image so it spans the full

                                                      intensity range of the recording tool or display device

                                                      a b c d Fig5

                                                      Digital Image Processing

                                                      Week 1

                                                      11

                                                      1

                                                      2 1 1 21 2

                                                      2 1 2 1

                                                      22

                                                      2

                                                      [0 ]

                                                      ( ) ( )( ) [ ]( ) ( )

                                                      ( 1 ) [ 1]( 1 )

                                                      s r r rrs r r s r rT r r r r

                                                      r r r rs L r r r L

                                                      L r

                                                      Digital Image Processing

                                                      Week 1

                                                      1 1 2 2r s r s identity transformation (no change)

                                                      1 2 1 2 0 1r r s s L thresholding function

                                                      Figure 5(b) shows an 8-bit image with low contrast

                                                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                      in the image respectively Thus the transformation function stretched the levels linearly

                                                      from their original range to the full range [0 L-1]

                                                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                      2 2 1r s m L where m is the mean gray level in the image

                                                      The original image on which these results are based is a scanning electron microscope

                                                      image of pollen magnified approximately 700 times

                                                      Digital Image Processing

                                                      Week 1

                                                      Intensity-level slicing

                                                      - highlighting a specific range of intensities in an image

                                                      There are two approaches for intensity-level slicing

                                                      1 display in one value (white for example) all the values in the range of interest and in

                                                      another (say black) all other intensities (Figure 311 (a))

                                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                      intensities in the image (Figure 311 (b))

                                                      Digital Image Processing

                                                      Week 1

                                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                      the top of the scale of intensities This type of enhancement produces a binary image

                                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                      Highlights range [A B] and preserves all other intensities

                                                      Digital Image Processing

                                                      Week 1

                                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                                      blockageshellip)

                                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                      image around the mean intensity was set to black the other intensities remain unchanged

                                                      Fig 6 - Aortic angiogram and intensity sliced versions

                                                      Digital Image Processing

                                                      Week 1

                                                      Bit-plane slicing

                                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                      This technique highlights the contribution made to the whole image appearances by each

                                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                      Digital Image Processing

                                                      Week 1

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                                                      Week 1

                                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                      • DIP 1 2017
                                                      • DIP 02 (2017)

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        Satellite images of Washington DC area in spectral bands of the Table 1

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        Examples of light microscopy

                                                        Taxol (anticancer agent)magnified 250X

                                                        Cholesterol(40X)

                                                        Microprocessor(60X)

                                                        Nickel oxidethin film(600X)

                                                        Surface of audio CD(1750X)

                                                        Organicsuperconductor(450X)

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        Automated visual inspection of manufactured goods

                                                        a bc de f

                                                        a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

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                                                        Imaging in the Microwave Band

                                                        The dominant aplication of imaging in the microwave band ndash radar

                                                        bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                                        bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                                        bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                                        An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                                        Digital Image ProcessingDigital Image Processing

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                                                        Spaceborne radar image of mountains in southeast Tibet

                                                        Digital Image ProcessingDigital Image Processing

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                                                        Imaging in the Radio Band

                                                        medicine astronomy

                                                        MRI = Magnetic Resonance Imaging

                                                        This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                                        Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                                        The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                                        Digital Image ProcessingDigital Image Processing

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                                                        MRI images of a human knee (left) and spine (right)

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                                                        Images of the Crab Pulsar covering the electromagnetic spectrum

                                                        Gamma X-ray Optical Infrared Radio

                                                        Digital Image ProcessingDigital Image Processing

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                                                        Other Imaging Modalities

                                                        acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                                        Imaging using sound geological explorations industry medicine

                                                        Mineral and oil exploration

                                                        For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

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                                                        Biometry - iris

                                                        Digital Image ProcessingDigital Image Processing

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                                                        Biometry - fingerprint

                                                        Digital Image ProcessingDigital Image Processing

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                                                        Face detection and recognition

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        Gender identification

                                                        Digital Image ProcessingDigital Image Processing

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                                                        Image morphing

                                                        Digital Image ProcessingDigital Image Processing

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                                                        Fundamental Steps in DIP

                                                        methods whose input and output are images

                                                        methods whose inputs are images but whose outputs are attributes extracted from those images

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        Outputs are images

                                                        bull image acquisition

                                                        bull image filtering and enhancement

                                                        bull image restoration

                                                        bull color image processing

                                                        bull wavelets and multiresolution processing

                                                        bull compression

                                                        bull morphological processing

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        Outputs are attributes

                                                        bull morphological processing

                                                        bull segmentation

                                                        bull representation and description

                                                        bull object recognition

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        Image acquisition - may involve preprocessing such as scaling

                                                        Image enhancement

                                                        bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                        bull enhancement is problem oriented

                                                        bull there is no general sbquotheoryrsquo of image enhancement

                                                        bull enhancement use subjective methods for image emprovement

                                                        bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        Image restoration

                                                        bull improving the appearance of an image

                                                        bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                        Color image processing

                                                        bull fundamental concept in color models

                                                        bull basic color processing in a digital domain

                                                        Wavelets and multiresolution processing

                                                        representing images in various degree of resolution

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                                                        Week 1Week 1

                                                        Compression

                                                        reducing the storage required to save an image or the bandwidth required to transmit it

                                                        Morphological processing

                                                        bull tools for extracting image components that are useful in the representation and description of shape

                                                        bull a transition from processes that output images to processes that outputimage attributes

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        Segmentation

                                                        bull partitioning an image into its constituents parts or objects

                                                        bull autonomous segmentation is one of the most difficult tasks of DIP

                                                        bull the more accurate the segmentation the more likley recognition is to succeed

                                                        Representation and description (almost always follows segmentation)

                                                        bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                        bull converting the data produced by segmentation to a form suitable for computer processing

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                        bull complete region the focus is on internal properties such as texture or skeletal shape

                                                        bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                        Object recognition

                                                        the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                        Knowledge database

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                                                        Week 1Week 1

                                                        Simplified diagramof a cross sectionof the human eye

                                                        Digital Image ProcessingDigital Image Processing

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                                                        Week 1Week 1

                                                        Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                        The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                        Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                        The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                        The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                        Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                        Fovea = the place where the image of the object of interest falls on

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                        Blind spot region without receptors

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        Image formation in the eye

                                                        Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                        Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                        distance between lens and retina along visual axix = 17 mm

                                                        range of focal length = 14 mm to 17 mm

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

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                                                        Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        Optical illusions

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                        gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                        quantities that describe the quality of a chromatic light source radiance

                                                        the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                        measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                        brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        the physical meaning is determined by the source of the image

                                                        ( )f D f x y

                                                        Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                        f(xy) ndash characterized by two components

                                                        i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                        r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                        ( ) ( ) ( )

                                                        0 ( ) 0 ( ) 1

                                                        f x y i x y r x y

                                                        i x y r x y

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                        i(xy) ndash determined by the illumination source

                                                        r(xy) ndash determined by the characteristics of the imaged objects

                                                        is called gray (or intensity) scale

                                                        In practice

                                                        min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                        indoor values without additional illuminationmin max10 1000L L

                                                        black whitemin max0 1 0 1 0 1L L L L l l L

                                                        min maxL L

                                                        Digital Image ProcessingDigital Image Processing

                                                        Week 1Week 1

                                                        Digital Image Processing

                                                        Week 1

                                                        Image Sampling and Quantization

                                                        - the output of the sensors is a continuous voltage waveform related to the sensed

                                                        scene

                                                        converting a continuous image f to digital form

                                                        - digitizing (x y) is called sampling

                                                        - digitizing f(x y) is called quantization

                                                        Digital Image Processing

                                                        Week 1

                                                        Digital Image Processing

                                                        Week 1

                                                        Continuous image projected onto a sensor array Result of image sampling and quantization

                                                        Digital Image Processing

                                                        Week 1

                                                        Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                        (00) (01) (0 1)(10) (11) (1 1)

                                                        ( )

                                                        ( 10) ( 11) ( 1 1)

                                                        f f f Nf f f N

                                                        f x y

                                                        f M f M f M N

                                                        image element pixel

                                                        00 01 0 1

                                                        10 11 1 1

                                                        10 11 1 1

                                                        ( ) ( )

                                                        N

                                                        i jN M N

                                                        i j

                                                        M M M N

                                                        a a aa f x i y j f i ja a a

                                                        Aa

                                                        a a a

                                                        f(00) ndash the upper left corner of the image

                                                        Digital Image Processing

                                                        Week 1

                                                        M N ge 0 L=2k

                                                        [0 1]i j i ja a L

                                                        Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                        Digital Image Processing

                                                        Week 1

                                                        Digital Image Processing

                                                        Week 1

                                                        Number of bits required to store a digitized image

                                                        for 2 b M N k M N b N k

                                                        When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                        Digital Image Processing

                                                        Week 1

                                                        Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                        Measures line pairs per unit distance dots (pixels) per unit distance

                                                        Image resolution = the largest number of discernible line pairs per unit distance

                                                        (eg 100 line pairs per mm)

                                                        Dots per unit distance are commonly used in printing and publishing

                                                        In US the measure is expressed in dots per inch (dpi)

                                                        (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                        Intensity resolution ndash the smallest discernible change in intensity level

                                                        The number of intensity levels (L) is determined by hardware considerations

                                                        L=2k ndash most common k = 8

                                                        Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                        Digital Image Processing

                                                        Week 1

                                                        Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                        150 dpi (lower left) 72 dpi (lower right)

                                                        Digital Image Processing

                                                        Week 1

                                                        Reducing the number of gray levels 256 128 64 32

                                                        Digital Image Processing

                                                        Week 1

                                                        Reducing the number of gray levels 16 8 4 2

                                                        Digital Image Processing

                                                        Week 1

                                                        Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                        Shrinking zooming ndash image resizing ndash image resampling methods

                                                        Interpolation is the process of using known data to estimate values at unknown locations

                                                        Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                        750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                        same spacing as the original and then shrink it so that it fits exactly over the original

                                                        image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                        Problem assignment of intensity-level in the new 750 times 750 grid

                                                        Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                        intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                        This technique has the tendency to produce undesirable effects like severe distortion of

                                                        straight edges

                                                        Digital Image Processing

                                                        Week 1

                                                        Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                        where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                        be written using the 4 nearest neighbors of point (x y)

                                                        Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                        modest increase in computational effort

                                                        Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                        nearest neighbors of the point 3 3

                                                        0 0

                                                        ( ) i ji j

                                                        i jv x y c x y

                                                        The coefficients cij are obtained solving a 16x16 linear system

                                                        intensity levels of the 16 nearest neighbors of 3 3

                                                        0 0

                                                        ( )i ji j

                                                        i jc x y x y

                                                        Digital Image Processing

                                                        Week 1

                                                        Generally bicubic interpolation does a better job of preserving fine detail than the

                                                        bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                        programs such as Adobe Photoshop and Corel Photopaint

                                                        Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                        the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                        then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                        neighbor interpolation was used (both for shrinking and zooming)

                                                        Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                        interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                        from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                        Digital Image Processing

                                                        Week 1

                                                        Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                        Digital Image Processing

                                                        Week 1

                                                        Neighbors of a Pixel

                                                        A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                        This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                        The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                        and are denoted ND(p)

                                                        The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                        N8 (p)

                                                        If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                        fall outside the image

                                                        Digital Image Processing

                                                        Week 1

                                                        Adjacency Connectivity Regions Boundaries

                                                        Denote by V the set of intensity levels used to define adjacency

                                                        - in a binary image V 01 (V=0 V=1)

                                                        - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                        We consider 3 types of adjacency

                                                        (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                        (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                        (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                        m-adjacent if

                                                        4( )q N p or

                                                        ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                        Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                        ambiguities that often arise when 8-adjacency is used Consider the example

                                                        Digital Image Processing

                                                        Week 1

                                                        binary image

                                                        0 1 1 0 1 1 0 1 1

                                                        1 0 1 0 0 1 0 0 1 0

                                                        0 0 1 0 0 1 0 0 1

                                                        V

                                                        The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                        8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                        m-adjacency

                                                        Digital Image Processing

                                                        Week 1

                                                        A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                        is a sequence of distinct pixels with coordinates

                                                        and are adjacent 0 0 1 1

                                                        1 1

                                                        ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                        n n

                                                        i i i i

                                                        x y x y x y x y s tx y x y i n

                                                        The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                        Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                        Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                        in S if there exists a path between them consisting only of pixels from S

                                                        S is a connected set if there is a path in S between any 2 pixels in S

                                                        Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                        Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                        that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                        8-adjacency are considered

                                                        Digital Image Processing

                                                        Week 1

                                                        Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                        touches the image border

                                                        the complement of 1

                                                        ( )K

                                                        cu k u u

                                                        k

                                                        R R R R

                                                        We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                        background of the image

                                                        The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                        points in the complement of R (R)c The border of an image is the set of pixels in the

                                                        region that have at least one background neighbor This definition is referred to as the

                                                        inner border to distinguish it from the notion of outer border which is the corresponding

                                                        border in the background

                                                        Digital Image Processing

                                                        Week 1

                                                        Distance measures

                                                        For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                        function or metric if

                                                        (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                        (b) D(p q) = D(q p)

                                                        (c) D(p z) le D(p q) + D(q z)

                                                        The Euclidean distance between p and q is defined as 1

                                                        2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                        The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                        centered at (x y)

                                                        Digital Image Processing

                                                        Week 1

                                                        The D4 distance (also called city-block distance) between p and q is defined as

                                                        4( ) | | | |D p q x s y t

                                                        The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                        4

                                                        22 1 2

                                                        2 2 1 0 1 22 1 2

                                                        2

                                                        D

                                                        The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                        The D8 distance (called the chessboard distance) between p and q is defined as

                                                        8( ) max| | | |D p q x s y t

                                                        The pixels q for which 8( )D p q r form a square centered at (x y)

                                                        Digital Image Processing

                                                        Week 1

                                                        8

                                                        2 2 2 2 22 1 1 1 2

                                                        2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                        D

                                                        The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                        D4 and D8 distances are independent of any paths that might exist between p and q

                                                        because these distances involve only the coordinates of the point

                                                        Digital Image Processing

                                                        Week 1

                                                        Array versus Matrix Operations

                                                        An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                        11 12 11 12

                                                        21 22 21 22

                                                        a a b ba a b b

                                                        Array product

                                                        11 12 11 12 11 11 12 12

                                                        21 22 21 22 21 21 22 21

                                                        a a b b a b a ba a b b a b a b

                                                        Matrix product

                                                        11 12 11 12 11 11 12 21 11 12 12 21

                                                        21 22 21 22 21 11 22 21 21 12 22 22

                                                        a a b b a b a b a b a ba a b b a b a b a b a b

                                                        We assume array operations unless stated otherwise

                                                        Digital Image Processing

                                                        Week 1

                                                        Linear versus Nonlinear Operations

                                                        One of the most important classifications of image-processing methods is whether it is

                                                        linear or nonlinear

                                                        ( ) ( )H f x y g x y

                                                        H is said to be a linear operator if

                                                        images1 2 1 2

                                                        1 2

                                                        ( ) ( ) ( ) ( )

                                                        H a f x y b f x y a H f x y b H f x y

                                                        a b f f

                                                        Example of nonlinear operator

                                                        the maximum value of the pixels of image max ( )H f f x y f

                                                        1 2

                                                        0 2 6 5 1 1

                                                        2 3 4 7f f a b

                                                        Digital Image Processing

                                                        Week 1

                                                        1 2

                                                        0 2 6 5 6 3max max 1 ( 1) max 2

                                                        2 3 4 7 2 4a f b f

                                                        0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                        2 3 4 7

                                                        Arithmetic Operations in Image Processing

                                                        Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                        f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                        For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                        value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                        The two random variables are uncorrelated when their covariance is 0

                                                        Digital Image Processing

                                                        Week 1

                                                        Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                        used in image enhancement)

                                                        1

                                                        1( ) ( )K

                                                        ii

                                                        g x y g x yK

                                                        If the noise satisfies the properties stated above we have

                                                        2 2( ) ( )

                                                        1( ) ( ) g x y x yE g x y f x yK

                                                        ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                        and g respectively The standard deviation (square root of the variance) at any point in

                                                        the average image is

                                                        ( ) ( )1

                                                        g x y x yK

                                                        Digital Image Processing

                                                        Week 1

                                                        As K increases the variability (as measured by the variance or the standard deviation) of

                                                        the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                        means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                        averaging process increases

                                                        An important application of image averaging is in the field of astronomy where imaging

                                                        under very low light levels frequently causes sensor noise to render single images

                                                        virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                        was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                        64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                        images respectively

                                                        Digital Image Processing

                                                        Week 1

                                                        Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                        100 noisy images

                                                        a b c d e f

                                                        Digital Image Processing

                                                        Week 1

                                                        A frequent application of image subtraction is in the enhancement of differences between

                                                        images

                                                        (a) (b) (c)

                                                        Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                        significant bit of each pixel (c) the difference between the two images

                                                        Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                        Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                        images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                        difference between images (a) and (b)

                                                        Digital Image Processing

                                                        Week 1

                                                        Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                        h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                        intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                        source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                        bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                        anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                        images after injection of the contrast medium

                                                        In g(x y) we can find the differences between h and f as enhanced detail

                                                        Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                        propagates through the various arteries in the area being observed

                                                        Digital Image Processing

                                                        Week 1

                                                        a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                        Digital Image Processing

                                                        Week 1

                                                        An important application of image multiplication (and division) is shading correction

                                                        Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                        f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                        When the shading function is known

                                                        ( )( )( )

                                                        g x yf x yh x y

                                                        h(x y) is unknown but we have access to the imaging system we can obtain an

                                                        approximation to the shading function by imaging a target of constant intensity When the

                                                        sensor is not available often the shading pattern can be estimated from the image

                                                        Digital Image Processing

                                                        Week 1

                                                        (a) (b) (c)

                                                        Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                        Digital Image Processing

                                                        Week 1

                                                        Another use of image multiplication is in masking also called region of interest (ROI)

                                                        operations The process consists of multiplying a given image by a mask image that has

                                                        1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                        image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                        (a) (b) (c)

                                                        Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                        Digital Image Processing

                                                        Week 1

                                                        In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                        min( )mf f f

                                                        0 ( 255)max( )

                                                        ms

                                                        m

                                                        ff K K K

                                                        f

                                                        Digital Image Processing

                                                        Week 1

                                                        Spatial Operations

                                                        - are performed directly on the pixels of a given image

                                                        There are three categories of spatial operations

                                                        single-pixel operations

                                                        neighborhood operations

                                                        geometric spatial transformations

                                                        Single-pixel operations

                                                        - change the values of intensity for the individual pixels ( )s T z

                                                        where z is the intensity of a pixel in the original image and s is the intensity of the

                                                        corresponding pixel in the processed image

                                                        Digital Image Processing

                                                        Week 1

                                                        Neighborhood operations

                                                        Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                        in an image f Neighborhood processing generates new intensity level at point (x y)

                                                        based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                        rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                        intensity by computing the average value of the pixels in Sxy

                                                        ( )

                                                        1( ) ( )xyr c S

                                                        g x y f r cm n

                                                        The net effect is to perform local blurring in the original image This type of process is

                                                        used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                        largest region of an image

                                                        Digital Image Processing

                                                        Week 1

                                                        Geometric spatial transformations and image registration

                                                        - modify the spatial relationship between pixels in an image

                                                        - these transformations are often called rubber-sheet transformations (analogous to

                                                        printing an image on a sheet of rubber and then stretching the sheet according to a

                                                        predefined set of rules

                                                        A geometric transformation consists of 2 basic operations

                                                        1 a spatial transformation of coordinates

                                                        2 intensity interpolation that assign intensity values to the spatial transformed

                                                        pixels

                                                        The coordinate system transformation ( ) [( )]x y T v w

                                                        (v w) ndash pixel coordinates in the original image

                                                        (x y) ndash pixel coordinates in the transformed image

                                                        Digital Image Processing

                                                        Week 1

                                                        [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                        Affine transform

                                                        11 1211 21 31

                                                        21 2212 22 33

                                                        31 32

                                                        0[ 1] [ 1] [ 1] 0

                                                        1

                                                        t tx t v t w t

                                                        x y v w T v w t ty t v t w t

                                                        t t

                                                        (AT)

                                                        This transform can scale rotate translate or shear a set of coordinate points depending

                                                        on the elements of the matrix T If we want to resize an image rotate it and move the

                                                        result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                        scaling rotation and translation matrices from Table 1

                                                        Digital Image Processing

                                                        Week 1

                                                        Affine transformations

                                                        Digital Image Processing

                                                        Week 1

                                                        The preceding transformations relocate pixels on an image to new locations To complete

                                                        the process we have to assign intensity values to those locations This task is done by

                                                        using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                        In practice we can use equation (AT) in two basic ways

                                                        forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                        location (x y) of the corresponding pixel in the new image using (AT) directly

                                                        Problems

                                                        - intensity assignment when 2 or more pixels in the original image are transformed to

                                                        the same location in the output image

                                                        - some output locations have no correspondent in the original image (no intensity

                                                        assignment)

                                                        Digital Image Processing

                                                        Week 1

                                                        inverse mapping scans the output pixel locations and at each location (x y)

                                                        computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                        It then interpolates among the nearest input pixels to determine the intensity of the output

                                                        pixel value

                                                        Inverse mappings are more efficient to implement than forward mappings and are used in

                                                        numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                        Digital Image Processing

                                                        Week 1

                                                        Digital Image Processing

                                                        Week 1

                                                        Image registration ndash align two or more images of the same scene

                                                        In image registration we have available the input and output images but the specific

                                                        transformation that produced the output image from the input is generally unknown

                                                        The problem is to estimate the transformation function and then use it to register the two

                                                        images

                                                        - it may be of interest to align (register) two or more image taken at approximately the

                                                        same time but using different imaging systems (MRI scanner and a PET scanner)

                                                        - align images of a given location taken by the same instrument at different moments

                                                        of time (satellite images)

                                                        Solving the problem using tie points (also called control points) which are

                                                        corresponding points whose locations are known precisely in the input and reference

                                                        image

                                                        Digital Image Processing

                                                        Week 1

                                                        How to select tie points

                                                        - interactively selecting them

                                                        - use of algorithms that try to detect these points

                                                        - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                        the imaging sensors These objects produce a set of known points (called reseau

                                                        marks) directly on all images captured by the system which can be used as guides

                                                        for establishing tie points

                                                        The problem of estimating the transformation is one of modeling Suppose we have a set

                                                        of 4 tie points both on the input image and the reference image A simple model based on

                                                        a bilinear approximation is given by

                                                        1 2 3 4

                                                        5 6 7 8

                                                        x c v c w c v w cy c v c w c v w c

                                                        (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                        Digital Image Processing

                                                        Week 1

                                                        When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                        frequently is to select a larger number of tie points and using this new set of tie points

                                                        subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                        subregions marked by 4 tie points we applied the transformation model described above

                                                        The number of tie points and the sophistication of the model required to solve the register

                                                        problem depend on the severity of the geometrical distortion

                                                        Digital Image Processing

                                                        Week 1

                                                        a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                        Digital Image Processing

                                                        Week 1

                                                        Probabilistic Methods

                                                        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                        p(zk) = the probability that the intensity level zk occurs in the given image

                                                        ( ) kk

                                                        np zM N

                                                        nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                        pixels in the image) 1

                                                        0( ) 1

                                                        L

                                                        kk

                                                        p z

                                                        The mean (average) intensity of an image is given by 1

                                                        0( )

                                                        L

                                                        k kk

                                                        m z p z

                                                        Digital Image Processing

                                                        Week 1

                                                        The variance of the intensities is 1

                                                        2 2

                                                        0( ) ( )

                                                        L

                                                        k kk

                                                        z m p z

                                                        The variance is a measure of the spread of the values of z about the mean so it is a

                                                        measure of image contrast Usually for measuring image contrast the standard deviation

                                                        ( ) is used

                                                        The n-th moment of a random variable z about the mean is defined as 1

                                                        0( ) ( ) ( )

                                                        Ln

                                                        n k kk

                                                        z z m p z

                                                        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                        3( ) 0z the intensities are biased to values higher than the mean

                                                        ( 3( ) 0z the intensities are biased to values lower than the mean

                                                        Digital Image Processing

                                                        Week 1

                                                        3( ) 0z the intensities are distributed approximately equally on both side of the

                                                        mean

                                                        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                        Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                        Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                        Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                        Digital Image Processing

                                                        Week 1

                                                        Intensity Transformations and Spatial Filtering

                                                        ( ) ( )g x y T f x y

                                                        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                        neighborhood of (x y)

                                                        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                        and much smaller in size than the image

                                                        Digital Image Processing

                                                        Week 1

                                                        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                        called spatial filter (spatial mask kernel template or window)

                                                        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                        ( )s T r

                                                        s and r are denoting respectively the intensity of g and f at (x y)

                                                        Figure 2 left - T produces an output image of higher contrast than the original by

                                                        darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                        is called contrast stretching

                                                        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                        Digital Image Processing

                                                        Week 1

                                                        Figure 2 right - T produces a binary output image A mapping of this form is called

                                                        thresholding function

                                                        Some Basic Intensity Transformation Functions

                                                        Image Negatives

                                                        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                        - equivalent of a photographic negative

                                                        - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                        image

                                                        Digital Image Processing

                                                        Week 1

                                                        Original Negative image

                                                        Digital Image Processing

                                                        Week 1

                                                        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                        Some basic intensity transformation functions

                                                        Digital Image Processing

                                                        Week 1

                                                        This transformation maps a narrow range of low intensity values in the input into a wider

                                                        range An operator of this type is used to expand the values of dark pixels in an image

                                                        while compressing the higher-level values The opposite is true for the inverse log

                                                        transformation The log functions compress the dynamic range of images with large

                                                        variations in pixel values

                                                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                        Digital Image Processing

                                                        Week 1

                                                        Power-Law (Gamma) Transformations

                                                        - positive constants( ) ( ( ) )s T r c r c s c r

                                                        Plots of gamma transformation for different values of γ (c=1)

                                                        Digital Image Processing

                                                        Week 1

                                                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                        of output values with the opposite being true for higher values of input values The

                                                        curves with 1 have the opposite effect of those generated with values of 1

                                                        1c - identity transformation

                                                        A variety of devices used for image capture printing and display respond according to a

                                                        power law The process used to correct these power-law response phenomena is called

                                                        gamma correction

                                                        Digital Image Processing

                                                        Week 1

                                                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                        Digital Image Processing

                                                        Week 1

                                                        Piecewise-Linear Transformations Functions

                                                        Contrast stretching

                                                        - a process that expands the range of intensity levels in an image so it spans the full

                                                        intensity range of the recording tool or display device

                                                        a b c d Fig5

                                                        Digital Image Processing

                                                        Week 1

                                                        11

                                                        1

                                                        2 1 1 21 2

                                                        2 1 2 1

                                                        22

                                                        2

                                                        [0 ]

                                                        ( ) ( )( ) [ ]( ) ( )

                                                        ( 1 ) [ 1]( 1 )

                                                        s r r rrs r r s r rT r r r r

                                                        r r r rs L r r r L

                                                        L r

                                                        Digital Image Processing

                                                        Week 1

                                                        1 1 2 2r s r s identity transformation (no change)

                                                        1 2 1 2 0 1r r s s L thresholding function

                                                        Figure 5(b) shows an 8-bit image with low contrast

                                                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                        in the image respectively Thus the transformation function stretched the levels linearly

                                                        from their original range to the full range [0 L-1]

                                                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                        2 2 1r s m L where m is the mean gray level in the image

                                                        The original image on which these results are based is a scanning electron microscope

                                                        image of pollen magnified approximately 700 times

                                                        Digital Image Processing

                                                        Week 1

                                                        Intensity-level slicing

                                                        - highlighting a specific range of intensities in an image

                                                        There are two approaches for intensity-level slicing

                                                        1 display in one value (white for example) all the values in the range of interest and in

                                                        another (say black) all other intensities (Figure 311 (a))

                                                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                        intensities in the image (Figure 311 (b))

                                                        Digital Image Processing

                                                        Week 1

                                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                        the top of the scale of intensities This type of enhancement produces a binary image

                                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                        Highlights range [A B] and preserves all other intensities

                                                        Digital Image Processing

                                                        Week 1

                                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                                        blockageshellip)

                                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                        image around the mean intensity was set to black the other intensities remain unchanged

                                                        Fig 6 - Aortic angiogram and intensity sliced versions

                                                        Digital Image Processing

                                                        Week 1

                                                        Bit-plane slicing

                                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                        This technique highlights the contribution made to the whole image appearances by each

                                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                        Digital Image Processing

                                                        Week 1

                                                        Digital Image Processing

                                                        Week 1

                                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                        • DIP 1 2017
                                                        • DIP 02 (2017)

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Examples of light microscopy

                                                          Taxol (anticancer agent)magnified 250X

                                                          Cholesterol(40X)

                                                          Microprocessor(60X)

                                                          Nickel oxidethin film(600X)

                                                          Surface of audio CD(1750X)

                                                          Organicsuperconductor(450X)

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Automated visual inspection of manufactured goods

                                                          a bc de f

                                                          a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Imaging in the Microwave Band

                                                          The dominant aplication of imaging in the microwave band ndash radar

                                                          bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                                          bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                                          bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                                          An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Spaceborne radar image of mountains in southeast Tibet

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Imaging in the Radio Band

                                                          medicine astronomy

                                                          MRI = Magnetic Resonance Imaging

                                                          This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                                          Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                                          The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          MRI images of a human knee (left) and spine (right)

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Images of the Crab Pulsar covering the electromagnetic spectrum

                                                          Gamma X-ray Optical Infrared Radio

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Other Imaging Modalities

                                                          acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                                          Imaging using sound geological explorations industry medicine

                                                          Mineral and oil exploration

                                                          For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Biometry - iris

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Biometry - fingerprint

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Face detection and recognition

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Gender identification

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Image morphing

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Fundamental Steps in DIP

                                                          methods whose input and output are images

                                                          methods whose inputs are images but whose outputs are attributes extracted from those images

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Outputs are images

                                                          bull image acquisition

                                                          bull image filtering and enhancement

                                                          bull image restoration

                                                          bull color image processing

                                                          bull wavelets and multiresolution processing

                                                          bull compression

                                                          bull morphological processing

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Outputs are attributes

                                                          bull morphological processing

                                                          bull segmentation

                                                          bull representation and description

                                                          bull object recognition

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Image acquisition - may involve preprocessing such as scaling

                                                          Image enhancement

                                                          bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                          bull enhancement is problem oriented

                                                          bull there is no general sbquotheoryrsquo of image enhancement

                                                          bull enhancement use subjective methods for image emprovement

                                                          bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Image restoration

                                                          bull improving the appearance of an image

                                                          bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                          Color image processing

                                                          bull fundamental concept in color models

                                                          bull basic color processing in a digital domain

                                                          Wavelets and multiresolution processing

                                                          representing images in various degree of resolution

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Compression

                                                          reducing the storage required to save an image or the bandwidth required to transmit it

                                                          Morphological processing

                                                          bull tools for extracting image components that are useful in the representation and description of shape

                                                          bull a transition from processes that output images to processes that outputimage attributes

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Segmentation

                                                          bull partitioning an image into its constituents parts or objects

                                                          bull autonomous segmentation is one of the most difficult tasks of DIP

                                                          bull the more accurate the segmentation the more likley recognition is to succeed

                                                          Representation and description (almost always follows segmentation)

                                                          bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                          bull converting the data produced by segmentation to a form suitable for computer processing

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                          bull complete region the focus is on internal properties such as texture or skeletal shape

                                                          bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                          Object recognition

                                                          the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                          Knowledge database

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Simplified diagramof a cross sectionof the human eye

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                          The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                          Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                          The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                          The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                          Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                          Fovea = the place where the image of the object of interest falls on

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                          Blind spot region without receptors

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Image formation in the eye

                                                          Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                          Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                          distance between lens and retina along visual axix = 17 mm

                                                          range of focal length = 14 mm to 17 mm

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

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                                                          Week 1Week 1

                                                          Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Optical illusions

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                          gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                          quantities that describe the quality of a chromatic light source radiance

                                                          the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                          measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                          brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          the physical meaning is determined by the source of the image

                                                          ( )f D f x y

                                                          Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                          f(xy) ndash characterized by two components

                                                          i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                          r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                          ( ) ( ) ( )

                                                          0 ( ) 0 ( ) 1

                                                          f x y i x y r x y

                                                          i x y r x y

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                          i(xy) ndash determined by the illumination source

                                                          r(xy) ndash determined by the characteristics of the imaged objects

                                                          is called gray (or intensity) scale

                                                          In practice

                                                          min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                          indoor values without additional illuminationmin max10 1000L L

                                                          black whitemin max0 1 0 1 0 1L L L L l l L

                                                          min maxL L

                                                          Digital Image ProcessingDigital Image Processing

                                                          Week 1Week 1

                                                          Digital Image Processing

                                                          Week 1

                                                          Image Sampling and Quantization

                                                          - the output of the sensors is a continuous voltage waveform related to the sensed

                                                          scene

                                                          converting a continuous image f to digital form

                                                          - digitizing (x y) is called sampling

                                                          - digitizing f(x y) is called quantization

                                                          Digital Image Processing

                                                          Week 1

                                                          Digital Image Processing

                                                          Week 1

                                                          Continuous image projected onto a sensor array Result of image sampling and quantization

                                                          Digital Image Processing

                                                          Week 1

                                                          Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                          (00) (01) (0 1)(10) (11) (1 1)

                                                          ( )

                                                          ( 10) ( 11) ( 1 1)

                                                          f f f Nf f f N

                                                          f x y

                                                          f M f M f M N

                                                          image element pixel

                                                          00 01 0 1

                                                          10 11 1 1

                                                          10 11 1 1

                                                          ( ) ( )

                                                          N

                                                          i jN M N

                                                          i j

                                                          M M M N

                                                          a a aa f x i y j f i ja a a

                                                          Aa

                                                          a a a

                                                          f(00) ndash the upper left corner of the image

                                                          Digital Image Processing

                                                          Week 1

                                                          M N ge 0 L=2k

                                                          [0 1]i j i ja a L

                                                          Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                          Digital Image Processing

                                                          Week 1

                                                          Digital Image Processing

                                                          Week 1

                                                          Number of bits required to store a digitized image

                                                          for 2 b M N k M N b N k

                                                          When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                          Digital Image Processing

                                                          Week 1

                                                          Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                          Measures line pairs per unit distance dots (pixels) per unit distance

                                                          Image resolution = the largest number of discernible line pairs per unit distance

                                                          (eg 100 line pairs per mm)

                                                          Dots per unit distance are commonly used in printing and publishing

                                                          In US the measure is expressed in dots per inch (dpi)

                                                          (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                          Intensity resolution ndash the smallest discernible change in intensity level

                                                          The number of intensity levels (L) is determined by hardware considerations

                                                          L=2k ndash most common k = 8

                                                          Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                          Digital Image Processing

                                                          Week 1

                                                          Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                          150 dpi (lower left) 72 dpi (lower right)

                                                          Digital Image Processing

                                                          Week 1

                                                          Reducing the number of gray levels 256 128 64 32

                                                          Digital Image Processing

                                                          Week 1

                                                          Reducing the number of gray levels 16 8 4 2

                                                          Digital Image Processing

                                                          Week 1

                                                          Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                          Shrinking zooming ndash image resizing ndash image resampling methods

                                                          Interpolation is the process of using known data to estimate values at unknown locations

                                                          Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                          750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                          same spacing as the original and then shrink it so that it fits exactly over the original

                                                          image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                          Problem assignment of intensity-level in the new 750 times 750 grid

                                                          Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                          intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                          This technique has the tendency to produce undesirable effects like severe distortion of

                                                          straight edges

                                                          Digital Image Processing

                                                          Week 1

                                                          Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                          where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                          be written using the 4 nearest neighbors of point (x y)

                                                          Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                          modest increase in computational effort

                                                          Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                          nearest neighbors of the point 3 3

                                                          0 0

                                                          ( ) i ji j

                                                          i jv x y c x y

                                                          The coefficients cij are obtained solving a 16x16 linear system

                                                          intensity levels of the 16 nearest neighbors of 3 3

                                                          0 0

                                                          ( )i ji j

                                                          i jc x y x y

                                                          Digital Image Processing

                                                          Week 1

                                                          Generally bicubic interpolation does a better job of preserving fine detail than the

                                                          bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                          programs such as Adobe Photoshop and Corel Photopaint

                                                          Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                          the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                          then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                          neighbor interpolation was used (both for shrinking and zooming)

                                                          Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                          interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                          from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                          Digital Image Processing

                                                          Week 1

                                                          Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                          Digital Image Processing

                                                          Week 1

                                                          Neighbors of a Pixel

                                                          A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                          This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                          The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                          and are denoted ND(p)

                                                          The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                          N8 (p)

                                                          If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                          fall outside the image

                                                          Digital Image Processing

                                                          Week 1

                                                          Adjacency Connectivity Regions Boundaries

                                                          Denote by V the set of intensity levels used to define adjacency

                                                          - in a binary image V 01 (V=0 V=1)

                                                          - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                          We consider 3 types of adjacency

                                                          (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                          (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                          (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                          m-adjacent if

                                                          4( )q N p or

                                                          ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                          Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                          ambiguities that often arise when 8-adjacency is used Consider the example

                                                          Digital Image Processing

                                                          Week 1

                                                          binary image

                                                          0 1 1 0 1 1 0 1 1

                                                          1 0 1 0 0 1 0 0 1 0

                                                          0 0 1 0 0 1 0 0 1

                                                          V

                                                          The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                          8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                          m-adjacency

                                                          Digital Image Processing

                                                          Week 1

                                                          A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                          is a sequence of distinct pixels with coordinates

                                                          and are adjacent 0 0 1 1

                                                          1 1

                                                          ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                          n n

                                                          i i i i

                                                          x y x y x y x y s tx y x y i n

                                                          The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                          Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                          Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                          in S if there exists a path between them consisting only of pixels from S

                                                          S is a connected set if there is a path in S between any 2 pixels in S

                                                          Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                          Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                          that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                          8-adjacency are considered

                                                          Digital Image Processing

                                                          Week 1

                                                          Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                          touches the image border

                                                          the complement of 1

                                                          ( )K

                                                          cu k u u

                                                          k

                                                          R R R R

                                                          We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                          background of the image

                                                          The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                          points in the complement of R (R)c The border of an image is the set of pixels in the

                                                          region that have at least one background neighbor This definition is referred to as the

                                                          inner border to distinguish it from the notion of outer border which is the corresponding

                                                          border in the background

                                                          Digital Image Processing

                                                          Week 1

                                                          Distance measures

                                                          For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                          function or metric if

                                                          (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                          (b) D(p q) = D(q p)

                                                          (c) D(p z) le D(p q) + D(q z)

                                                          The Euclidean distance between p and q is defined as 1

                                                          2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                          The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                          centered at (x y)

                                                          Digital Image Processing

                                                          Week 1

                                                          The D4 distance (also called city-block distance) between p and q is defined as

                                                          4( ) | | | |D p q x s y t

                                                          The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                          4

                                                          22 1 2

                                                          2 2 1 0 1 22 1 2

                                                          2

                                                          D

                                                          The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                          The D8 distance (called the chessboard distance) between p and q is defined as

                                                          8( ) max| | | |D p q x s y t

                                                          The pixels q for which 8( )D p q r form a square centered at (x y)

                                                          Digital Image Processing

                                                          Week 1

                                                          8

                                                          2 2 2 2 22 1 1 1 2

                                                          2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                          D

                                                          The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                          D4 and D8 distances are independent of any paths that might exist between p and q

                                                          because these distances involve only the coordinates of the point

                                                          Digital Image Processing

                                                          Week 1

                                                          Array versus Matrix Operations

                                                          An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                          11 12 11 12

                                                          21 22 21 22

                                                          a a b ba a b b

                                                          Array product

                                                          11 12 11 12 11 11 12 12

                                                          21 22 21 22 21 21 22 21

                                                          a a b b a b a ba a b b a b a b

                                                          Matrix product

                                                          11 12 11 12 11 11 12 21 11 12 12 21

                                                          21 22 21 22 21 11 22 21 21 12 22 22

                                                          a a b b a b a b a b a ba a b b a b a b a b a b

                                                          We assume array operations unless stated otherwise

                                                          Digital Image Processing

                                                          Week 1

                                                          Linear versus Nonlinear Operations

                                                          One of the most important classifications of image-processing methods is whether it is

                                                          linear or nonlinear

                                                          ( ) ( )H f x y g x y

                                                          H is said to be a linear operator if

                                                          images1 2 1 2

                                                          1 2

                                                          ( ) ( ) ( ) ( )

                                                          H a f x y b f x y a H f x y b H f x y

                                                          a b f f

                                                          Example of nonlinear operator

                                                          the maximum value of the pixels of image max ( )H f f x y f

                                                          1 2

                                                          0 2 6 5 1 1

                                                          2 3 4 7f f a b

                                                          Digital Image Processing

                                                          Week 1

                                                          1 2

                                                          0 2 6 5 6 3max max 1 ( 1) max 2

                                                          2 3 4 7 2 4a f b f

                                                          0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                          2 3 4 7

                                                          Arithmetic Operations in Image Processing

                                                          Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                          f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                          For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                          value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                          The two random variables are uncorrelated when their covariance is 0

                                                          Digital Image Processing

                                                          Week 1

                                                          Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                          used in image enhancement)

                                                          1

                                                          1( ) ( )K

                                                          ii

                                                          g x y g x yK

                                                          If the noise satisfies the properties stated above we have

                                                          2 2( ) ( )

                                                          1( ) ( ) g x y x yE g x y f x yK

                                                          ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                          and g respectively The standard deviation (square root of the variance) at any point in

                                                          the average image is

                                                          ( ) ( )1

                                                          g x y x yK

                                                          Digital Image Processing

                                                          Week 1

                                                          As K increases the variability (as measured by the variance or the standard deviation) of

                                                          the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                          means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                          averaging process increases

                                                          An important application of image averaging is in the field of astronomy where imaging

                                                          under very low light levels frequently causes sensor noise to render single images

                                                          virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                          was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                          64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                          images respectively

                                                          Digital Image Processing

                                                          Week 1

                                                          Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                          100 noisy images

                                                          a b c d e f

                                                          Digital Image Processing

                                                          Week 1

                                                          A frequent application of image subtraction is in the enhancement of differences between

                                                          images

                                                          (a) (b) (c)

                                                          Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                          significant bit of each pixel (c) the difference between the two images

                                                          Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                          Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                          images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                          difference between images (a) and (b)

                                                          Digital Image Processing

                                                          Week 1

                                                          Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                          h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                          intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                          source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                          bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                          anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                          images after injection of the contrast medium

                                                          In g(x y) we can find the differences between h and f as enhanced detail

                                                          Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                          propagates through the various arteries in the area being observed

                                                          Digital Image Processing

                                                          Week 1

                                                          a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                          Digital Image Processing

                                                          Week 1

                                                          An important application of image multiplication (and division) is shading correction

                                                          Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                          f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                          When the shading function is known

                                                          ( )( )( )

                                                          g x yf x yh x y

                                                          h(x y) is unknown but we have access to the imaging system we can obtain an

                                                          approximation to the shading function by imaging a target of constant intensity When the

                                                          sensor is not available often the shading pattern can be estimated from the image

                                                          Digital Image Processing

                                                          Week 1

                                                          (a) (b) (c)

                                                          Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                          Digital Image Processing

                                                          Week 1

                                                          Another use of image multiplication is in masking also called region of interest (ROI)

                                                          operations The process consists of multiplying a given image by a mask image that has

                                                          1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                          image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                          (a) (b) (c)

                                                          Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                          Digital Image Processing

                                                          Week 1

                                                          In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                          min( )mf f f

                                                          0 ( 255)max( )

                                                          ms

                                                          m

                                                          ff K K K

                                                          f

                                                          Digital Image Processing

                                                          Week 1

                                                          Spatial Operations

                                                          - are performed directly on the pixels of a given image

                                                          There are three categories of spatial operations

                                                          single-pixel operations

                                                          neighborhood operations

                                                          geometric spatial transformations

                                                          Single-pixel operations

                                                          - change the values of intensity for the individual pixels ( )s T z

                                                          where z is the intensity of a pixel in the original image and s is the intensity of the

                                                          corresponding pixel in the processed image

                                                          Digital Image Processing

                                                          Week 1

                                                          Neighborhood operations

                                                          Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                          in an image f Neighborhood processing generates new intensity level at point (x y)

                                                          based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                          rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                          intensity by computing the average value of the pixels in Sxy

                                                          ( )

                                                          1( ) ( )xyr c S

                                                          g x y f r cm n

                                                          The net effect is to perform local blurring in the original image This type of process is

                                                          used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                          largest region of an image

                                                          Digital Image Processing

                                                          Week 1

                                                          Geometric spatial transformations and image registration

                                                          - modify the spatial relationship between pixels in an image

                                                          - these transformations are often called rubber-sheet transformations (analogous to

                                                          printing an image on a sheet of rubber and then stretching the sheet according to a

                                                          predefined set of rules

                                                          A geometric transformation consists of 2 basic operations

                                                          1 a spatial transformation of coordinates

                                                          2 intensity interpolation that assign intensity values to the spatial transformed

                                                          pixels

                                                          The coordinate system transformation ( ) [( )]x y T v w

                                                          (v w) ndash pixel coordinates in the original image

                                                          (x y) ndash pixel coordinates in the transformed image

                                                          Digital Image Processing

                                                          Week 1

                                                          [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                          Affine transform

                                                          11 1211 21 31

                                                          21 2212 22 33

                                                          31 32

                                                          0[ 1] [ 1] [ 1] 0

                                                          1

                                                          t tx t v t w t

                                                          x y v w T v w t ty t v t w t

                                                          t t

                                                          (AT)

                                                          This transform can scale rotate translate or shear a set of coordinate points depending

                                                          on the elements of the matrix T If we want to resize an image rotate it and move the

                                                          result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                          scaling rotation and translation matrices from Table 1

                                                          Digital Image Processing

                                                          Week 1

                                                          Affine transformations

                                                          Digital Image Processing

                                                          Week 1

                                                          The preceding transformations relocate pixels on an image to new locations To complete

                                                          the process we have to assign intensity values to those locations This task is done by

                                                          using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                          In practice we can use equation (AT) in two basic ways

                                                          forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                          location (x y) of the corresponding pixel in the new image using (AT) directly

                                                          Problems

                                                          - intensity assignment when 2 or more pixels in the original image are transformed to

                                                          the same location in the output image

                                                          - some output locations have no correspondent in the original image (no intensity

                                                          assignment)

                                                          Digital Image Processing

                                                          Week 1

                                                          inverse mapping scans the output pixel locations and at each location (x y)

                                                          computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                          It then interpolates among the nearest input pixels to determine the intensity of the output

                                                          pixel value

                                                          Inverse mappings are more efficient to implement than forward mappings and are used in

                                                          numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                          Digital Image Processing

                                                          Week 1

                                                          Digital Image Processing

                                                          Week 1

                                                          Image registration ndash align two or more images of the same scene

                                                          In image registration we have available the input and output images but the specific

                                                          transformation that produced the output image from the input is generally unknown

                                                          The problem is to estimate the transformation function and then use it to register the two

                                                          images

                                                          - it may be of interest to align (register) two or more image taken at approximately the

                                                          same time but using different imaging systems (MRI scanner and a PET scanner)

                                                          - align images of a given location taken by the same instrument at different moments

                                                          of time (satellite images)

                                                          Solving the problem using tie points (also called control points) which are

                                                          corresponding points whose locations are known precisely in the input and reference

                                                          image

                                                          Digital Image Processing

                                                          Week 1

                                                          How to select tie points

                                                          - interactively selecting them

                                                          - use of algorithms that try to detect these points

                                                          - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                          the imaging sensors These objects produce a set of known points (called reseau

                                                          marks) directly on all images captured by the system which can be used as guides

                                                          for establishing tie points

                                                          The problem of estimating the transformation is one of modeling Suppose we have a set

                                                          of 4 tie points both on the input image and the reference image A simple model based on

                                                          a bilinear approximation is given by

                                                          1 2 3 4

                                                          5 6 7 8

                                                          x c v c w c v w cy c v c w c v w c

                                                          (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                          Digital Image Processing

                                                          Week 1

                                                          When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                          frequently is to select a larger number of tie points and using this new set of tie points

                                                          subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                          subregions marked by 4 tie points we applied the transformation model described above

                                                          The number of tie points and the sophistication of the model required to solve the register

                                                          problem depend on the severity of the geometrical distortion

                                                          Digital Image Processing

                                                          Week 1

                                                          a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                          Digital Image Processing

                                                          Week 1

                                                          Probabilistic Methods

                                                          zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                          p(zk) = the probability that the intensity level zk occurs in the given image

                                                          ( ) kk

                                                          np zM N

                                                          nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                          pixels in the image) 1

                                                          0( ) 1

                                                          L

                                                          kk

                                                          p z

                                                          The mean (average) intensity of an image is given by 1

                                                          0( )

                                                          L

                                                          k kk

                                                          m z p z

                                                          Digital Image Processing

                                                          Week 1

                                                          The variance of the intensities is 1

                                                          2 2

                                                          0( ) ( )

                                                          L

                                                          k kk

                                                          z m p z

                                                          The variance is a measure of the spread of the values of z about the mean so it is a

                                                          measure of image contrast Usually for measuring image contrast the standard deviation

                                                          ( ) is used

                                                          The n-th moment of a random variable z about the mean is defined as 1

                                                          0( ) ( ) ( )

                                                          Ln

                                                          n k kk

                                                          z z m p z

                                                          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                          3( ) 0z the intensities are biased to values higher than the mean

                                                          ( 3( ) 0z the intensities are biased to values lower than the mean

                                                          Digital Image Processing

                                                          Week 1

                                                          3( ) 0z the intensities are distributed approximately equally on both side of the

                                                          mean

                                                          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                          Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                          Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                          Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                          Digital Image Processing

                                                          Week 1

                                                          Intensity Transformations and Spatial Filtering

                                                          ( ) ( )g x y T f x y

                                                          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                          neighborhood of (x y)

                                                          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                          and much smaller in size than the image

                                                          Digital Image Processing

                                                          Week 1

                                                          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                          called spatial filter (spatial mask kernel template or window)

                                                          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                          ( )s T r

                                                          s and r are denoting respectively the intensity of g and f at (x y)

                                                          Figure 2 left - T produces an output image of higher contrast than the original by

                                                          darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                          is called contrast stretching

                                                          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                          Digital Image Processing

                                                          Week 1

                                                          Figure 2 right - T produces a binary output image A mapping of this form is called

                                                          thresholding function

                                                          Some Basic Intensity Transformation Functions

                                                          Image Negatives

                                                          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                          - equivalent of a photographic negative

                                                          - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                          image

                                                          Digital Image Processing

                                                          Week 1

                                                          Original Negative image

                                                          Digital Image Processing

                                                          Week 1

                                                          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                          Some basic intensity transformation functions

                                                          Digital Image Processing

                                                          Week 1

                                                          This transformation maps a narrow range of low intensity values in the input into a wider

                                                          range An operator of this type is used to expand the values of dark pixels in an image

                                                          while compressing the higher-level values The opposite is true for the inverse log

                                                          transformation The log functions compress the dynamic range of images with large

                                                          variations in pixel values

                                                          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                          Digital Image Processing

                                                          Week 1

                                                          Power-Law (Gamma) Transformations

                                                          - positive constants( ) ( ( ) )s T r c r c s c r

                                                          Plots of gamma transformation for different values of γ (c=1)

                                                          Digital Image Processing

                                                          Week 1

                                                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                          of output values with the opposite being true for higher values of input values The

                                                          curves with 1 have the opposite effect of those generated with values of 1

                                                          1c - identity transformation

                                                          A variety of devices used for image capture printing and display respond according to a

                                                          power law The process used to correct these power-law response phenomena is called

                                                          gamma correction

                                                          Digital Image Processing

                                                          Week 1

                                                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                          Digital Image Processing

                                                          Week 1

                                                          Piecewise-Linear Transformations Functions

                                                          Contrast stretching

                                                          - a process that expands the range of intensity levels in an image so it spans the full

                                                          intensity range of the recording tool or display device

                                                          a b c d Fig5

                                                          Digital Image Processing

                                                          Week 1

                                                          11

                                                          1

                                                          2 1 1 21 2

                                                          2 1 2 1

                                                          22

                                                          2

                                                          [0 ]

                                                          ( ) ( )( ) [ ]( ) ( )

                                                          ( 1 ) [ 1]( 1 )

                                                          s r r rrs r r s r rT r r r r

                                                          r r r rs L r r r L

                                                          L r

                                                          Digital Image Processing

                                                          Week 1

                                                          1 1 2 2r s r s identity transformation (no change)

                                                          1 2 1 2 0 1r r s s L thresholding function

                                                          Figure 5(b) shows an 8-bit image with low contrast

                                                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                          in the image respectively Thus the transformation function stretched the levels linearly

                                                          from their original range to the full range [0 L-1]

                                                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                          2 2 1r s m L where m is the mean gray level in the image

                                                          The original image on which these results are based is a scanning electron microscope

                                                          image of pollen magnified approximately 700 times

                                                          Digital Image Processing

                                                          Week 1

                                                          Intensity-level slicing

                                                          - highlighting a specific range of intensities in an image

                                                          There are two approaches for intensity-level slicing

                                                          1 display in one value (white for example) all the values in the range of interest and in

                                                          another (say black) all other intensities (Figure 311 (a))

                                                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                          intensities in the image (Figure 311 (b))

                                                          Digital Image Processing

                                                          Week 1

                                                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                          the top of the scale of intensities This type of enhancement produces a binary image

                                                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                          Highlights range [A B] and preserves all other intensities

                                                          Digital Image Processing

                                                          Week 1

                                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                                          blockageshellip)

                                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                          image around the mean intensity was set to black the other intensities remain unchanged

                                                          Fig 6 - Aortic angiogram and intensity sliced versions

                                                          Digital Image Processing

                                                          Week 1

                                                          Bit-plane slicing

                                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                          This technique highlights the contribution made to the whole image appearances by each

                                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                          Digital Image Processing

                                                          Week 1

                                                          Digital Image Processing

                                                          Week 1

                                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                          • DIP 1 2017
                                                          • DIP 02 (2017)

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Automated visual inspection of manufactured goods

                                                            a bc de f

                                                            a ndash a circuit board controllerb ndash packaged pillesc ndash bottlesd ndash air bubbles in clear-plastic producte ndash cerealf ndash image of intraocular implant

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Imaging in the Microwave Band

                                                            The dominant aplication of imaging in the microwave band ndash radar

                                                            bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                                            bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                                            bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                                            An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Spaceborne radar image of mountains in southeast Tibet

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Imaging in the Radio Band

                                                            medicine astronomy

                                                            MRI = Magnetic Resonance Imaging

                                                            This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                                            Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                                            The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            MRI images of a human knee (left) and spine (right)

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Images of the Crab Pulsar covering the electromagnetic spectrum

                                                            Gamma X-ray Optical Infrared Radio

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Other Imaging Modalities

                                                            acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                                            Imaging using sound geological explorations industry medicine

                                                            Mineral and oil exploration

                                                            For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Biometry - iris

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Biometry - fingerprint

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Face detection and recognition

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Gender identification

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Image morphing

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Fundamental Steps in DIP

                                                            methods whose input and output are images

                                                            methods whose inputs are images but whose outputs are attributes extracted from those images

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Outputs are images

                                                            bull image acquisition

                                                            bull image filtering and enhancement

                                                            bull image restoration

                                                            bull color image processing

                                                            bull wavelets and multiresolution processing

                                                            bull compression

                                                            bull morphological processing

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Outputs are attributes

                                                            bull morphological processing

                                                            bull segmentation

                                                            bull representation and description

                                                            bull object recognition

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Image acquisition - may involve preprocessing such as scaling

                                                            Image enhancement

                                                            bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                            bull enhancement is problem oriented

                                                            bull there is no general sbquotheoryrsquo of image enhancement

                                                            bull enhancement use subjective methods for image emprovement

                                                            bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Image restoration

                                                            bull improving the appearance of an image

                                                            bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                            Color image processing

                                                            bull fundamental concept in color models

                                                            bull basic color processing in a digital domain

                                                            Wavelets and multiresolution processing

                                                            representing images in various degree of resolution

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Compression

                                                            reducing the storage required to save an image or the bandwidth required to transmit it

                                                            Morphological processing

                                                            bull tools for extracting image components that are useful in the representation and description of shape

                                                            bull a transition from processes that output images to processes that outputimage attributes

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Segmentation

                                                            bull partitioning an image into its constituents parts or objects

                                                            bull autonomous segmentation is one of the most difficult tasks of DIP

                                                            bull the more accurate the segmentation the more likley recognition is to succeed

                                                            Representation and description (almost always follows segmentation)

                                                            bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                            bull converting the data produced by segmentation to a form suitable for computer processing

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                            bull complete region the focus is on internal properties such as texture or skeletal shape

                                                            bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                            Object recognition

                                                            the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                            Knowledge database

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Simplified diagramof a cross sectionof the human eye

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                            The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                            Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                            The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                            The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                            Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                            Fovea = the place where the image of the object of interest falls on

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                            Blind spot region without receptors

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Image formation in the eye

                                                            Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                            Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                            distance between lens and retina along visual axix = 17 mm

                                                            range of focal length = 14 mm to 17 mm

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Optical illusions

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                            gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                            quantities that describe the quality of a chromatic light source radiance

                                                            the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                            measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                            brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            the physical meaning is determined by the source of the image

                                                            ( )f D f x y

                                                            Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                            f(xy) ndash characterized by two components

                                                            i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                            r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                            ( ) ( ) ( )

                                                            0 ( ) 0 ( ) 1

                                                            f x y i x y r x y

                                                            i x y r x y

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                            i(xy) ndash determined by the illumination source

                                                            r(xy) ndash determined by the characteristics of the imaged objects

                                                            is called gray (or intensity) scale

                                                            In practice

                                                            min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                            indoor values without additional illuminationmin max10 1000L L

                                                            black whitemin max0 1 0 1 0 1L L L L l l L

                                                            min maxL L

                                                            Digital Image ProcessingDigital Image Processing

                                                            Week 1Week 1

                                                            Digital Image Processing

                                                            Week 1

                                                            Image Sampling and Quantization

                                                            - the output of the sensors is a continuous voltage waveform related to the sensed

                                                            scene

                                                            converting a continuous image f to digital form

                                                            - digitizing (x y) is called sampling

                                                            - digitizing f(x y) is called quantization

                                                            Digital Image Processing

                                                            Week 1

                                                            Digital Image Processing

                                                            Week 1

                                                            Continuous image projected onto a sensor array Result of image sampling and quantization

                                                            Digital Image Processing

                                                            Week 1

                                                            Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                            (00) (01) (0 1)(10) (11) (1 1)

                                                            ( )

                                                            ( 10) ( 11) ( 1 1)

                                                            f f f Nf f f N

                                                            f x y

                                                            f M f M f M N

                                                            image element pixel

                                                            00 01 0 1

                                                            10 11 1 1

                                                            10 11 1 1

                                                            ( ) ( )

                                                            N

                                                            i jN M N

                                                            i j

                                                            M M M N

                                                            a a aa f x i y j f i ja a a

                                                            Aa

                                                            a a a

                                                            f(00) ndash the upper left corner of the image

                                                            Digital Image Processing

                                                            Week 1

                                                            M N ge 0 L=2k

                                                            [0 1]i j i ja a L

                                                            Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                            Digital Image Processing

                                                            Week 1

                                                            Digital Image Processing

                                                            Week 1

                                                            Number of bits required to store a digitized image

                                                            for 2 b M N k M N b N k

                                                            When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                            Digital Image Processing

                                                            Week 1

                                                            Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                            Measures line pairs per unit distance dots (pixels) per unit distance

                                                            Image resolution = the largest number of discernible line pairs per unit distance

                                                            (eg 100 line pairs per mm)

                                                            Dots per unit distance are commonly used in printing and publishing

                                                            In US the measure is expressed in dots per inch (dpi)

                                                            (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                            Intensity resolution ndash the smallest discernible change in intensity level

                                                            The number of intensity levels (L) is determined by hardware considerations

                                                            L=2k ndash most common k = 8

                                                            Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                            Digital Image Processing

                                                            Week 1

                                                            Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                            150 dpi (lower left) 72 dpi (lower right)

                                                            Digital Image Processing

                                                            Week 1

                                                            Reducing the number of gray levels 256 128 64 32

                                                            Digital Image Processing

                                                            Week 1

                                                            Reducing the number of gray levels 16 8 4 2

                                                            Digital Image Processing

                                                            Week 1

                                                            Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                            Shrinking zooming ndash image resizing ndash image resampling methods

                                                            Interpolation is the process of using known data to estimate values at unknown locations

                                                            Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                            750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                            same spacing as the original and then shrink it so that it fits exactly over the original

                                                            image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                            Problem assignment of intensity-level in the new 750 times 750 grid

                                                            Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                            intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                            This technique has the tendency to produce undesirable effects like severe distortion of

                                                            straight edges

                                                            Digital Image Processing

                                                            Week 1

                                                            Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                            where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                            be written using the 4 nearest neighbors of point (x y)

                                                            Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                            modest increase in computational effort

                                                            Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                            nearest neighbors of the point 3 3

                                                            0 0

                                                            ( ) i ji j

                                                            i jv x y c x y

                                                            The coefficients cij are obtained solving a 16x16 linear system

                                                            intensity levels of the 16 nearest neighbors of 3 3

                                                            0 0

                                                            ( )i ji j

                                                            i jc x y x y

                                                            Digital Image Processing

                                                            Week 1

                                                            Generally bicubic interpolation does a better job of preserving fine detail than the

                                                            bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                            programs such as Adobe Photoshop and Corel Photopaint

                                                            Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                            the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                            then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                            neighbor interpolation was used (both for shrinking and zooming)

                                                            Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                            interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                            from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                            Digital Image Processing

                                                            Week 1

                                                            Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                            Digital Image Processing

                                                            Week 1

                                                            Neighbors of a Pixel

                                                            A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                            This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                            The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                            and are denoted ND(p)

                                                            The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                            N8 (p)

                                                            If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                            fall outside the image

                                                            Digital Image Processing

                                                            Week 1

                                                            Adjacency Connectivity Regions Boundaries

                                                            Denote by V the set of intensity levels used to define adjacency

                                                            - in a binary image V 01 (V=0 V=1)

                                                            - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                            We consider 3 types of adjacency

                                                            (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                            (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                            (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                            m-adjacent if

                                                            4( )q N p or

                                                            ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                            Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                            ambiguities that often arise when 8-adjacency is used Consider the example

                                                            Digital Image Processing

                                                            Week 1

                                                            binary image

                                                            0 1 1 0 1 1 0 1 1

                                                            1 0 1 0 0 1 0 0 1 0

                                                            0 0 1 0 0 1 0 0 1

                                                            V

                                                            The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                            8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                            m-adjacency

                                                            Digital Image Processing

                                                            Week 1

                                                            A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                            is a sequence of distinct pixels with coordinates

                                                            and are adjacent 0 0 1 1

                                                            1 1

                                                            ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                            n n

                                                            i i i i

                                                            x y x y x y x y s tx y x y i n

                                                            The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                            Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                            Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                            in S if there exists a path between them consisting only of pixels from S

                                                            S is a connected set if there is a path in S between any 2 pixels in S

                                                            Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                            Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                            that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                            8-adjacency are considered

                                                            Digital Image Processing

                                                            Week 1

                                                            Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                            touches the image border

                                                            the complement of 1

                                                            ( )K

                                                            cu k u u

                                                            k

                                                            R R R R

                                                            We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                            background of the image

                                                            The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                            points in the complement of R (R)c The border of an image is the set of pixels in the

                                                            region that have at least one background neighbor This definition is referred to as the

                                                            inner border to distinguish it from the notion of outer border which is the corresponding

                                                            border in the background

                                                            Digital Image Processing

                                                            Week 1

                                                            Distance measures

                                                            For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                            function or metric if

                                                            (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                            (b) D(p q) = D(q p)

                                                            (c) D(p z) le D(p q) + D(q z)

                                                            The Euclidean distance between p and q is defined as 1

                                                            2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                            The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                            centered at (x y)

                                                            Digital Image Processing

                                                            Week 1

                                                            The D4 distance (also called city-block distance) between p and q is defined as

                                                            4( ) | | | |D p q x s y t

                                                            The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                            4

                                                            22 1 2

                                                            2 2 1 0 1 22 1 2

                                                            2

                                                            D

                                                            The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                            The D8 distance (called the chessboard distance) between p and q is defined as

                                                            8( ) max| | | |D p q x s y t

                                                            The pixels q for which 8( )D p q r form a square centered at (x y)

                                                            Digital Image Processing

                                                            Week 1

                                                            8

                                                            2 2 2 2 22 1 1 1 2

                                                            2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                            D

                                                            The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                            D4 and D8 distances are independent of any paths that might exist between p and q

                                                            because these distances involve only the coordinates of the point

                                                            Digital Image Processing

                                                            Week 1

                                                            Array versus Matrix Operations

                                                            An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                            11 12 11 12

                                                            21 22 21 22

                                                            a a b ba a b b

                                                            Array product

                                                            11 12 11 12 11 11 12 12

                                                            21 22 21 22 21 21 22 21

                                                            a a b b a b a ba a b b a b a b

                                                            Matrix product

                                                            11 12 11 12 11 11 12 21 11 12 12 21

                                                            21 22 21 22 21 11 22 21 21 12 22 22

                                                            a a b b a b a b a b a ba a b b a b a b a b a b

                                                            We assume array operations unless stated otherwise

                                                            Digital Image Processing

                                                            Week 1

                                                            Linear versus Nonlinear Operations

                                                            One of the most important classifications of image-processing methods is whether it is

                                                            linear or nonlinear

                                                            ( ) ( )H f x y g x y

                                                            H is said to be a linear operator if

                                                            images1 2 1 2

                                                            1 2

                                                            ( ) ( ) ( ) ( )

                                                            H a f x y b f x y a H f x y b H f x y

                                                            a b f f

                                                            Example of nonlinear operator

                                                            the maximum value of the pixels of image max ( )H f f x y f

                                                            1 2

                                                            0 2 6 5 1 1

                                                            2 3 4 7f f a b

                                                            Digital Image Processing

                                                            Week 1

                                                            1 2

                                                            0 2 6 5 6 3max max 1 ( 1) max 2

                                                            2 3 4 7 2 4a f b f

                                                            0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                            2 3 4 7

                                                            Arithmetic Operations in Image Processing

                                                            Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                            f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                            For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                            value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                            The two random variables are uncorrelated when their covariance is 0

                                                            Digital Image Processing

                                                            Week 1

                                                            Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                            used in image enhancement)

                                                            1

                                                            1( ) ( )K

                                                            ii

                                                            g x y g x yK

                                                            If the noise satisfies the properties stated above we have

                                                            2 2( ) ( )

                                                            1( ) ( ) g x y x yE g x y f x yK

                                                            ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                            and g respectively The standard deviation (square root of the variance) at any point in

                                                            the average image is

                                                            ( ) ( )1

                                                            g x y x yK

                                                            Digital Image Processing

                                                            Week 1

                                                            As K increases the variability (as measured by the variance or the standard deviation) of

                                                            the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                            means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                            averaging process increases

                                                            An important application of image averaging is in the field of astronomy where imaging

                                                            under very low light levels frequently causes sensor noise to render single images

                                                            virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                            was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                            64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                            images respectively

                                                            Digital Image Processing

                                                            Week 1

                                                            Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                            100 noisy images

                                                            a b c d e f

                                                            Digital Image Processing

                                                            Week 1

                                                            A frequent application of image subtraction is in the enhancement of differences between

                                                            images

                                                            (a) (b) (c)

                                                            Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                            significant bit of each pixel (c) the difference between the two images

                                                            Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                            Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                            images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                            difference between images (a) and (b)

                                                            Digital Image Processing

                                                            Week 1

                                                            Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                            h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                            intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                            source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                            bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                            anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                            images after injection of the contrast medium

                                                            In g(x y) we can find the differences between h and f as enhanced detail

                                                            Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                            propagates through the various arteries in the area being observed

                                                            Digital Image Processing

                                                            Week 1

                                                            a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                            Digital Image Processing

                                                            Week 1

                                                            An important application of image multiplication (and division) is shading correction

                                                            Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                            f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                            When the shading function is known

                                                            ( )( )( )

                                                            g x yf x yh x y

                                                            h(x y) is unknown but we have access to the imaging system we can obtain an

                                                            approximation to the shading function by imaging a target of constant intensity When the

                                                            sensor is not available often the shading pattern can be estimated from the image

                                                            Digital Image Processing

                                                            Week 1

                                                            (a) (b) (c)

                                                            Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                            Digital Image Processing

                                                            Week 1

                                                            Another use of image multiplication is in masking also called region of interest (ROI)

                                                            operations The process consists of multiplying a given image by a mask image that has

                                                            1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                            image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                            (a) (b) (c)

                                                            Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                            Digital Image Processing

                                                            Week 1

                                                            In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                            min( )mf f f

                                                            0 ( 255)max( )

                                                            ms

                                                            m

                                                            ff K K K

                                                            f

                                                            Digital Image Processing

                                                            Week 1

                                                            Spatial Operations

                                                            - are performed directly on the pixels of a given image

                                                            There are three categories of spatial operations

                                                            single-pixel operations

                                                            neighborhood operations

                                                            geometric spatial transformations

                                                            Single-pixel operations

                                                            - change the values of intensity for the individual pixels ( )s T z

                                                            where z is the intensity of a pixel in the original image and s is the intensity of the

                                                            corresponding pixel in the processed image

                                                            Digital Image Processing

                                                            Week 1

                                                            Neighborhood operations

                                                            Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                            in an image f Neighborhood processing generates new intensity level at point (x y)

                                                            based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                            rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                            intensity by computing the average value of the pixels in Sxy

                                                            ( )

                                                            1( ) ( )xyr c S

                                                            g x y f r cm n

                                                            The net effect is to perform local blurring in the original image This type of process is

                                                            used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                            largest region of an image

                                                            Digital Image Processing

                                                            Week 1

                                                            Geometric spatial transformations and image registration

                                                            - modify the spatial relationship between pixels in an image

                                                            - these transformations are often called rubber-sheet transformations (analogous to

                                                            printing an image on a sheet of rubber and then stretching the sheet according to a

                                                            predefined set of rules

                                                            A geometric transformation consists of 2 basic operations

                                                            1 a spatial transformation of coordinates

                                                            2 intensity interpolation that assign intensity values to the spatial transformed

                                                            pixels

                                                            The coordinate system transformation ( ) [( )]x y T v w

                                                            (v w) ndash pixel coordinates in the original image

                                                            (x y) ndash pixel coordinates in the transformed image

                                                            Digital Image Processing

                                                            Week 1

                                                            [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                            Affine transform

                                                            11 1211 21 31

                                                            21 2212 22 33

                                                            31 32

                                                            0[ 1] [ 1] [ 1] 0

                                                            1

                                                            t tx t v t w t

                                                            x y v w T v w t ty t v t w t

                                                            t t

                                                            (AT)

                                                            This transform can scale rotate translate or shear a set of coordinate points depending

                                                            on the elements of the matrix T If we want to resize an image rotate it and move the

                                                            result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                            scaling rotation and translation matrices from Table 1

                                                            Digital Image Processing

                                                            Week 1

                                                            Affine transformations

                                                            Digital Image Processing

                                                            Week 1

                                                            The preceding transformations relocate pixels on an image to new locations To complete

                                                            the process we have to assign intensity values to those locations This task is done by

                                                            using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                            In practice we can use equation (AT) in two basic ways

                                                            forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                            location (x y) of the corresponding pixel in the new image using (AT) directly

                                                            Problems

                                                            - intensity assignment when 2 or more pixels in the original image are transformed to

                                                            the same location in the output image

                                                            - some output locations have no correspondent in the original image (no intensity

                                                            assignment)

                                                            Digital Image Processing

                                                            Week 1

                                                            inverse mapping scans the output pixel locations and at each location (x y)

                                                            computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                            It then interpolates among the nearest input pixels to determine the intensity of the output

                                                            pixel value

                                                            Inverse mappings are more efficient to implement than forward mappings and are used in

                                                            numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                            Digital Image Processing

                                                            Week 1

                                                            Digital Image Processing

                                                            Week 1

                                                            Image registration ndash align two or more images of the same scene

                                                            In image registration we have available the input and output images but the specific

                                                            transformation that produced the output image from the input is generally unknown

                                                            The problem is to estimate the transformation function and then use it to register the two

                                                            images

                                                            - it may be of interest to align (register) two or more image taken at approximately the

                                                            same time but using different imaging systems (MRI scanner and a PET scanner)

                                                            - align images of a given location taken by the same instrument at different moments

                                                            of time (satellite images)

                                                            Solving the problem using tie points (also called control points) which are

                                                            corresponding points whose locations are known precisely in the input and reference

                                                            image

                                                            Digital Image Processing

                                                            Week 1

                                                            How to select tie points

                                                            - interactively selecting them

                                                            - use of algorithms that try to detect these points

                                                            - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                            the imaging sensors These objects produce a set of known points (called reseau

                                                            marks) directly on all images captured by the system which can be used as guides

                                                            for establishing tie points

                                                            The problem of estimating the transformation is one of modeling Suppose we have a set

                                                            of 4 tie points both on the input image and the reference image A simple model based on

                                                            a bilinear approximation is given by

                                                            1 2 3 4

                                                            5 6 7 8

                                                            x c v c w c v w cy c v c w c v w c

                                                            (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                            Digital Image Processing

                                                            Week 1

                                                            When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                            frequently is to select a larger number of tie points and using this new set of tie points

                                                            subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                            subregions marked by 4 tie points we applied the transformation model described above

                                                            The number of tie points and the sophistication of the model required to solve the register

                                                            problem depend on the severity of the geometrical distortion

                                                            Digital Image Processing

                                                            Week 1

                                                            a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                            Digital Image Processing

                                                            Week 1

                                                            Probabilistic Methods

                                                            zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                            p(zk) = the probability that the intensity level zk occurs in the given image

                                                            ( ) kk

                                                            np zM N

                                                            nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                            pixels in the image) 1

                                                            0( ) 1

                                                            L

                                                            kk

                                                            p z

                                                            The mean (average) intensity of an image is given by 1

                                                            0( )

                                                            L

                                                            k kk

                                                            m z p z

                                                            Digital Image Processing

                                                            Week 1

                                                            The variance of the intensities is 1

                                                            2 2

                                                            0( ) ( )

                                                            L

                                                            k kk

                                                            z m p z

                                                            The variance is a measure of the spread of the values of z about the mean so it is a

                                                            measure of image contrast Usually for measuring image contrast the standard deviation

                                                            ( ) is used

                                                            The n-th moment of a random variable z about the mean is defined as 1

                                                            0( ) ( ) ( )

                                                            Ln

                                                            n k kk

                                                            z z m p z

                                                            ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                            3( ) 0z the intensities are biased to values higher than the mean

                                                            ( 3( ) 0z the intensities are biased to values lower than the mean

                                                            Digital Image Processing

                                                            Week 1

                                                            3( ) 0z the intensities are distributed approximately equally on both side of the

                                                            mean

                                                            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                            Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                            Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                            Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                            Digital Image Processing

                                                            Week 1

                                                            Intensity Transformations and Spatial Filtering

                                                            ( ) ( )g x y T f x y

                                                            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                            neighborhood of (x y)

                                                            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                            and much smaller in size than the image

                                                            Digital Image Processing

                                                            Week 1

                                                            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                            called spatial filter (spatial mask kernel template or window)

                                                            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                            ( )s T r

                                                            s and r are denoting respectively the intensity of g and f at (x y)

                                                            Figure 2 left - T produces an output image of higher contrast than the original by

                                                            darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                            is called contrast stretching

                                                            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                            Digital Image Processing

                                                            Week 1

                                                            Figure 2 right - T produces a binary output image A mapping of this form is called

                                                            thresholding function

                                                            Some Basic Intensity Transformation Functions

                                                            Image Negatives

                                                            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                            - equivalent of a photographic negative

                                                            - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                            image

                                                            Digital Image Processing

                                                            Week 1

                                                            Original Negative image

                                                            Digital Image Processing

                                                            Week 1

                                                            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                            Some basic intensity transformation functions

                                                            Digital Image Processing

                                                            Week 1

                                                            This transformation maps a narrow range of low intensity values in the input into a wider

                                                            range An operator of this type is used to expand the values of dark pixels in an image

                                                            while compressing the higher-level values The opposite is true for the inverse log

                                                            transformation The log functions compress the dynamic range of images with large

                                                            variations in pixel values

                                                            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                            Digital Image Processing

                                                            Week 1

                                                            Power-Law (Gamma) Transformations

                                                            - positive constants( ) ( ( ) )s T r c r c s c r

                                                            Plots of gamma transformation for different values of γ (c=1)

                                                            Digital Image Processing

                                                            Week 1

                                                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                            of output values with the opposite being true for higher values of input values The

                                                            curves with 1 have the opposite effect of those generated with values of 1

                                                            1c - identity transformation

                                                            A variety of devices used for image capture printing and display respond according to a

                                                            power law The process used to correct these power-law response phenomena is called

                                                            gamma correction

                                                            Digital Image Processing

                                                            Week 1

                                                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                            Digital Image Processing

                                                            Week 1

                                                            Piecewise-Linear Transformations Functions

                                                            Contrast stretching

                                                            - a process that expands the range of intensity levels in an image so it spans the full

                                                            intensity range of the recording tool or display device

                                                            a b c d Fig5

                                                            Digital Image Processing

                                                            Week 1

                                                            11

                                                            1

                                                            2 1 1 21 2

                                                            2 1 2 1

                                                            22

                                                            2

                                                            [0 ]

                                                            ( ) ( )( ) [ ]( ) ( )

                                                            ( 1 ) [ 1]( 1 )

                                                            s r r rrs r r s r rT r r r r

                                                            r r r rs L r r r L

                                                            L r

                                                            Digital Image Processing

                                                            Week 1

                                                            1 1 2 2r s r s identity transformation (no change)

                                                            1 2 1 2 0 1r r s s L thresholding function

                                                            Figure 5(b) shows an 8-bit image with low contrast

                                                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                            in the image respectively Thus the transformation function stretched the levels linearly

                                                            from their original range to the full range [0 L-1]

                                                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                            2 2 1r s m L where m is the mean gray level in the image

                                                            The original image on which these results are based is a scanning electron microscope

                                                            image of pollen magnified approximately 700 times

                                                            Digital Image Processing

                                                            Week 1

                                                            Intensity-level slicing

                                                            - highlighting a specific range of intensities in an image

                                                            There are two approaches for intensity-level slicing

                                                            1 display in one value (white for example) all the values in the range of interest and in

                                                            another (say black) all other intensities (Figure 311 (a))

                                                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                            intensities in the image (Figure 311 (b))

                                                            Digital Image Processing

                                                            Week 1

                                                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                            the top of the scale of intensities This type of enhancement produces a binary image

                                                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                            Highlights range [A B] and preserves all other intensities

                                                            Digital Image Processing

                                                            Week 1

                                                            which is useful for studying the shape of the flow of the contrast substance (to detect

                                                            blockageshellip)

                                                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                            image around the mean intensity was set to black the other intensities remain unchanged

                                                            Fig 6 - Aortic angiogram and intensity sliced versions

                                                            Digital Image Processing

                                                            Week 1

                                                            Bit-plane slicing

                                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                            This technique highlights the contribution made to the whole image appearances by each

                                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                            Digital Image Processing

                                                            Week 1

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                                                            Week 1

                                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                            • DIP 1 2017
                                                            • DIP 02 (2017)

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Imaging in the Microwave Band

                                                              The dominant aplication of imaging in the microwave band ndash radar

                                                              bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                                              bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                                              bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                                              An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Spaceborne radar image of mountains in southeast Tibet

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Imaging in the Radio Band

                                                              medicine astronomy

                                                              MRI = Magnetic Resonance Imaging

                                                              This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                                              Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                                              The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              MRI images of a human knee (left) and spine (right)

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Images of the Crab Pulsar covering the electromagnetic spectrum

                                                              Gamma X-ray Optical Infrared Radio

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Other Imaging Modalities

                                                              acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                                              Imaging using sound geological explorations industry medicine

                                                              Mineral and oil exploration

                                                              For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                                              Digital Image ProcessingDigital Image Processing

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                                                              Digital Image ProcessingDigital Image Processing

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                                                              Biometry - iris

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Biometry - fingerprint

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Face detection and recognition

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Gender identification

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Image morphing

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Fundamental Steps in DIP

                                                              methods whose input and output are images

                                                              methods whose inputs are images but whose outputs are attributes extracted from those images

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Outputs are images

                                                              bull image acquisition

                                                              bull image filtering and enhancement

                                                              bull image restoration

                                                              bull color image processing

                                                              bull wavelets and multiresolution processing

                                                              bull compression

                                                              bull morphological processing

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Outputs are attributes

                                                              bull morphological processing

                                                              bull segmentation

                                                              bull representation and description

                                                              bull object recognition

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Image acquisition - may involve preprocessing such as scaling

                                                              Image enhancement

                                                              bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                              bull enhancement is problem oriented

                                                              bull there is no general sbquotheoryrsquo of image enhancement

                                                              bull enhancement use subjective methods for image emprovement

                                                              bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Image restoration

                                                              bull improving the appearance of an image

                                                              bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                              Color image processing

                                                              bull fundamental concept in color models

                                                              bull basic color processing in a digital domain

                                                              Wavelets and multiresolution processing

                                                              representing images in various degree of resolution

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Compression

                                                              reducing the storage required to save an image or the bandwidth required to transmit it

                                                              Morphological processing

                                                              bull tools for extracting image components that are useful in the representation and description of shape

                                                              bull a transition from processes that output images to processes that outputimage attributes

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Segmentation

                                                              bull partitioning an image into its constituents parts or objects

                                                              bull autonomous segmentation is one of the most difficult tasks of DIP

                                                              bull the more accurate the segmentation the more likley recognition is to succeed

                                                              Representation and description (almost always follows segmentation)

                                                              bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                              bull converting the data produced by segmentation to a form suitable for computer processing

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                              bull complete region the focus is on internal properties such as texture or skeletal shape

                                                              bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                              Object recognition

                                                              the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                              Knowledge database

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Simplified diagramof a cross sectionof the human eye

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                              The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                              Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                              The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                              The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                              Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                              Fovea = the place where the image of the object of interest falls on

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                              Blind spot region without receptors

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Image formation in the eye

                                                              Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                              Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                              distance between lens and retina along visual axix = 17 mm

                                                              range of focal length = 14 mm to 17 mm

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

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                                                              Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Optical illusions

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                              gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                              quantities that describe the quality of a chromatic light source radiance

                                                              the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                              measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                              brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              the physical meaning is determined by the source of the image

                                                              ( )f D f x y

                                                              Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                              f(xy) ndash characterized by two components

                                                              i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                              r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                              ( ) ( ) ( )

                                                              0 ( ) 0 ( ) 1

                                                              f x y i x y r x y

                                                              i x y r x y

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                              i(xy) ndash determined by the illumination source

                                                              r(xy) ndash determined by the characteristics of the imaged objects

                                                              is called gray (or intensity) scale

                                                              In practice

                                                              min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                              indoor values without additional illuminationmin max10 1000L L

                                                              black whitemin max0 1 0 1 0 1L L L L l l L

                                                              min maxL L

                                                              Digital Image ProcessingDigital Image Processing

                                                              Week 1Week 1

                                                              Digital Image Processing

                                                              Week 1

                                                              Image Sampling and Quantization

                                                              - the output of the sensors is a continuous voltage waveform related to the sensed

                                                              scene

                                                              converting a continuous image f to digital form

                                                              - digitizing (x y) is called sampling

                                                              - digitizing f(x y) is called quantization

                                                              Digital Image Processing

                                                              Week 1

                                                              Digital Image Processing

                                                              Week 1

                                                              Continuous image projected onto a sensor array Result of image sampling and quantization

                                                              Digital Image Processing

                                                              Week 1

                                                              Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                              (00) (01) (0 1)(10) (11) (1 1)

                                                              ( )

                                                              ( 10) ( 11) ( 1 1)

                                                              f f f Nf f f N

                                                              f x y

                                                              f M f M f M N

                                                              image element pixel

                                                              00 01 0 1

                                                              10 11 1 1

                                                              10 11 1 1

                                                              ( ) ( )

                                                              N

                                                              i jN M N

                                                              i j

                                                              M M M N

                                                              a a aa f x i y j f i ja a a

                                                              Aa

                                                              a a a

                                                              f(00) ndash the upper left corner of the image

                                                              Digital Image Processing

                                                              Week 1

                                                              M N ge 0 L=2k

                                                              [0 1]i j i ja a L

                                                              Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                              Digital Image Processing

                                                              Week 1

                                                              Digital Image Processing

                                                              Week 1

                                                              Number of bits required to store a digitized image

                                                              for 2 b M N k M N b N k

                                                              When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                              Digital Image Processing

                                                              Week 1

                                                              Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                              Measures line pairs per unit distance dots (pixels) per unit distance

                                                              Image resolution = the largest number of discernible line pairs per unit distance

                                                              (eg 100 line pairs per mm)

                                                              Dots per unit distance are commonly used in printing and publishing

                                                              In US the measure is expressed in dots per inch (dpi)

                                                              (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                              Intensity resolution ndash the smallest discernible change in intensity level

                                                              The number of intensity levels (L) is determined by hardware considerations

                                                              L=2k ndash most common k = 8

                                                              Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                              Digital Image Processing

                                                              Week 1

                                                              Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                              150 dpi (lower left) 72 dpi (lower right)

                                                              Digital Image Processing

                                                              Week 1

                                                              Reducing the number of gray levels 256 128 64 32

                                                              Digital Image Processing

                                                              Week 1

                                                              Reducing the number of gray levels 16 8 4 2

                                                              Digital Image Processing

                                                              Week 1

                                                              Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                              Shrinking zooming ndash image resizing ndash image resampling methods

                                                              Interpolation is the process of using known data to estimate values at unknown locations

                                                              Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                              750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                              same spacing as the original and then shrink it so that it fits exactly over the original

                                                              image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                              Problem assignment of intensity-level in the new 750 times 750 grid

                                                              Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                              intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                              This technique has the tendency to produce undesirable effects like severe distortion of

                                                              straight edges

                                                              Digital Image Processing

                                                              Week 1

                                                              Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                              where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                              be written using the 4 nearest neighbors of point (x y)

                                                              Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                              modest increase in computational effort

                                                              Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                              nearest neighbors of the point 3 3

                                                              0 0

                                                              ( ) i ji j

                                                              i jv x y c x y

                                                              The coefficients cij are obtained solving a 16x16 linear system

                                                              intensity levels of the 16 nearest neighbors of 3 3

                                                              0 0

                                                              ( )i ji j

                                                              i jc x y x y

                                                              Digital Image Processing

                                                              Week 1

                                                              Generally bicubic interpolation does a better job of preserving fine detail than the

                                                              bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                              programs such as Adobe Photoshop and Corel Photopaint

                                                              Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                              the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                              then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                              neighbor interpolation was used (both for shrinking and zooming)

                                                              Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                              interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                              from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                              Digital Image Processing

                                                              Week 1

                                                              Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                              Digital Image Processing

                                                              Week 1

                                                              Neighbors of a Pixel

                                                              A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                              This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                              The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                              and are denoted ND(p)

                                                              The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                              N8 (p)

                                                              If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                              fall outside the image

                                                              Digital Image Processing

                                                              Week 1

                                                              Adjacency Connectivity Regions Boundaries

                                                              Denote by V the set of intensity levels used to define adjacency

                                                              - in a binary image V 01 (V=0 V=1)

                                                              - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                              We consider 3 types of adjacency

                                                              (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                              (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                              (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                              m-adjacent if

                                                              4( )q N p or

                                                              ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                              Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                              ambiguities that often arise when 8-adjacency is used Consider the example

                                                              Digital Image Processing

                                                              Week 1

                                                              binary image

                                                              0 1 1 0 1 1 0 1 1

                                                              1 0 1 0 0 1 0 0 1 0

                                                              0 0 1 0 0 1 0 0 1

                                                              V

                                                              The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                              8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                              m-adjacency

                                                              Digital Image Processing

                                                              Week 1

                                                              A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                              is a sequence of distinct pixels with coordinates

                                                              and are adjacent 0 0 1 1

                                                              1 1

                                                              ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                              n n

                                                              i i i i

                                                              x y x y x y x y s tx y x y i n

                                                              The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                              Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                              Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                              in S if there exists a path between them consisting only of pixels from S

                                                              S is a connected set if there is a path in S between any 2 pixels in S

                                                              Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                              Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                              that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                              8-adjacency are considered

                                                              Digital Image Processing

                                                              Week 1

                                                              Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                              touches the image border

                                                              the complement of 1

                                                              ( )K

                                                              cu k u u

                                                              k

                                                              R R R R

                                                              We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                              background of the image

                                                              The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                              points in the complement of R (R)c The border of an image is the set of pixels in the

                                                              region that have at least one background neighbor This definition is referred to as the

                                                              inner border to distinguish it from the notion of outer border which is the corresponding

                                                              border in the background

                                                              Digital Image Processing

                                                              Week 1

                                                              Distance measures

                                                              For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                              function or metric if

                                                              (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                              (b) D(p q) = D(q p)

                                                              (c) D(p z) le D(p q) + D(q z)

                                                              The Euclidean distance between p and q is defined as 1

                                                              2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                              The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                              centered at (x y)

                                                              Digital Image Processing

                                                              Week 1

                                                              The D4 distance (also called city-block distance) between p and q is defined as

                                                              4( ) | | | |D p q x s y t

                                                              The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                              4

                                                              22 1 2

                                                              2 2 1 0 1 22 1 2

                                                              2

                                                              D

                                                              The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                              The D8 distance (called the chessboard distance) between p and q is defined as

                                                              8( ) max| | | |D p q x s y t

                                                              The pixels q for which 8( )D p q r form a square centered at (x y)

                                                              Digital Image Processing

                                                              Week 1

                                                              8

                                                              2 2 2 2 22 1 1 1 2

                                                              2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                              D

                                                              The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                              D4 and D8 distances are independent of any paths that might exist between p and q

                                                              because these distances involve only the coordinates of the point

                                                              Digital Image Processing

                                                              Week 1

                                                              Array versus Matrix Operations

                                                              An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                              11 12 11 12

                                                              21 22 21 22

                                                              a a b ba a b b

                                                              Array product

                                                              11 12 11 12 11 11 12 12

                                                              21 22 21 22 21 21 22 21

                                                              a a b b a b a ba a b b a b a b

                                                              Matrix product

                                                              11 12 11 12 11 11 12 21 11 12 12 21

                                                              21 22 21 22 21 11 22 21 21 12 22 22

                                                              a a b b a b a b a b a ba a b b a b a b a b a b

                                                              We assume array operations unless stated otherwise

                                                              Digital Image Processing

                                                              Week 1

                                                              Linear versus Nonlinear Operations

                                                              One of the most important classifications of image-processing methods is whether it is

                                                              linear or nonlinear

                                                              ( ) ( )H f x y g x y

                                                              H is said to be a linear operator if

                                                              images1 2 1 2

                                                              1 2

                                                              ( ) ( ) ( ) ( )

                                                              H a f x y b f x y a H f x y b H f x y

                                                              a b f f

                                                              Example of nonlinear operator

                                                              the maximum value of the pixels of image max ( )H f f x y f

                                                              1 2

                                                              0 2 6 5 1 1

                                                              2 3 4 7f f a b

                                                              Digital Image Processing

                                                              Week 1

                                                              1 2

                                                              0 2 6 5 6 3max max 1 ( 1) max 2

                                                              2 3 4 7 2 4a f b f

                                                              0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                              2 3 4 7

                                                              Arithmetic Operations in Image Processing

                                                              Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                              f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                              For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                              value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                              The two random variables are uncorrelated when their covariance is 0

                                                              Digital Image Processing

                                                              Week 1

                                                              Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                              used in image enhancement)

                                                              1

                                                              1( ) ( )K

                                                              ii

                                                              g x y g x yK

                                                              If the noise satisfies the properties stated above we have

                                                              2 2( ) ( )

                                                              1( ) ( ) g x y x yE g x y f x yK

                                                              ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                              and g respectively The standard deviation (square root of the variance) at any point in

                                                              the average image is

                                                              ( ) ( )1

                                                              g x y x yK

                                                              Digital Image Processing

                                                              Week 1

                                                              As K increases the variability (as measured by the variance or the standard deviation) of

                                                              the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                              means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                              averaging process increases

                                                              An important application of image averaging is in the field of astronomy where imaging

                                                              under very low light levels frequently causes sensor noise to render single images

                                                              virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                              was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                              64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                              images respectively

                                                              Digital Image Processing

                                                              Week 1

                                                              Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                              100 noisy images

                                                              a b c d e f

                                                              Digital Image Processing

                                                              Week 1

                                                              A frequent application of image subtraction is in the enhancement of differences between

                                                              images

                                                              (a) (b) (c)

                                                              Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                              significant bit of each pixel (c) the difference between the two images

                                                              Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                              Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                              images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                              difference between images (a) and (b)

                                                              Digital Image Processing

                                                              Week 1

                                                              Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                              h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                              intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                              source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                              bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                              anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                              images after injection of the contrast medium

                                                              In g(x y) we can find the differences between h and f as enhanced detail

                                                              Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                              propagates through the various arteries in the area being observed

                                                              Digital Image Processing

                                                              Week 1

                                                              a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                              Digital Image Processing

                                                              Week 1

                                                              An important application of image multiplication (and division) is shading correction

                                                              Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                              f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                              When the shading function is known

                                                              ( )( )( )

                                                              g x yf x yh x y

                                                              h(x y) is unknown but we have access to the imaging system we can obtain an

                                                              approximation to the shading function by imaging a target of constant intensity When the

                                                              sensor is not available often the shading pattern can be estimated from the image

                                                              Digital Image Processing

                                                              Week 1

                                                              (a) (b) (c)

                                                              Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                              Digital Image Processing

                                                              Week 1

                                                              Another use of image multiplication is in masking also called region of interest (ROI)

                                                              operations The process consists of multiplying a given image by a mask image that has

                                                              1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                              image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                              (a) (b) (c)

                                                              Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                              Digital Image Processing

                                                              Week 1

                                                              In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                              min( )mf f f

                                                              0 ( 255)max( )

                                                              ms

                                                              m

                                                              ff K K K

                                                              f

                                                              Digital Image Processing

                                                              Week 1

                                                              Spatial Operations

                                                              - are performed directly on the pixels of a given image

                                                              There are three categories of spatial operations

                                                              single-pixel operations

                                                              neighborhood operations

                                                              geometric spatial transformations

                                                              Single-pixel operations

                                                              - change the values of intensity for the individual pixels ( )s T z

                                                              where z is the intensity of a pixel in the original image and s is the intensity of the

                                                              corresponding pixel in the processed image

                                                              Digital Image Processing

                                                              Week 1

                                                              Neighborhood operations

                                                              Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                              in an image f Neighborhood processing generates new intensity level at point (x y)

                                                              based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                              rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                              intensity by computing the average value of the pixels in Sxy

                                                              ( )

                                                              1( ) ( )xyr c S

                                                              g x y f r cm n

                                                              The net effect is to perform local blurring in the original image This type of process is

                                                              used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                              largest region of an image

                                                              Digital Image Processing

                                                              Week 1

                                                              Geometric spatial transformations and image registration

                                                              - modify the spatial relationship between pixels in an image

                                                              - these transformations are often called rubber-sheet transformations (analogous to

                                                              printing an image on a sheet of rubber and then stretching the sheet according to a

                                                              predefined set of rules

                                                              A geometric transformation consists of 2 basic operations

                                                              1 a spatial transformation of coordinates

                                                              2 intensity interpolation that assign intensity values to the spatial transformed

                                                              pixels

                                                              The coordinate system transformation ( ) [( )]x y T v w

                                                              (v w) ndash pixel coordinates in the original image

                                                              (x y) ndash pixel coordinates in the transformed image

                                                              Digital Image Processing

                                                              Week 1

                                                              [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                              Affine transform

                                                              11 1211 21 31

                                                              21 2212 22 33

                                                              31 32

                                                              0[ 1] [ 1] [ 1] 0

                                                              1

                                                              t tx t v t w t

                                                              x y v w T v w t ty t v t w t

                                                              t t

                                                              (AT)

                                                              This transform can scale rotate translate or shear a set of coordinate points depending

                                                              on the elements of the matrix T If we want to resize an image rotate it and move the

                                                              result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                              scaling rotation and translation matrices from Table 1

                                                              Digital Image Processing

                                                              Week 1

                                                              Affine transformations

                                                              Digital Image Processing

                                                              Week 1

                                                              The preceding transformations relocate pixels on an image to new locations To complete

                                                              the process we have to assign intensity values to those locations This task is done by

                                                              using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                              In practice we can use equation (AT) in two basic ways

                                                              forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                              location (x y) of the corresponding pixel in the new image using (AT) directly

                                                              Problems

                                                              - intensity assignment when 2 or more pixels in the original image are transformed to

                                                              the same location in the output image

                                                              - some output locations have no correspondent in the original image (no intensity

                                                              assignment)

                                                              Digital Image Processing

                                                              Week 1

                                                              inverse mapping scans the output pixel locations and at each location (x y)

                                                              computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                              It then interpolates among the nearest input pixels to determine the intensity of the output

                                                              pixel value

                                                              Inverse mappings are more efficient to implement than forward mappings and are used in

                                                              numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                              Digital Image Processing

                                                              Week 1

                                                              Digital Image Processing

                                                              Week 1

                                                              Image registration ndash align two or more images of the same scene

                                                              In image registration we have available the input and output images but the specific

                                                              transformation that produced the output image from the input is generally unknown

                                                              The problem is to estimate the transformation function and then use it to register the two

                                                              images

                                                              - it may be of interest to align (register) two or more image taken at approximately the

                                                              same time but using different imaging systems (MRI scanner and a PET scanner)

                                                              - align images of a given location taken by the same instrument at different moments

                                                              of time (satellite images)

                                                              Solving the problem using tie points (also called control points) which are

                                                              corresponding points whose locations are known precisely in the input and reference

                                                              image

                                                              Digital Image Processing

                                                              Week 1

                                                              How to select tie points

                                                              - interactively selecting them

                                                              - use of algorithms that try to detect these points

                                                              - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                              the imaging sensors These objects produce a set of known points (called reseau

                                                              marks) directly on all images captured by the system which can be used as guides

                                                              for establishing tie points

                                                              The problem of estimating the transformation is one of modeling Suppose we have a set

                                                              of 4 tie points both on the input image and the reference image A simple model based on

                                                              a bilinear approximation is given by

                                                              1 2 3 4

                                                              5 6 7 8

                                                              x c v c w c v w cy c v c w c v w c

                                                              (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                              Digital Image Processing

                                                              Week 1

                                                              When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                              frequently is to select a larger number of tie points and using this new set of tie points

                                                              subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                              subregions marked by 4 tie points we applied the transformation model described above

                                                              The number of tie points and the sophistication of the model required to solve the register

                                                              problem depend on the severity of the geometrical distortion

                                                              Digital Image Processing

                                                              Week 1

                                                              a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                              Digital Image Processing

                                                              Week 1

                                                              Probabilistic Methods

                                                              zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                              p(zk) = the probability that the intensity level zk occurs in the given image

                                                              ( ) kk

                                                              np zM N

                                                              nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                              pixels in the image) 1

                                                              0( ) 1

                                                              L

                                                              kk

                                                              p z

                                                              The mean (average) intensity of an image is given by 1

                                                              0( )

                                                              L

                                                              k kk

                                                              m z p z

                                                              Digital Image Processing

                                                              Week 1

                                                              The variance of the intensities is 1

                                                              2 2

                                                              0( ) ( )

                                                              L

                                                              k kk

                                                              z m p z

                                                              The variance is a measure of the spread of the values of z about the mean so it is a

                                                              measure of image contrast Usually for measuring image contrast the standard deviation

                                                              ( ) is used

                                                              The n-th moment of a random variable z about the mean is defined as 1

                                                              0( ) ( ) ( )

                                                              Ln

                                                              n k kk

                                                              z z m p z

                                                              ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                              3( ) 0z the intensities are biased to values higher than the mean

                                                              ( 3( ) 0z the intensities are biased to values lower than the mean

                                                              Digital Image Processing

                                                              Week 1

                                                              3( ) 0z the intensities are distributed approximately equally on both side of the

                                                              mean

                                                              Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                              Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                              Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                              Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                              Digital Image Processing

                                                              Week 1

                                                              Intensity Transformations and Spatial Filtering

                                                              ( ) ( )g x y T f x y

                                                              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                              neighborhood of (x y)

                                                              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                              and much smaller in size than the image

                                                              Digital Image Processing

                                                              Week 1

                                                              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                              called spatial filter (spatial mask kernel template or window)

                                                              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                              ( )s T r

                                                              s and r are denoting respectively the intensity of g and f at (x y)

                                                              Figure 2 left - T produces an output image of higher contrast than the original by

                                                              darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                              is called contrast stretching

                                                              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                              Digital Image Processing

                                                              Week 1

                                                              Figure 2 right - T produces a binary output image A mapping of this form is called

                                                              thresholding function

                                                              Some Basic Intensity Transformation Functions

                                                              Image Negatives

                                                              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                              - equivalent of a photographic negative

                                                              - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                              image

                                                              Digital Image Processing

                                                              Week 1

                                                              Original Negative image

                                                              Digital Image Processing

                                                              Week 1

                                                              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                              Some basic intensity transformation functions

                                                              Digital Image Processing

                                                              Week 1

                                                              This transformation maps a narrow range of low intensity values in the input into a wider

                                                              range An operator of this type is used to expand the values of dark pixels in an image

                                                              while compressing the higher-level values The opposite is true for the inverse log

                                                              transformation The log functions compress the dynamic range of images with large

                                                              variations in pixel values

                                                              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                              Digital Image Processing

                                                              Week 1

                                                              Power-Law (Gamma) Transformations

                                                              - positive constants( ) ( ( ) )s T r c r c s c r

                                                              Plots of gamma transformation for different values of γ (c=1)

                                                              Digital Image Processing

                                                              Week 1

                                                              Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                              of output values with the opposite being true for higher values of input values The

                                                              curves with 1 have the opposite effect of those generated with values of 1

                                                              1c - identity transformation

                                                              A variety of devices used for image capture printing and display respond according to a

                                                              power law The process used to correct these power-law response phenomena is called

                                                              gamma correction

                                                              Digital Image Processing

                                                              Week 1

                                                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                              Digital Image Processing

                                                              Week 1

                                                              Piecewise-Linear Transformations Functions

                                                              Contrast stretching

                                                              - a process that expands the range of intensity levels in an image so it spans the full

                                                              intensity range of the recording tool or display device

                                                              a b c d Fig5

                                                              Digital Image Processing

                                                              Week 1

                                                              11

                                                              1

                                                              2 1 1 21 2

                                                              2 1 2 1

                                                              22

                                                              2

                                                              [0 ]

                                                              ( ) ( )( ) [ ]( ) ( )

                                                              ( 1 ) [ 1]( 1 )

                                                              s r r rrs r r s r rT r r r r

                                                              r r r rs L r r r L

                                                              L r

                                                              Digital Image Processing

                                                              Week 1

                                                              1 1 2 2r s r s identity transformation (no change)

                                                              1 2 1 2 0 1r r s s L thresholding function

                                                              Figure 5(b) shows an 8-bit image with low contrast

                                                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                              in the image respectively Thus the transformation function stretched the levels linearly

                                                              from their original range to the full range [0 L-1]

                                                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                              2 2 1r s m L where m is the mean gray level in the image

                                                              The original image on which these results are based is a scanning electron microscope

                                                              image of pollen magnified approximately 700 times

                                                              Digital Image Processing

                                                              Week 1

                                                              Intensity-level slicing

                                                              - highlighting a specific range of intensities in an image

                                                              There are two approaches for intensity-level slicing

                                                              1 display in one value (white for example) all the values in the range of interest and in

                                                              another (say black) all other intensities (Figure 311 (a))

                                                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                              intensities in the image (Figure 311 (b))

                                                              Digital Image Processing

                                                              Week 1

                                                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                              the top of the scale of intensities This type of enhancement produces a binary image

                                                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                              Highlights range [A B] and preserves all other intensities

                                                              Digital Image Processing

                                                              Week 1

                                                              which is useful for studying the shape of the flow of the contrast substance (to detect

                                                              blockageshellip)

                                                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                              image around the mean intensity was set to black the other intensities remain unchanged

                                                              Fig 6 - Aortic angiogram and intensity sliced versions

                                                              Digital Image Processing

                                                              Week 1

                                                              Bit-plane slicing

                                                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                              This technique highlights the contribution made to the whole image appearances by each

                                                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                              Digital Image Processing

                                                              Week 1

                                                              Digital Image Processing

                                                              Week 1

                                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                              • DIP 1 2017
                                                              • DIP 02 (2017)

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Imaging in the Microwave Band

                                                                The dominant aplication of imaging in the microwave band ndash radar

                                                                bull radar has the ability to collect data over virtually any region at any time regardless of weather or ambient light conditions

                                                                bull some radar waves can penetrate clouds under certain conditions canpenetrate vegetation ice dry sand

                                                                bull sometimes radar is the only way to explore inaccessible regions of theEarthrsquos surface

                                                                An imaging radar works like a flash camera it provides its own illumination(microwave pulses) to light an area on the ground and take a snapshot image Instead of a camera lens a radar uses an antenna and a digital device torecord the images In a radar image one can see only the microwave energythat was reflected back toward the radar antenna

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Spaceborne radar image of mountains in southeast Tibet

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Imaging in the Radio Band

                                                                medicine astronomy

                                                                MRI = Magnetic Resonance Imaging

                                                                This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                                                Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                                                The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                MRI images of a human knee (left) and spine (right)

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Images of the Crab Pulsar covering the electromagnetic spectrum

                                                                Gamma X-ray Optical Infrared Radio

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Other Imaging Modalities

                                                                acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                                                Imaging using sound geological explorations industry medicine

                                                                Mineral and oil exploration

                                                                For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Biometry - iris

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Biometry - fingerprint

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Face detection and recognition

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Gender identification

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Image morphing

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Fundamental Steps in DIP

                                                                methods whose input and output are images

                                                                methods whose inputs are images but whose outputs are attributes extracted from those images

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Outputs are images

                                                                bull image acquisition

                                                                bull image filtering and enhancement

                                                                bull image restoration

                                                                bull color image processing

                                                                bull wavelets and multiresolution processing

                                                                bull compression

                                                                bull morphological processing

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Outputs are attributes

                                                                bull morphological processing

                                                                bull segmentation

                                                                bull representation and description

                                                                bull object recognition

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Image acquisition - may involve preprocessing such as scaling

                                                                Image enhancement

                                                                bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                bull enhancement is problem oriented

                                                                bull there is no general sbquotheoryrsquo of image enhancement

                                                                bull enhancement use subjective methods for image emprovement

                                                                bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Image restoration

                                                                bull improving the appearance of an image

                                                                bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                Color image processing

                                                                bull fundamental concept in color models

                                                                bull basic color processing in a digital domain

                                                                Wavelets and multiresolution processing

                                                                representing images in various degree of resolution

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Compression

                                                                reducing the storage required to save an image or the bandwidth required to transmit it

                                                                Morphological processing

                                                                bull tools for extracting image components that are useful in the representation and description of shape

                                                                bull a transition from processes that output images to processes that outputimage attributes

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                                                                Segmentation

                                                                bull partitioning an image into its constituents parts or objects

                                                                bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                bull the more accurate the segmentation the more likley recognition is to succeed

                                                                Representation and description (almost always follows segmentation)

                                                                bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                bull converting the data produced by segmentation to a form suitable for computer processing

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                                                                Week 1Week 1

                                                                bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                Object recognition

                                                                the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                Knowledge database

                                                                Digital Image ProcessingDigital Image Processing

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                                                                Simplified diagramof a cross sectionof the human eye

                                                                Digital Image ProcessingDigital Image Processing

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                                                                Digital Image ProcessingDigital Image Processing

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                                                                Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                Fovea = the place where the image of the object of interest falls on

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                Blind spot region without receptors

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Image formation in the eye

                                                                Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                distance between lens and retina along visual axix = 17 mm

                                                                range of focal length = 14 mm to 17 mm

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                                                                Week 1Week 1

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                                                                Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Optical illusions

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                Digital Image ProcessingDigital Image Processing

                                                                Week 1Week 1

                                                                Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                quantities that describe the quality of a chromatic light source radiance

                                                                the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

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                                                                Week 1Week 1

                                                                For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

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                                                                Week 1Week 1

                                                                the physical meaning is determined by the source of the image

                                                                ( )f D f x y

                                                                Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                f(xy) ndash characterized by two components

                                                                i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                ( ) ( ) ( )

                                                                0 ( ) 0 ( ) 1

                                                                f x y i x y r x y

                                                                i x y r x y

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                                                                Week 1Week 1

                                                                r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                i(xy) ndash determined by the illumination source

                                                                r(xy) ndash determined by the characteristics of the imaged objects

                                                                is called gray (or intensity) scale

                                                                In practice

                                                                min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                indoor values without additional illuminationmin max10 1000L L

                                                                black whitemin max0 1 0 1 0 1L L L L l l L

                                                                min maxL L

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                                                                Week 1Week 1

                                                                Digital Image Processing

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                                                                Image Sampling and Quantization

                                                                - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                scene

                                                                converting a continuous image f to digital form

                                                                - digitizing (x y) is called sampling

                                                                - digitizing f(x y) is called quantization

                                                                Digital Image Processing

                                                                Week 1

                                                                Digital Image Processing

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                                                                Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                Digital Image Processing

                                                                Week 1

                                                                Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                (00) (01) (0 1)(10) (11) (1 1)

                                                                ( )

                                                                ( 10) ( 11) ( 1 1)

                                                                f f f Nf f f N

                                                                f x y

                                                                f M f M f M N

                                                                image element pixel

                                                                00 01 0 1

                                                                10 11 1 1

                                                                10 11 1 1

                                                                ( ) ( )

                                                                N

                                                                i jN M N

                                                                i j

                                                                M M M N

                                                                a a aa f x i y j f i ja a a

                                                                Aa

                                                                a a a

                                                                f(00) ndash the upper left corner of the image

                                                                Digital Image Processing

                                                                Week 1

                                                                M N ge 0 L=2k

                                                                [0 1]i j i ja a L

                                                                Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                Digital Image Processing

                                                                Week 1

                                                                Digital Image Processing

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                                                                Number of bits required to store a digitized image

                                                                for 2 b M N k M N b N k

                                                                When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                Digital Image Processing

                                                                Week 1

                                                                Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                Measures line pairs per unit distance dots (pixels) per unit distance

                                                                Image resolution = the largest number of discernible line pairs per unit distance

                                                                (eg 100 line pairs per mm)

                                                                Dots per unit distance are commonly used in printing and publishing

                                                                In US the measure is expressed in dots per inch (dpi)

                                                                (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                Intensity resolution ndash the smallest discernible change in intensity level

                                                                The number of intensity levels (L) is determined by hardware considerations

                                                                L=2k ndash most common k = 8

                                                                Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                Digital Image Processing

                                                                Week 1

                                                                Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                150 dpi (lower left) 72 dpi (lower right)

                                                                Digital Image Processing

                                                                Week 1

                                                                Reducing the number of gray levels 256 128 64 32

                                                                Digital Image Processing

                                                                Week 1

                                                                Reducing the number of gray levels 16 8 4 2

                                                                Digital Image Processing

                                                                Week 1

                                                                Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                Shrinking zooming ndash image resizing ndash image resampling methods

                                                                Interpolation is the process of using known data to estimate values at unknown locations

                                                                Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                same spacing as the original and then shrink it so that it fits exactly over the original

                                                                image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                Problem assignment of intensity-level in the new 750 times 750 grid

                                                                Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                This technique has the tendency to produce undesirable effects like severe distortion of

                                                                straight edges

                                                                Digital Image Processing

                                                                Week 1

                                                                Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                be written using the 4 nearest neighbors of point (x y)

                                                                Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                modest increase in computational effort

                                                                Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                nearest neighbors of the point 3 3

                                                                0 0

                                                                ( ) i ji j

                                                                i jv x y c x y

                                                                The coefficients cij are obtained solving a 16x16 linear system

                                                                intensity levels of the 16 nearest neighbors of 3 3

                                                                0 0

                                                                ( )i ji j

                                                                i jc x y x y

                                                                Digital Image Processing

                                                                Week 1

                                                                Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                programs such as Adobe Photoshop and Corel Photopaint

                                                                Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                neighbor interpolation was used (both for shrinking and zooming)

                                                                Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                Digital Image Processing

                                                                Week 1

                                                                Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                Digital Image Processing

                                                                Week 1

                                                                Neighbors of a Pixel

                                                                A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                and are denoted ND(p)

                                                                The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                N8 (p)

                                                                If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                fall outside the image

                                                                Digital Image Processing

                                                                Week 1

                                                                Adjacency Connectivity Regions Boundaries

                                                                Denote by V the set of intensity levels used to define adjacency

                                                                - in a binary image V 01 (V=0 V=1)

                                                                - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                We consider 3 types of adjacency

                                                                (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                m-adjacent if

                                                                4( )q N p or

                                                                ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                ambiguities that often arise when 8-adjacency is used Consider the example

                                                                Digital Image Processing

                                                                Week 1

                                                                binary image

                                                                0 1 1 0 1 1 0 1 1

                                                                1 0 1 0 0 1 0 0 1 0

                                                                0 0 1 0 0 1 0 0 1

                                                                V

                                                                The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                m-adjacency

                                                                Digital Image Processing

                                                                Week 1

                                                                A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                is a sequence of distinct pixels with coordinates

                                                                and are adjacent 0 0 1 1

                                                                1 1

                                                                ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                n n

                                                                i i i i

                                                                x y x y x y x y s tx y x y i n

                                                                The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                in S if there exists a path between them consisting only of pixels from S

                                                                S is a connected set if there is a path in S between any 2 pixels in S

                                                                Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                8-adjacency are considered

                                                                Digital Image Processing

                                                                Week 1

                                                                Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                touches the image border

                                                                the complement of 1

                                                                ( )K

                                                                cu k u u

                                                                k

                                                                R R R R

                                                                We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                background of the image

                                                                The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                region that have at least one background neighbor This definition is referred to as the

                                                                inner border to distinguish it from the notion of outer border which is the corresponding

                                                                border in the background

                                                                Digital Image Processing

                                                                Week 1

                                                                Distance measures

                                                                For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                function or metric if

                                                                (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                (b) D(p q) = D(q p)

                                                                (c) D(p z) le D(p q) + D(q z)

                                                                The Euclidean distance between p and q is defined as 1

                                                                2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                centered at (x y)

                                                                Digital Image Processing

                                                                Week 1

                                                                The D4 distance (also called city-block distance) between p and q is defined as

                                                                4( ) | | | |D p q x s y t

                                                                The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                4

                                                                22 1 2

                                                                2 2 1 0 1 22 1 2

                                                                2

                                                                D

                                                                The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                The D8 distance (called the chessboard distance) between p and q is defined as

                                                                8( ) max| | | |D p q x s y t

                                                                The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                Digital Image Processing

                                                                Week 1

                                                                8

                                                                2 2 2 2 22 1 1 1 2

                                                                2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                D

                                                                The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                D4 and D8 distances are independent of any paths that might exist between p and q

                                                                because these distances involve only the coordinates of the point

                                                                Digital Image Processing

                                                                Week 1

                                                                Array versus Matrix Operations

                                                                An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                11 12 11 12

                                                                21 22 21 22

                                                                a a b ba a b b

                                                                Array product

                                                                11 12 11 12 11 11 12 12

                                                                21 22 21 22 21 21 22 21

                                                                a a b b a b a ba a b b a b a b

                                                                Matrix product

                                                                11 12 11 12 11 11 12 21 11 12 12 21

                                                                21 22 21 22 21 11 22 21 21 12 22 22

                                                                a a b b a b a b a b a ba a b b a b a b a b a b

                                                                We assume array operations unless stated otherwise

                                                                Digital Image Processing

                                                                Week 1

                                                                Linear versus Nonlinear Operations

                                                                One of the most important classifications of image-processing methods is whether it is

                                                                linear or nonlinear

                                                                ( ) ( )H f x y g x y

                                                                H is said to be a linear operator if

                                                                images1 2 1 2

                                                                1 2

                                                                ( ) ( ) ( ) ( )

                                                                H a f x y b f x y a H f x y b H f x y

                                                                a b f f

                                                                Example of nonlinear operator

                                                                the maximum value of the pixels of image max ( )H f f x y f

                                                                1 2

                                                                0 2 6 5 1 1

                                                                2 3 4 7f f a b

                                                                Digital Image Processing

                                                                Week 1

                                                                1 2

                                                                0 2 6 5 6 3max max 1 ( 1) max 2

                                                                2 3 4 7 2 4a f b f

                                                                0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                2 3 4 7

                                                                Arithmetic Operations in Image Processing

                                                                Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                The two random variables are uncorrelated when their covariance is 0

                                                                Digital Image Processing

                                                                Week 1

                                                                Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                used in image enhancement)

                                                                1

                                                                1( ) ( )K

                                                                ii

                                                                g x y g x yK

                                                                If the noise satisfies the properties stated above we have

                                                                2 2( ) ( )

                                                                1( ) ( ) g x y x yE g x y f x yK

                                                                ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                and g respectively The standard deviation (square root of the variance) at any point in

                                                                the average image is

                                                                ( ) ( )1

                                                                g x y x yK

                                                                Digital Image Processing

                                                                Week 1

                                                                As K increases the variability (as measured by the variance or the standard deviation) of

                                                                the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                averaging process increases

                                                                An important application of image averaging is in the field of astronomy where imaging

                                                                under very low light levels frequently causes sensor noise to render single images

                                                                virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                images respectively

                                                                Digital Image Processing

                                                                Week 1

                                                                Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                100 noisy images

                                                                a b c d e f

                                                                Digital Image Processing

                                                                Week 1

                                                                A frequent application of image subtraction is in the enhancement of differences between

                                                                images

                                                                (a) (b) (c)

                                                                Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                significant bit of each pixel (c) the difference between the two images

                                                                Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                difference between images (a) and (b)

                                                                Digital Image Processing

                                                                Week 1

                                                                Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                images after injection of the contrast medium

                                                                In g(x y) we can find the differences between h and f as enhanced detail

                                                                Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                propagates through the various arteries in the area being observed

                                                                Digital Image Processing

                                                                Week 1

                                                                a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                Digital Image Processing

                                                                Week 1

                                                                An important application of image multiplication (and division) is shading correction

                                                                Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                When the shading function is known

                                                                ( )( )( )

                                                                g x yf x yh x y

                                                                h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                approximation to the shading function by imaging a target of constant intensity When the

                                                                sensor is not available often the shading pattern can be estimated from the image

                                                                Digital Image Processing

                                                                Week 1

                                                                (a) (b) (c)

                                                                Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                Digital Image Processing

                                                                Week 1

                                                                Another use of image multiplication is in masking also called region of interest (ROI)

                                                                operations The process consists of multiplying a given image by a mask image that has

                                                                1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                (a) (b) (c)

                                                                Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                Digital Image Processing

                                                                Week 1

                                                                In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                min( )mf f f

                                                                0 ( 255)max( )

                                                                ms

                                                                m

                                                                ff K K K

                                                                f

                                                                Digital Image Processing

                                                                Week 1

                                                                Spatial Operations

                                                                - are performed directly on the pixels of a given image

                                                                There are three categories of spatial operations

                                                                single-pixel operations

                                                                neighborhood operations

                                                                geometric spatial transformations

                                                                Single-pixel operations

                                                                - change the values of intensity for the individual pixels ( )s T z

                                                                where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                corresponding pixel in the processed image

                                                                Digital Image Processing

                                                                Week 1

                                                                Neighborhood operations

                                                                Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                intensity by computing the average value of the pixels in Sxy

                                                                ( )

                                                                1( ) ( )xyr c S

                                                                g x y f r cm n

                                                                The net effect is to perform local blurring in the original image This type of process is

                                                                used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                largest region of an image

                                                                Digital Image Processing

                                                                Week 1

                                                                Geometric spatial transformations and image registration

                                                                - modify the spatial relationship between pixels in an image

                                                                - these transformations are often called rubber-sheet transformations (analogous to

                                                                printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                predefined set of rules

                                                                A geometric transformation consists of 2 basic operations

                                                                1 a spatial transformation of coordinates

                                                                2 intensity interpolation that assign intensity values to the spatial transformed

                                                                pixels

                                                                The coordinate system transformation ( ) [( )]x y T v w

                                                                (v w) ndash pixel coordinates in the original image

                                                                (x y) ndash pixel coordinates in the transformed image

                                                                Digital Image Processing

                                                                Week 1

                                                                [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                Affine transform

                                                                11 1211 21 31

                                                                21 2212 22 33

                                                                31 32

                                                                0[ 1] [ 1] [ 1] 0

                                                                1

                                                                t tx t v t w t

                                                                x y v w T v w t ty t v t w t

                                                                t t

                                                                (AT)

                                                                This transform can scale rotate translate or shear a set of coordinate points depending

                                                                on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                scaling rotation and translation matrices from Table 1

                                                                Digital Image Processing

                                                                Week 1

                                                                Affine transformations

                                                                Digital Image Processing

                                                                Week 1

                                                                The preceding transformations relocate pixels on an image to new locations To complete

                                                                the process we have to assign intensity values to those locations This task is done by

                                                                using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                In practice we can use equation (AT) in two basic ways

                                                                forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                Problems

                                                                - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                the same location in the output image

                                                                - some output locations have no correspondent in the original image (no intensity

                                                                assignment)

                                                                Digital Image Processing

                                                                Week 1

                                                                inverse mapping scans the output pixel locations and at each location (x y)

                                                                computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                pixel value

                                                                Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                Digital Image Processing

                                                                Week 1

                                                                Digital Image Processing

                                                                Week 1

                                                                Image registration ndash align two or more images of the same scene

                                                                In image registration we have available the input and output images but the specific

                                                                transformation that produced the output image from the input is generally unknown

                                                                The problem is to estimate the transformation function and then use it to register the two

                                                                images

                                                                - it may be of interest to align (register) two or more image taken at approximately the

                                                                same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                - align images of a given location taken by the same instrument at different moments

                                                                of time (satellite images)

                                                                Solving the problem using tie points (also called control points) which are

                                                                corresponding points whose locations are known precisely in the input and reference

                                                                image

                                                                Digital Image Processing

                                                                Week 1

                                                                How to select tie points

                                                                - interactively selecting them

                                                                - use of algorithms that try to detect these points

                                                                - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                the imaging sensors These objects produce a set of known points (called reseau

                                                                marks) directly on all images captured by the system which can be used as guides

                                                                for establishing tie points

                                                                The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                of 4 tie points both on the input image and the reference image A simple model based on

                                                                a bilinear approximation is given by

                                                                1 2 3 4

                                                                5 6 7 8

                                                                x c v c w c v w cy c v c w c v w c

                                                                (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                Digital Image Processing

                                                                Week 1

                                                                When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                frequently is to select a larger number of tie points and using this new set of tie points

                                                                subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                subregions marked by 4 tie points we applied the transformation model described above

                                                                The number of tie points and the sophistication of the model required to solve the register

                                                                problem depend on the severity of the geometrical distortion

                                                                Digital Image Processing

                                                                Week 1

                                                                a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                Digital Image Processing

                                                                Week 1

                                                                Probabilistic Methods

                                                                zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                p(zk) = the probability that the intensity level zk occurs in the given image

                                                                ( ) kk

                                                                np zM N

                                                                nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                pixels in the image) 1

                                                                0( ) 1

                                                                L

                                                                kk

                                                                p z

                                                                The mean (average) intensity of an image is given by 1

                                                                0( )

                                                                L

                                                                k kk

                                                                m z p z

                                                                Digital Image Processing

                                                                Week 1

                                                                The variance of the intensities is 1

                                                                2 2

                                                                0( ) ( )

                                                                L

                                                                k kk

                                                                z m p z

                                                                The variance is a measure of the spread of the values of z about the mean so it is a

                                                                measure of image contrast Usually for measuring image contrast the standard deviation

                                                                ( ) is used

                                                                The n-th moment of a random variable z about the mean is defined as 1

                                                                0( ) ( ) ( )

                                                                Ln

                                                                n k kk

                                                                z z m p z

                                                                ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                3( ) 0z the intensities are biased to values higher than the mean

                                                                ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                Digital Image Processing

                                                                Week 1

                                                                3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                mean

                                                                Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                Digital Image Processing

                                                                Week 1

                                                                Intensity Transformations and Spatial Filtering

                                                                ( ) ( )g x y T f x y

                                                                f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                neighborhood of (x y)

                                                                - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                and much smaller in size than the image

                                                                Digital Image Processing

                                                                Week 1

                                                                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                called spatial filter (spatial mask kernel template or window)

                                                                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                ( )s T r

                                                                s and r are denoting respectively the intensity of g and f at (x y)

                                                                Figure 2 left - T produces an output image of higher contrast than the original by

                                                                darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                is called contrast stretching

                                                                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                Digital Image Processing

                                                                Week 1

                                                                Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                thresholding function

                                                                Some Basic Intensity Transformation Functions

                                                                Image Negatives

                                                                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                - equivalent of a photographic negative

                                                                - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                image

                                                                Digital Image Processing

                                                                Week 1

                                                                Original Negative image

                                                                Digital Image Processing

                                                                Week 1

                                                                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                Some basic intensity transformation functions

                                                                Digital Image Processing

                                                                Week 1

                                                                This transformation maps a narrow range of low intensity values in the input into a wider

                                                                range An operator of this type is used to expand the values of dark pixels in an image

                                                                while compressing the higher-level values The opposite is true for the inverse log

                                                                transformation The log functions compress the dynamic range of images with large

                                                                variations in pixel values

                                                                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                Digital Image Processing

                                                                Week 1

                                                                Power-Law (Gamma) Transformations

                                                                - positive constants( ) ( ( ) )s T r c r c s c r

                                                                Plots of gamma transformation for different values of γ (c=1)

                                                                Digital Image Processing

                                                                Week 1

                                                                Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                of output values with the opposite being true for higher values of input values The

                                                                curves with 1 have the opposite effect of those generated with values of 1

                                                                1c - identity transformation

                                                                A variety of devices used for image capture printing and display respond according to a

                                                                power law The process used to correct these power-law response phenomena is called

                                                                gamma correction

                                                                Digital Image Processing

                                                                Week 1

                                                                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                Digital Image Processing

                                                                Week 1

                                                                Piecewise-Linear Transformations Functions

                                                                Contrast stretching

                                                                - a process that expands the range of intensity levels in an image so it spans the full

                                                                intensity range of the recording tool or display device

                                                                a b c d Fig5

                                                                Digital Image Processing

                                                                Week 1

                                                                11

                                                                1

                                                                2 1 1 21 2

                                                                2 1 2 1

                                                                22

                                                                2

                                                                [0 ]

                                                                ( ) ( )( ) [ ]( ) ( )

                                                                ( 1 ) [ 1]( 1 )

                                                                s r r rrs r r s r rT r r r r

                                                                r r r rs L r r r L

                                                                L r

                                                                Digital Image Processing

                                                                Week 1

                                                                1 1 2 2r s r s identity transformation (no change)

                                                                1 2 1 2 0 1r r s s L thresholding function

                                                                Figure 5(b) shows an 8-bit image with low contrast

                                                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                in the image respectively Thus the transformation function stretched the levels linearly

                                                                from their original range to the full range [0 L-1]

                                                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                2 2 1r s m L where m is the mean gray level in the image

                                                                The original image on which these results are based is a scanning electron microscope

                                                                image of pollen magnified approximately 700 times

                                                                Digital Image Processing

                                                                Week 1

                                                                Intensity-level slicing

                                                                - highlighting a specific range of intensities in an image

                                                                There are two approaches for intensity-level slicing

                                                                1 display in one value (white for example) all the values in the range of interest and in

                                                                another (say black) all other intensities (Figure 311 (a))

                                                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                intensities in the image (Figure 311 (b))

                                                                Digital Image Processing

                                                                Week 1

                                                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                the top of the scale of intensities This type of enhancement produces a binary image

                                                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                Highlights range [A B] and preserves all other intensities

                                                                Digital Image Processing

                                                                Week 1

                                                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                blockageshellip)

                                                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                image around the mean intensity was set to black the other intensities remain unchanged

                                                                Fig 6 - Aortic angiogram and intensity sliced versions

                                                                Digital Image Processing

                                                                Week 1

                                                                Bit-plane slicing

                                                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                This technique highlights the contribution made to the whole image appearances by each

                                                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                Digital Image Processing

                                                                Week 1

                                                                Digital Image Processing

                                                                Week 1

                                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                • DIP 1 2017
                                                                • DIP 02 (2017)

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Spaceborne radar image of mountains in southeast Tibet

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Imaging in the Radio Band

                                                                  medicine astronomy

                                                                  MRI = Magnetic Resonance Imaging

                                                                  This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                                                  Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                                                  The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

                                                                  Digital Image ProcessingDigital Image Processing

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                                                                  MRI images of a human knee (left) and spine (right)

                                                                  Digital Image ProcessingDigital Image Processing

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                                                                  Images of the Crab Pulsar covering the electromagnetic spectrum

                                                                  Gamma X-ray Optical Infrared Radio

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Other Imaging Modalities

                                                                  acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                                                  Imaging using sound geological explorations industry medicine

                                                                  Mineral and oil exploration

                                                                  For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

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                                                                  Biometry - iris

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                                                                  Biometry - fingerprint

                                                                  Digital Image ProcessingDigital Image Processing

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                                                                  Face detection and recognition

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Gender identification

                                                                  Digital Image ProcessingDigital Image Processing

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                                                                  Image morphing

                                                                  Digital Image ProcessingDigital Image Processing

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                                                                  Fundamental Steps in DIP

                                                                  methods whose input and output are images

                                                                  methods whose inputs are images but whose outputs are attributes extracted from those images

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Outputs are images

                                                                  bull image acquisition

                                                                  bull image filtering and enhancement

                                                                  bull image restoration

                                                                  bull color image processing

                                                                  bull wavelets and multiresolution processing

                                                                  bull compression

                                                                  bull morphological processing

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Outputs are attributes

                                                                  bull morphological processing

                                                                  bull segmentation

                                                                  bull representation and description

                                                                  bull object recognition

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Image acquisition - may involve preprocessing such as scaling

                                                                  Image enhancement

                                                                  bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                  bull enhancement is problem oriented

                                                                  bull there is no general sbquotheoryrsquo of image enhancement

                                                                  bull enhancement use subjective methods for image emprovement

                                                                  bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Image restoration

                                                                  bull improving the appearance of an image

                                                                  bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                  Color image processing

                                                                  bull fundamental concept in color models

                                                                  bull basic color processing in a digital domain

                                                                  Wavelets and multiresolution processing

                                                                  representing images in various degree of resolution

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Compression

                                                                  reducing the storage required to save an image or the bandwidth required to transmit it

                                                                  Morphological processing

                                                                  bull tools for extracting image components that are useful in the representation and description of shape

                                                                  bull a transition from processes that output images to processes that outputimage attributes

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Segmentation

                                                                  bull partitioning an image into its constituents parts or objects

                                                                  bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                  bull the more accurate the segmentation the more likley recognition is to succeed

                                                                  Representation and description (almost always follows segmentation)

                                                                  bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                  bull converting the data produced by segmentation to a form suitable for computer processing

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                  bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                  bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                  Object recognition

                                                                  the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                  Knowledge database

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Simplified diagramof a cross sectionof the human eye

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                  The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                  Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                  The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                  The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                  Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                  Fovea = the place where the image of the object of interest falls on

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                  Blind spot region without receptors

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Image formation in the eye

                                                                  Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                  Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                  distance between lens and retina along visual axix = 17 mm

                                                                  range of focal length = 14 mm to 17 mm

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Optical illusions

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                  gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                  quantities that describe the quality of a chromatic light source radiance

                                                                  the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                  measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                  brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  the physical meaning is determined by the source of the image

                                                                  ( )f D f x y

                                                                  Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                  f(xy) ndash characterized by two components

                                                                  i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                  r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                  ( ) ( ) ( )

                                                                  0 ( ) 0 ( ) 1

                                                                  f x y i x y r x y

                                                                  i x y r x y

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                  i(xy) ndash determined by the illumination source

                                                                  r(xy) ndash determined by the characteristics of the imaged objects

                                                                  is called gray (or intensity) scale

                                                                  In practice

                                                                  min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                  indoor values without additional illuminationmin max10 1000L L

                                                                  black whitemin max0 1 0 1 0 1L L L L l l L

                                                                  min maxL L

                                                                  Digital Image ProcessingDigital Image Processing

                                                                  Week 1Week 1

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Image Sampling and Quantization

                                                                  - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                  scene

                                                                  converting a continuous image f to digital form

                                                                  - digitizing (x y) is called sampling

                                                                  - digitizing f(x y) is called quantization

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                  (00) (01) (0 1)(10) (11) (1 1)

                                                                  ( )

                                                                  ( 10) ( 11) ( 1 1)

                                                                  f f f Nf f f N

                                                                  f x y

                                                                  f M f M f M N

                                                                  image element pixel

                                                                  00 01 0 1

                                                                  10 11 1 1

                                                                  10 11 1 1

                                                                  ( ) ( )

                                                                  N

                                                                  i jN M N

                                                                  i j

                                                                  M M M N

                                                                  a a aa f x i y j f i ja a a

                                                                  Aa

                                                                  a a a

                                                                  f(00) ndash the upper left corner of the image

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  M N ge 0 L=2k

                                                                  [0 1]i j i ja a L

                                                                  Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Number of bits required to store a digitized image

                                                                  for 2 b M N k M N b N k

                                                                  When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                  Measures line pairs per unit distance dots (pixels) per unit distance

                                                                  Image resolution = the largest number of discernible line pairs per unit distance

                                                                  (eg 100 line pairs per mm)

                                                                  Dots per unit distance are commonly used in printing and publishing

                                                                  In US the measure is expressed in dots per inch (dpi)

                                                                  (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                  Intensity resolution ndash the smallest discernible change in intensity level

                                                                  The number of intensity levels (L) is determined by hardware considerations

                                                                  L=2k ndash most common k = 8

                                                                  Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                  150 dpi (lower left) 72 dpi (lower right)

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Reducing the number of gray levels 256 128 64 32

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Reducing the number of gray levels 16 8 4 2

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                  Shrinking zooming ndash image resizing ndash image resampling methods

                                                                  Interpolation is the process of using known data to estimate values at unknown locations

                                                                  Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                  750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                  same spacing as the original and then shrink it so that it fits exactly over the original

                                                                  image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                  Problem assignment of intensity-level in the new 750 times 750 grid

                                                                  Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                  intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                  This technique has the tendency to produce undesirable effects like severe distortion of

                                                                  straight edges

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                  where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                  be written using the 4 nearest neighbors of point (x y)

                                                                  Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                  modest increase in computational effort

                                                                  Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                  nearest neighbors of the point 3 3

                                                                  0 0

                                                                  ( ) i ji j

                                                                  i jv x y c x y

                                                                  The coefficients cij are obtained solving a 16x16 linear system

                                                                  intensity levels of the 16 nearest neighbors of 3 3

                                                                  0 0

                                                                  ( )i ji j

                                                                  i jc x y x y

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                  bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                  programs such as Adobe Photoshop and Corel Photopaint

                                                                  Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                  the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                  then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                  neighbor interpolation was used (both for shrinking and zooming)

                                                                  Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                  interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                  from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Neighbors of a Pixel

                                                                  A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                  This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                  The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                  and are denoted ND(p)

                                                                  The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                  N8 (p)

                                                                  If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                  fall outside the image

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Adjacency Connectivity Regions Boundaries

                                                                  Denote by V the set of intensity levels used to define adjacency

                                                                  - in a binary image V 01 (V=0 V=1)

                                                                  - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                  We consider 3 types of adjacency

                                                                  (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                  (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                  (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                  m-adjacent if

                                                                  4( )q N p or

                                                                  ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                  Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                  ambiguities that often arise when 8-adjacency is used Consider the example

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  binary image

                                                                  0 1 1 0 1 1 0 1 1

                                                                  1 0 1 0 0 1 0 0 1 0

                                                                  0 0 1 0 0 1 0 0 1

                                                                  V

                                                                  The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                  8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                  m-adjacency

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                  is a sequence of distinct pixels with coordinates

                                                                  and are adjacent 0 0 1 1

                                                                  1 1

                                                                  ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                  n n

                                                                  i i i i

                                                                  x y x y x y x y s tx y x y i n

                                                                  The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                  Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                  Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                  in S if there exists a path between them consisting only of pixels from S

                                                                  S is a connected set if there is a path in S between any 2 pixels in S

                                                                  Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                  Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                  that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                  8-adjacency are considered

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                  touches the image border

                                                                  the complement of 1

                                                                  ( )K

                                                                  cu k u u

                                                                  k

                                                                  R R R R

                                                                  We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                  background of the image

                                                                  The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                  points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                  region that have at least one background neighbor This definition is referred to as the

                                                                  inner border to distinguish it from the notion of outer border which is the corresponding

                                                                  border in the background

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Distance measures

                                                                  For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                  function or metric if

                                                                  (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                  (b) D(p q) = D(q p)

                                                                  (c) D(p z) le D(p q) + D(q z)

                                                                  The Euclidean distance between p and q is defined as 1

                                                                  2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                  The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                  centered at (x y)

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  The D4 distance (also called city-block distance) between p and q is defined as

                                                                  4( ) | | | |D p q x s y t

                                                                  The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                  4

                                                                  22 1 2

                                                                  2 2 1 0 1 22 1 2

                                                                  2

                                                                  D

                                                                  The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                  The D8 distance (called the chessboard distance) between p and q is defined as

                                                                  8( ) max| | | |D p q x s y t

                                                                  The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  8

                                                                  2 2 2 2 22 1 1 1 2

                                                                  2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                  D

                                                                  The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                  D4 and D8 distances are independent of any paths that might exist between p and q

                                                                  because these distances involve only the coordinates of the point

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Array versus Matrix Operations

                                                                  An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                  11 12 11 12

                                                                  21 22 21 22

                                                                  a a b ba a b b

                                                                  Array product

                                                                  11 12 11 12 11 11 12 12

                                                                  21 22 21 22 21 21 22 21

                                                                  a a b b a b a ba a b b a b a b

                                                                  Matrix product

                                                                  11 12 11 12 11 11 12 21 11 12 12 21

                                                                  21 22 21 22 21 11 22 21 21 12 22 22

                                                                  a a b b a b a b a b a ba a b b a b a b a b a b

                                                                  We assume array operations unless stated otherwise

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Linear versus Nonlinear Operations

                                                                  One of the most important classifications of image-processing methods is whether it is

                                                                  linear or nonlinear

                                                                  ( ) ( )H f x y g x y

                                                                  H is said to be a linear operator if

                                                                  images1 2 1 2

                                                                  1 2

                                                                  ( ) ( ) ( ) ( )

                                                                  H a f x y b f x y a H f x y b H f x y

                                                                  a b f f

                                                                  Example of nonlinear operator

                                                                  the maximum value of the pixels of image max ( )H f f x y f

                                                                  1 2

                                                                  0 2 6 5 1 1

                                                                  2 3 4 7f f a b

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  1 2

                                                                  0 2 6 5 6 3max max 1 ( 1) max 2

                                                                  2 3 4 7 2 4a f b f

                                                                  0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                  2 3 4 7

                                                                  Arithmetic Operations in Image Processing

                                                                  Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                  f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                  For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                  value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                  The two random variables are uncorrelated when their covariance is 0

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                  used in image enhancement)

                                                                  1

                                                                  1( ) ( )K

                                                                  ii

                                                                  g x y g x yK

                                                                  If the noise satisfies the properties stated above we have

                                                                  2 2( ) ( )

                                                                  1( ) ( ) g x y x yE g x y f x yK

                                                                  ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                  and g respectively The standard deviation (square root of the variance) at any point in

                                                                  the average image is

                                                                  ( ) ( )1

                                                                  g x y x yK

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  As K increases the variability (as measured by the variance or the standard deviation) of

                                                                  the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                  means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                  averaging process increases

                                                                  An important application of image averaging is in the field of astronomy where imaging

                                                                  under very low light levels frequently causes sensor noise to render single images

                                                                  virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                  was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                  64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                  images respectively

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                  100 noisy images

                                                                  a b c d e f

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  A frequent application of image subtraction is in the enhancement of differences between

                                                                  images

                                                                  (a) (b) (c)

                                                                  Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                  significant bit of each pixel (c) the difference between the two images

                                                                  Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                  Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                  images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                  difference between images (a) and (b)

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                  h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                  intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                  source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                  bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                  anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                  images after injection of the contrast medium

                                                                  In g(x y) we can find the differences between h and f as enhanced detail

                                                                  Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                  propagates through the various arteries in the area being observed

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  An important application of image multiplication (and division) is shading correction

                                                                  Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                  f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                  When the shading function is known

                                                                  ( )( )( )

                                                                  g x yf x yh x y

                                                                  h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                  approximation to the shading function by imaging a target of constant intensity When the

                                                                  sensor is not available often the shading pattern can be estimated from the image

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  (a) (b) (c)

                                                                  Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Another use of image multiplication is in masking also called region of interest (ROI)

                                                                  operations The process consists of multiplying a given image by a mask image that has

                                                                  1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                  image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                  (a) (b) (c)

                                                                  Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                  min( )mf f f

                                                                  0 ( 255)max( )

                                                                  ms

                                                                  m

                                                                  ff K K K

                                                                  f

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Spatial Operations

                                                                  - are performed directly on the pixels of a given image

                                                                  There are three categories of spatial operations

                                                                  single-pixel operations

                                                                  neighborhood operations

                                                                  geometric spatial transformations

                                                                  Single-pixel operations

                                                                  - change the values of intensity for the individual pixels ( )s T z

                                                                  where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                  corresponding pixel in the processed image

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Neighborhood operations

                                                                  Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                  in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                  based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                  rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                  intensity by computing the average value of the pixels in Sxy

                                                                  ( )

                                                                  1( ) ( )xyr c S

                                                                  g x y f r cm n

                                                                  The net effect is to perform local blurring in the original image This type of process is

                                                                  used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                  largest region of an image

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Geometric spatial transformations and image registration

                                                                  - modify the spatial relationship between pixels in an image

                                                                  - these transformations are often called rubber-sheet transformations (analogous to

                                                                  printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                  predefined set of rules

                                                                  A geometric transformation consists of 2 basic operations

                                                                  1 a spatial transformation of coordinates

                                                                  2 intensity interpolation that assign intensity values to the spatial transformed

                                                                  pixels

                                                                  The coordinate system transformation ( ) [( )]x y T v w

                                                                  (v w) ndash pixel coordinates in the original image

                                                                  (x y) ndash pixel coordinates in the transformed image

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                  Affine transform

                                                                  11 1211 21 31

                                                                  21 2212 22 33

                                                                  31 32

                                                                  0[ 1] [ 1] [ 1] 0

                                                                  1

                                                                  t tx t v t w t

                                                                  x y v w T v w t ty t v t w t

                                                                  t t

                                                                  (AT)

                                                                  This transform can scale rotate translate or shear a set of coordinate points depending

                                                                  on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                  result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                  scaling rotation and translation matrices from Table 1

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Affine transformations

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  The preceding transformations relocate pixels on an image to new locations To complete

                                                                  the process we have to assign intensity values to those locations This task is done by

                                                                  using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                  In practice we can use equation (AT) in two basic ways

                                                                  forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                  location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                  Problems

                                                                  - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                  the same location in the output image

                                                                  - some output locations have no correspondent in the original image (no intensity

                                                                  assignment)

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  inverse mapping scans the output pixel locations and at each location (x y)

                                                                  computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                  It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                  pixel value

                                                                  Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                  numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Image registration ndash align two or more images of the same scene

                                                                  In image registration we have available the input and output images but the specific

                                                                  transformation that produced the output image from the input is generally unknown

                                                                  The problem is to estimate the transformation function and then use it to register the two

                                                                  images

                                                                  - it may be of interest to align (register) two or more image taken at approximately the

                                                                  same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                  - align images of a given location taken by the same instrument at different moments

                                                                  of time (satellite images)

                                                                  Solving the problem using tie points (also called control points) which are

                                                                  corresponding points whose locations are known precisely in the input and reference

                                                                  image

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  How to select tie points

                                                                  - interactively selecting them

                                                                  - use of algorithms that try to detect these points

                                                                  - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                  the imaging sensors These objects produce a set of known points (called reseau

                                                                  marks) directly on all images captured by the system which can be used as guides

                                                                  for establishing tie points

                                                                  The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                  of 4 tie points both on the input image and the reference image A simple model based on

                                                                  a bilinear approximation is given by

                                                                  1 2 3 4

                                                                  5 6 7 8

                                                                  x c v c w c v w cy c v c w c v w c

                                                                  (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                  frequently is to select a larger number of tie points and using this new set of tie points

                                                                  subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                  subregions marked by 4 tie points we applied the transformation model described above

                                                                  The number of tie points and the sophistication of the model required to solve the register

                                                                  problem depend on the severity of the geometrical distortion

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Probabilistic Methods

                                                                  zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                  p(zk) = the probability that the intensity level zk occurs in the given image

                                                                  ( ) kk

                                                                  np zM N

                                                                  nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                  pixels in the image) 1

                                                                  0( ) 1

                                                                  L

                                                                  kk

                                                                  p z

                                                                  The mean (average) intensity of an image is given by 1

                                                                  0( )

                                                                  L

                                                                  k kk

                                                                  m z p z

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  The variance of the intensities is 1

                                                                  2 2

                                                                  0( ) ( )

                                                                  L

                                                                  k kk

                                                                  z m p z

                                                                  The variance is a measure of the spread of the values of z about the mean so it is a

                                                                  measure of image contrast Usually for measuring image contrast the standard deviation

                                                                  ( ) is used

                                                                  The n-th moment of a random variable z about the mean is defined as 1

                                                                  0( ) ( ) ( )

                                                                  Ln

                                                                  n k kk

                                                                  z z m p z

                                                                  ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                  3( ) 0z the intensities are biased to values higher than the mean

                                                                  ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                  mean

                                                                  Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                  Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                  Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                  Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Intensity Transformations and Spatial Filtering

                                                                  ( ) ( )g x y T f x y

                                                                  f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                  neighborhood of (x y)

                                                                  - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                  and much smaller in size than the image

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                  called spatial filter (spatial mask kernel template or window)

                                                                  ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                  ( )s T r

                                                                  s and r are denoting respectively the intensity of g and f at (x y)

                                                                  Figure 2 left - T produces an output image of higher contrast than the original by

                                                                  darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                  is called contrast stretching

                                                                  Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                  thresholding function

                                                                  Some Basic Intensity Transformation Functions

                                                                  Image Negatives

                                                                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                  - equivalent of a photographic negative

                                                                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                  image

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Original Negative image

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                  Some basic intensity transformation functions

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  This transformation maps a narrow range of low intensity values in the input into a wider

                                                                  range An operator of this type is used to expand the values of dark pixels in an image

                                                                  while compressing the higher-level values The opposite is true for the inverse log

                                                                  transformation The log functions compress the dynamic range of images with large

                                                                  variations in pixel values

                                                                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Power-Law (Gamma) Transformations

                                                                  - positive constants( ) ( ( ) )s T r c r c s c r

                                                                  Plots of gamma transformation for different values of γ (c=1)

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                  of output values with the opposite being true for higher values of input values The

                                                                  curves with 1 have the opposite effect of those generated with values of 1

                                                                  1c - identity transformation

                                                                  A variety of devices used for image capture printing and display respond according to a

                                                                  power law The process used to correct these power-law response phenomena is called

                                                                  gamma correction

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Piecewise-Linear Transformations Functions

                                                                  Contrast stretching

                                                                  - a process that expands the range of intensity levels in an image so it spans the full

                                                                  intensity range of the recording tool or display device

                                                                  a b c d Fig5

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  11

                                                                  1

                                                                  2 1 1 21 2

                                                                  2 1 2 1

                                                                  22

                                                                  2

                                                                  [0 ]

                                                                  ( ) ( )( ) [ ]( ) ( )

                                                                  ( 1 ) [ 1]( 1 )

                                                                  s r r rrs r r s r rT r r r r

                                                                  r r r rs L r r r L

                                                                  L r

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  1 1 2 2r s r s identity transformation (no change)

                                                                  1 2 1 2 0 1r r s s L thresholding function

                                                                  Figure 5(b) shows an 8-bit image with low contrast

                                                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                  in the image respectively Thus the transformation function stretched the levels linearly

                                                                  from their original range to the full range [0 L-1]

                                                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                  2 2 1r s m L where m is the mean gray level in the image

                                                                  The original image on which these results are based is a scanning electron microscope

                                                                  image of pollen magnified approximately 700 times

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Intensity-level slicing

                                                                  - highlighting a specific range of intensities in an image

                                                                  There are two approaches for intensity-level slicing

                                                                  1 display in one value (white for example) all the values in the range of interest and in

                                                                  another (say black) all other intensities (Figure 311 (a))

                                                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                  intensities in the image (Figure 311 (b))

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                  the top of the scale of intensities This type of enhancement produces a binary image

                                                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                  Highlights range [A B] and preserves all other intensities

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                  blockageshellip)

                                                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                  image around the mean intensity was set to black the other intensities remain unchanged

                                                                  Fig 6 - Aortic angiogram and intensity sliced versions

                                                                  Digital Image Processing

                                                                  Week 1

                                                                  Bit-plane slicing

                                                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                  This technique highlights the contribution made to the whole image appearances by each

                                                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                  Digital Image Processing

                                                                  Week 1

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                                                                  Week 1

                                                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                  • DIP 1 2017
                                                                  • DIP 02 (2017)

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    Imaging in the Radio Band

                                                                    medicine astronomy

                                                                    MRI = Magnetic Resonance Imaging

                                                                    This technique places the patient in a powerful magnet and passes short pulses of radio waves through his or her body

                                                                    Each pulse causes a responding pulse of radio waves to be emited by the patinet tissues

                                                                    The location from which these signals originate and their strength are determined by a computer which produces a 2D picture of a section of the patient

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                                                                    Week 1Week 1

                                                                    MRI images of a human knee (left) and spine (right)

                                                                    Digital Image ProcessingDigital Image Processing

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                                                                    Images of the Crab Pulsar covering the electromagnetic spectrum

                                                                    Gamma X-ray Optical Infrared Radio

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    Other Imaging Modalities

                                                                    acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                                                    Imaging using sound geological explorations industry medicine

                                                                    Mineral and oil exploration

                                                                    For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

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                                                                    Biometry - iris

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                                                                    Biometry - fingerprint

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                                                                    Face detection and recognition

                                                                    Digital Image ProcessingDigital Image Processing

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                                                                    Gender identification

                                                                    Digital Image ProcessingDigital Image Processing

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                                                                    Image morphing

                                                                    Digital Image ProcessingDigital Image Processing

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                                                                    Fundamental Steps in DIP

                                                                    methods whose input and output are images

                                                                    methods whose inputs are images but whose outputs are attributes extracted from those images

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    Outputs are images

                                                                    bull image acquisition

                                                                    bull image filtering and enhancement

                                                                    bull image restoration

                                                                    bull color image processing

                                                                    bull wavelets and multiresolution processing

                                                                    bull compression

                                                                    bull morphological processing

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                                                                    Outputs are attributes

                                                                    bull morphological processing

                                                                    bull segmentation

                                                                    bull representation and description

                                                                    bull object recognition

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                                                                    Week 1Week 1

                                                                    Image acquisition - may involve preprocessing such as scaling

                                                                    Image enhancement

                                                                    bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                    bull enhancement is problem oriented

                                                                    bull there is no general sbquotheoryrsquo of image enhancement

                                                                    bull enhancement use subjective methods for image emprovement

                                                                    bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

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                                                                    Week 1Week 1

                                                                    Image restoration

                                                                    bull improving the appearance of an image

                                                                    bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                    Color image processing

                                                                    bull fundamental concept in color models

                                                                    bull basic color processing in a digital domain

                                                                    Wavelets and multiresolution processing

                                                                    representing images in various degree of resolution

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    Compression

                                                                    reducing the storage required to save an image or the bandwidth required to transmit it

                                                                    Morphological processing

                                                                    bull tools for extracting image components that are useful in the representation and description of shape

                                                                    bull a transition from processes that output images to processes that outputimage attributes

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    Segmentation

                                                                    bull partitioning an image into its constituents parts or objects

                                                                    bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                    bull the more accurate the segmentation the more likley recognition is to succeed

                                                                    Representation and description (almost always follows segmentation)

                                                                    bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                    bull converting the data produced by segmentation to a form suitable for computer processing

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                    bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                    bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                    Object recognition

                                                                    the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                    Knowledge database

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    Simplified diagramof a cross sectionof the human eye

                                                                    Digital Image ProcessingDigital Image Processing

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                                                                    Week 1Week 1

                                                                    Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                    The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                    Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                    The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                    The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                    Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                    Fovea = the place where the image of the object of interest falls on

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                    Blind spot region without receptors

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    Image formation in the eye

                                                                    Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                    Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                    distance between lens and retina along visual axix = 17 mm

                                                                    range of focal length = 14 mm to 17 mm

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

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                                                                    Week 1Week 1

                                                                    Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    Optical illusions

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                    gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                    quantities that describe the quality of a chromatic light source radiance

                                                                    the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                    measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                    brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    the physical meaning is determined by the source of the image

                                                                    ( )f D f x y

                                                                    Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                    f(xy) ndash characterized by two components

                                                                    i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                    r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                    ( ) ( ) ( )

                                                                    0 ( ) 0 ( ) 1

                                                                    f x y i x y r x y

                                                                    i x y r x y

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                    i(xy) ndash determined by the illumination source

                                                                    r(xy) ndash determined by the characteristics of the imaged objects

                                                                    is called gray (or intensity) scale

                                                                    In practice

                                                                    min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                    indoor values without additional illuminationmin max10 1000L L

                                                                    black whitemin max0 1 0 1 0 1L L L L l l L

                                                                    min maxL L

                                                                    Digital Image ProcessingDigital Image Processing

                                                                    Week 1Week 1

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Image Sampling and Quantization

                                                                    - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                    scene

                                                                    converting a continuous image f to digital form

                                                                    - digitizing (x y) is called sampling

                                                                    - digitizing f(x y) is called quantization

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                    (00) (01) (0 1)(10) (11) (1 1)

                                                                    ( )

                                                                    ( 10) ( 11) ( 1 1)

                                                                    f f f Nf f f N

                                                                    f x y

                                                                    f M f M f M N

                                                                    image element pixel

                                                                    00 01 0 1

                                                                    10 11 1 1

                                                                    10 11 1 1

                                                                    ( ) ( )

                                                                    N

                                                                    i jN M N

                                                                    i j

                                                                    M M M N

                                                                    a a aa f x i y j f i ja a a

                                                                    Aa

                                                                    a a a

                                                                    f(00) ndash the upper left corner of the image

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    M N ge 0 L=2k

                                                                    [0 1]i j i ja a L

                                                                    Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Number of bits required to store a digitized image

                                                                    for 2 b M N k M N b N k

                                                                    When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                    Measures line pairs per unit distance dots (pixels) per unit distance

                                                                    Image resolution = the largest number of discernible line pairs per unit distance

                                                                    (eg 100 line pairs per mm)

                                                                    Dots per unit distance are commonly used in printing and publishing

                                                                    In US the measure is expressed in dots per inch (dpi)

                                                                    (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                    Intensity resolution ndash the smallest discernible change in intensity level

                                                                    The number of intensity levels (L) is determined by hardware considerations

                                                                    L=2k ndash most common k = 8

                                                                    Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                    150 dpi (lower left) 72 dpi (lower right)

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Reducing the number of gray levels 256 128 64 32

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Reducing the number of gray levels 16 8 4 2

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                    Shrinking zooming ndash image resizing ndash image resampling methods

                                                                    Interpolation is the process of using known data to estimate values at unknown locations

                                                                    Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                    750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                    same spacing as the original and then shrink it so that it fits exactly over the original

                                                                    image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                    Problem assignment of intensity-level in the new 750 times 750 grid

                                                                    Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                    intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                    This technique has the tendency to produce undesirable effects like severe distortion of

                                                                    straight edges

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                    where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                    be written using the 4 nearest neighbors of point (x y)

                                                                    Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                    modest increase in computational effort

                                                                    Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                    nearest neighbors of the point 3 3

                                                                    0 0

                                                                    ( ) i ji j

                                                                    i jv x y c x y

                                                                    The coefficients cij are obtained solving a 16x16 linear system

                                                                    intensity levels of the 16 nearest neighbors of 3 3

                                                                    0 0

                                                                    ( )i ji j

                                                                    i jc x y x y

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                    bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                    programs such as Adobe Photoshop and Corel Photopaint

                                                                    Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                    the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                    then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                    neighbor interpolation was used (both for shrinking and zooming)

                                                                    Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                    interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                    from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Neighbors of a Pixel

                                                                    A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                    This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                    The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                    and are denoted ND(p)

                                                                    The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                    N8 (p)

                                                                    If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                    fall outside the image

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Adjacency Connectivity Regions Boundaries

                                                                    Denote by V the set of intensity levels used to define adjacency

                                                                    - in a binary image V 01 (V=0 V=1)

                                                                    - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                    We consider 3 types of adjacency

                                                                    (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                    (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                    (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                    m-adjacent if

                                                                    4( )q N p or

                                                                    ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                    Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                    ambiguities that often arise when 8-adjacency is used Consider the example

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    binary image

                                                                    0 1 1 0 1 1 0 1 1

                                                                    1 0 1 0 0 1 0 0 1 0

                                                                    0 0 1 0 0 1 0 0 1

                                                                    V

                                                                    The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                    8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                    m-adjacency

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                    is a sequence of distinct pixels with coordinates

                                                                    and are adjacent 0 0 1 1

                                                                    1 1

                                                                    ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                    n n

                                                                    i i i i

                                                                    x y x y x y x y s tx y x y i n

                                                                    The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                    Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                    Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                    in S if there exists a path between them consisting only of pixels from S

                                                                    S is a connected set if there is a path in S between any 2 pixels in S

                                                                    Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                    Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                    that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                    8-adjacency are considered

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                    touches the image border

                                                                    the complement of 1

                                                                    ( )K

                                                                    cu k u u

                                                                    k

                                                                    R R R R

                                                                    We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                    background of the image

                                                                    The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                    points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                    region that have at least one background neighbor This definition is referred to as the

                                                                    inner border to distinguish it from the notion of outer border which is the corresponding

                                                                    border in the background

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Distance measures

                                                                    For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                    function or metric if

                                                                    (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                    (b) D(p q) = D(q p)

                                                                    (c) D(p z) le D(p q) + D(q z)

                                                                    The Euclidean distance between p and q is defined as 1

                                                                    2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                    The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                    centered at (x y)

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    The D4 distance (also called city-block distance) between p and q is defined as

                                                                    4( ) | | | |D p q x s y t

                                                                    The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                    4

                                                                    22 1 2

                                                                    2 2 1 0 1 22 1 2

                                                                    2

                                                                    D

                                                                    The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                    The D8 distance (called the chessboard distance) between p and q is defined as

                                                                    8( ) max| | | |D p q x s y t

                                                                    The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    8

                                                                    2 2 2 2 22 1 1 1 2

                                                                    2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                    D

                                                                    The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                    D4 and D8 distances are independent of any paths that might exist between p and q

                                                                    because these distances involve only the coordinates of the point

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Array versus Matrix Operations

                                                                    An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                    11 12 11 12

                                                                    21 22 21 22

                                                                    a a b ba a b b

                                                                    Array product

                                                                    11 12 11 12 11 11 12 12

                                                                    21 22 21 22 21 21 22 21

                                                                    a a b b a b a ba a b b a b a b

                                                                    Matrix product

                                                                    11 12 11 12 11 11 12 21 11 12 12 21

                                                                    21 22 21 22 21 11 22 21 21 12 22 22

                                                                    a a b b a b a b a b a ba a b b a b a b a b a b

                                                                    We assume array operations unless stated otherwise

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Linear versus Nonlinear Operations

                                                                    One of the most important classifications of image-processing methods is whether it is

                                                                    linear or nonlinear

                                                                    ( ) ( )H f x y g x y

                                                                    H is said to be a linear operator if

                                                                    images1 2 1 2

                                                                    1 2

                                                                    ( ) ( ) ( ) ( )

                                                                    H a f x y b f x y a H f x y b H f x y

                                                                    a b f f

                                                                    Example of nonlinear operator

                                                                    the maximum value of the pixels of image max ( )H f f x y f

                                                                    1 2

                                                                    0 2 6 5 1 1

                                                                    2 3 4 7f f a b

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    1 2

                                                                    0 2 6 5 6 3max max 1 ( 1) max 2

                                                                    2 3 4 7 2 4a f b f

                                                                    0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                    2 3 4 7

                                                                    Arithmetic Operations in Image Processing

                                                                    Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                    f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                    For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                    value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                    The two random variables are uncorrelated when their covariance is 0

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                    used in image enhancement)

                                                                    1

                                                                    1( ) ( )K

                                                                    ii

                                                                    g x y g x yK

                                                                    If the noise satisfies the properties stated above we have

                                                                    2 2( ) ( )

                                                                    1( ) ( ) g x y x yE g x y f x yK

                                                                    ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                    and g respectively The standard deviation (square root of the variance) at any point in

                                                                    the average image is

                                                                    ( ) ( )1

                                                                    g x y x yK

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    As K increases the variability (as measured by the variance or the standard deviation) of

                                                                    the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                    means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                    averaging process increases

                                                                    An important application of image averaging is in the field of astronomy where imaging

                                                                    under very low light levels frequently causes sensor noise to render single images

                                                                    virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                    was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                    64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                    images respectively

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                    100 noisy images

                                                                    a b c d e f

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    A frequent application of image subtraction is in the enhancement of differences between

                                                                    images

                                                                    (a) (b) (c)

                                                                    Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                    significant bit of each pixel (c) the difference between the two images

                                                                    Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                    Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                    images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                    difference between images (a) and (b)

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                    h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                    intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                    source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                    bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                    anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                    images after injection of the contrast medium

                                                                    In g(x y) we can find the differences between h and f as enhanced detail

                                                                    Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                    propagates through the various arteries in the area being observed

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    An important application of image multiplication (and division) is shading correction

                                                                    Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                    f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                    When the shading function is known

                                                                    ( )( )( )

                                                                    g x yf x yh x y

                                                                    h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                    approximation to the shading function by imaging a target of constant intensity When the

                                                                    sensor is not available often the shading pattern can be estimated from the image

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    (a) (b) (c)

                                                                    Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Another use of image multiplication is in masking also called region of interest (ROI)

                                                                    operations The process consists of multiplying a given image by a mask image that has

                                                                    1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                    image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                    (a) (b) (c)

                                                                    Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                    min( )mf f f

                                                                    0 ( 255)max( )

                                                                    ms

                                                                    m

                                                                    ff K K K

                                                                    f

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Spatial Operations

                                                                    - are performed directly on the pixels of a given image

                                                                    There are three categories of spatial operations

                                                                    single-pixel operations

                                                                    neighborhood operations

                                                                    geometric spatial transformations

                                                                    Single-pixel operations

                                                                    - change the values of intensity for the individual pixels ( )s T z

                                                                    where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                    corresponding pixel in the processed image

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Neighborhood operations

                                                                    Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                    in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                    based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                    rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                    intensity by computing the average value of the pixels in Sxy

                                                                    ( )

                                                                    1( ) ( )xyr c S

                                                                    g x y f r cm n

                                                                    The net effect is to perform local blurring in the original image This type of process is

                                                                    used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                    largest region of an image

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Geometric spatial transformations and image registration

                                                                    - modify the spatial relationship between pixels in an image

                                                                    - these transformations are often called rubber-sheet transformations (analogous to

                                                                    printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                    predefined set of rules

                                                                    A geometric transformation consists of 2 basic operations

                                                                    1 a spatial transformation of coordinates

                                                                    2 intensity interpolation that assign intensity values to the spatial transformed

                                                                    pixels

                                                                    The coordinate system transformation ( ) [( )]x y T v w

                                                                    (v w) ndash pixel coordinates in the original image

                                                                    (x y) ndash pixel coordinates in the transformed image

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                    Affine transform

                                                                    11 1211 21 31

                                                                    21 2212 22 33

                                                                    31 32

                                                                    0[ 1] [ 1] [ 1] 0

                                                                    1

                                                                    t tx t v t w t

                                                                    x y v w T v w t ty t v t w t

                                                                    t t

                                                                    (AT)

                                                                    This transform can scale rotate translate or shear a set of coordinate points depending

                                                                    on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                    result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                    scaling rotation and translation matrices from Table 1

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Affine transformations

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    The preceding transformations relocate pixels on an image to new locations To complete

                                                                    the process we have to assign intensity values to those locations This task is done by

                                                                    using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                    In practice we can use equation (AT) in two basic ways

                                                                    forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                    location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                    Problems

                                                                    - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                    the same location in the output image

                                                                    - some output locations have no correspondent in the original image (no intensity

                                                                    assignment)

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    inverse mapping scans the output pixel locations and at each location (x y)

                                                                    computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                    It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                    pixel value

                                                                    Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                    numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Image registration ndash align two or more images of the same scene

                                                                    In image registration we have available the input and output images but the specific

                                                                    transformation that produced the output image from the input is generally unknown

                                                                    The problem is to estimate the transformation function and then use it to register the two

                                                                    images

                                                                    - it may be of interest to align (register) two or more image taken at approximately the

                                                                    same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                    - align images of a given location taken by the same instrument at different moments

                                                                    of time (satellite images)

                                                                    Solving the problem using tie points (also called control points) which are

                                                                    corresponding points whose locations are known precisely in the input and reference

                                                                    image

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    How to select tie points

                                                                    - interactively selecting them

                                                                    - use of algorithms that try to detect these points

                                                                    - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                    the imaging sensors These objects produce a set of known points (called reseau

                                                                    marks) directly on all images captured by the system which can be used as guides

                                                                    for establishing tie points

                                                                    The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                    of 4 tie points both on the input image and the reference image A simple model based on

                                                                    a bilinear approximation is given by

                                                                    1 2 3 4

                                                                    5 6 7 8

                                                                    x c v c w c v w cy c v c w c v w c

                                                                    (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                    frequently is to select a larger number of tie points and using this new set of tie points

                                                                    subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                    subregions marked by 4 tie points we applied the transformation model described above

                                                                    The number of tie points and the sophistication of the model required to solve the register

                                                                    problem depend on the severity of the geometrical distortion

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Probabilistic Methods

                                                                    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                    p(zk) = the probability that the intensity level zk occurs in the given image

                                                                    ( ) kk

                                                                    np zM N

                                                                    nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                    pixels in the image) 1

                                                                    0( ) 1

                                                                    L

                                                                    kk

                                                                    p z

                                                                    The mean (average) intensity of an image is given by 1

                                                                    0( )

                                                                    L

                                                                    k kk

                                                                    m z p z

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    The variance of the intensities is 1

                                                                    2 2

                                                                    0( ) ( )

                                                                    L

                                                                    k kk

                                                                    z m p z

                                                                    The variance is a measure of the spread of the values of z about the mean so it is a

                                                                    measure of image contrast Usually for measuring image contrast the standard deviation

                                                                    ( ) is used

                                                                    The n-th moment of a random variable z about the mean is defined as 1

                                                                    0( ) ( ) ( )

                                                                    Ln

                                                                    n k kk

                                                                    z z m p z

                                                                    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                    3( ) 0z the intensities are biased to values higher than the mean

                                                                    ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                    mean

                                                                    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                    Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                    Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                    Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Intensity Transformations and Spatial Filtering

                                                                    ( ) ( )g x y T f x y

                                                                    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                    neighborhood of (x y)

                                                                    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                    and much smaller in size than the image

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                    called spatial filter (spatial mask kernel template or window)

                                                                    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                    ( )s T r

                                                                    s and r are denoting respectively the intensity of g and f at (x y)

                                                                    Figure 2 left - T produces an output image of higher contrast than the original by

                                                                    darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                    is called contrast stretching

                                                                    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                    thresholding function

                                                                    Some Basic Intensity Transformation Functions

                                                                    Image Negatives

                                                                    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                    - equivalent of a photographic negative

                                                                    - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                    image

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Original Negative image

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                    Some basic intensity transformation functions

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    This transformation maps a narrow range of low intensity values in the input into a wider

                                                                    range An operator of this type is used to expand the values of dark pixels in an image

                                                                    while compressing the higher-level values The opposite is true for the inverse log

                                                                    transformation The log functions compress the dynamic range of images with large

                                                                    variations in pixel values

                                                                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Power-Law (Gamma) Transformations

                                                                    - positive constants( ) ( ( ) )s T r c r c s c r

                                                                    Plots of gamma transformation for different values of γ (c=1)

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                    of output values with the opposite being true for higher values of input values The

                                                                    curves with 1 have the opposite effect of those generated with values of 1

                                                                    1c - identity transformation

                                                                    A variety of devices used for image capture printing and display respond according to a

                                                                    power law The process used to correct these power-law response phenomena is called

                                                                    gamma correction

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Piecewise-Linear Transformations Functions

                                                                    Contrast stretching

                                                                    - a process that expands the range of intensity levels in an image so it spans the full

                                                                    intensity range of the recording tool or display device

                                                                    a b c d Fig5

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    11

                                                                    1

                                                                    2 1 1 21 2

                                                                    2 1 2 1

                                                                    22

                                                                    2

                                                                    [0 ]

                                                                    ( ) ( )( ) [ ]( ) ( )

                                                                    ( 1 ) [ 1]( 1 )

                                                                    s r r rrs r r s r rT r r r r

                                                                    r r r rs L r r r L

                                                                    L r

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    1 1 2 2r s r s identity transformation (no change)

                                                                    1 2 1 2 0 1r r s s L thresholding function

                                                                    Figure 5(b) shows an 8-bit image with low contrast

                                                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                    in the image respectively Thus the transformation function stretched the levels linearly

                                                                    from their original range to the full range [0 L-1]

                                                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                    2 2 1r s m L where m is the mean gray level in the image

                                                                    The original image on which these results are based is a scanning electron microscope

                                                                    image of pollen magnified approximately 700 times

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Intensity-level slicing

                                                                    - highlighting a specific range of intensities in an image

                                                                    There are two approaches for intensity-level slicing

                                                                    1 display in one value (white for example) all the values in the range of interest and in

                                                                    another (say black) all other intensities (Figure 311 (a))

                                                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                    intensities in the image (Figure 311 (b))

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                    the top of the scale of intensities This type of enhancement produces a binary image

                                                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                    Highlights range [A B] and preserves all other intensities

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                    blockageshellip)

                                                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                    image around the mean intensity was set to black the other intensities remain unchanged

                                                                    Fig 6 - Aortic angiogram and intensity sliced versions

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Bit-plane slicing

                                                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                    This technique highlights the contribution made to the whole image appearances by each

                                                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    Digital Image Processing

                                                                    Week 1

                                                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                    • DIP 1 2017
                                                                    • DIP 02 (2017)

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      MRI images of a human knee (left) and spine (right)

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Images of the Crab Pulsar covering the electromagnetic spectrum

                                                                      Gamma X-ray Optical Infrared Radio

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Other Imaging Modalities

                                                                      acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                                                      Imaging using sound geological explorations industry medicine

                                                                      Mineral and oil exploration

                                                                      For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Biometry - iris

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Biometry - fingerprint

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Face detection and recognition

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Gender identification

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Image morphing

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Fundamental Steps in DIP

                                                                      methods whose input and output are images

                                                                      methods whose inputs are images but whose outputs are attributes extracted from those images

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Outputs are images

                                                                      bull image acquisition

                                                                      bull image filtering and enhancement

                                                                      bull image restoration

                                                                      bull color image processing

                                                                      bull wavelets and multiresolution processing

                                                                      bull compression

                                                                      bull morphological processing

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Outputs are attributes

                                                                      bull morphological processing

                                                                      bull segmentation

                                                                      bull representation and description

                                                                      bull object recognition

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Image acquisition - may involve preprocessing such as scaling

                                                                      Image enhancement

                                                                      bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                      bull enhancement is problem oriented

                                                                      bull there is no general sbquotheoryrsquo of image enhancement

                                                                      bull enhancement use subjective methods for image emprovement

                                                                      bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Image restoration

                                                                      bull improving the appearance of an image

                                                                      bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                      Color image processing

                                                                      bull fundamental concept in color models

                                                                      bull basic color processing in a digital domain

                                                                      Wavelets and multiresolution processing

                                                                      representing images in various degree of resolution

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Compression

                                                                      reducing the storage required to save an image or the bandwidth required to transmit it

                                                                      Morphological processing

                                                                      bull tools for extracting image components that are useful in the representation and description of shape

                                                                      bull a transition from processes that output images to processes that outputimage attributes

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Segmentation

                                                                      bull partitioning an image into its constituents parts or objects

                                                                      bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                      bull the more accurate the segmentation the more likley recognition is to succeed

                                                                      Representation and description (almost always follows segmentation)

                                                                      bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                      bull converting the data produced by segmentation to a form suitable for computer processing

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                      bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                      bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                      Object recognition

                                                                      the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                      Knowledge database

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Simplified diagramof a cross sectionof the human eye

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                      The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                      Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                      The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                      The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                      Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                      Fovea = the place where the image of the object of interest falls on

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                      Blind spot region without receptors

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Image formation in the eye

                                                                      Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                      Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                      distance between lens and retina along visual axix = 17 mm

                                                                      range of focal length = 14 mm to 17 mm

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      Optical illusions

                                                                      Digital Image ProcessingDigital Image Processing

                                                                      Week 1Week 1

                                                                      ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                      Digital Image ProcessingDigital Image Processing

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                                                                      Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                      gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                      quantities that describe the quality of a chromatic light source radiance

                                                                      the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                      measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

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                                                                      For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                      brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                      Digital Image ProcessingDigital Image Processing

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                                                                      the physical meaning is determined by the source of the image

                                                                      ( )f D f x y

                                                                      Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                      f(xy) ndash characterized by two components

                                                                      i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                      r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                      ( ) ( ) ( )

                                                                      0 ( ) 0 ( ) 1

                                                                      f x y i x y r x y

                                                                      i x y r x y

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                                                                      Week 1Week 1

                                                                      r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                      i(xy) ndash determined by the illumination source

                                                                      r(xy) ndash determined by the characteristics of the imaged objects

                                                                      is called gray (or intensity) scale

                                                                      In practice

                                                                      min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                      indoor values without additional illuminationmin max10 1000L L

                                                                      black whitemin max0 1 0 1 0 1L L L L l l L

                                                                      min maxL L

                                                                      Digital Image ProcessingDigital Image Processing

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                                                                      Digital Image Processing

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                                                                      Image Sampling and Quantization

                                                                      - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                      scene

                                                                      converting a continuous image f to digital form

                                                                      - digitizing (x y) is called sampling

                                                                      - digitizing f(x y) is called quantization

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                                                                      Continuous image projected onto a sensor array Result of image sampling and quantization

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                                                                      Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                      (00) (01) (0 1)(10) (11) (1 1)

                                                                      ( )

                                                                      ( 10) ( 11) ( 1 1)

                                                                      f f f Nf f f N

                                                                      f x y

                                                                      f M f M f M N

                                                                      image element pixel

                                                                      00 01 0 1

                                                                      10 11 1 1

                                                                      10 11 1 1

                                                                      ( ) ( )

                                                                      N

                                                                      i jN M N

                                                                      i j

                                                                      M M M N

                                                                      a a aa f x i y j f i ja a a

                                                                      Aa

                                                                      a a a

                                                                      f(00) ndash the upper left corner of the image

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                                                                      M N ge 0 L=2k

                                                                      [0 1]i j i ja a L

                                                                      Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

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                                                                      Digital Image Processing

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                                                                      Number of bits required to store a digitized image

                                                                      for 2 b M N k M N b N k

                                                                      When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

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                                                                      Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                      Measures line pairs per unit distance dots (pixels) per unit distance

                                                                      Image resolution = the largest number of discernible line pairs per unit distance

                                                                      (eg 100 line pairs per mm)

                                                                      Dots per unit distance are commonly used in printing and publishing

                                                                      In US the measure is expressed in dots per inch (dpi)

                                                                      (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                      Intensity resolution ndash the smallest discernible change in intensity level

                                                                      The number of intensity levels (L) is determined by hardware considerations

                                                                      L=2k ndash most common k = 8

                                                                      Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                      Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                      150 dpi (lower left) 72 dpi (lower right)

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                                                                      Reducing the number of gray levels 256 128 64 32

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                                                                      Reducing the number of gray levels 16 8 4 2

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                                                                      Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                      Shrinking zooming ndash image resizing ndash image resampling methods

                                                                      Interpolation is the process of using known data to estimate values at unknown locations

                                                                      Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                      750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                      same spacing as the original and then shrink it so that it fits exactly over the original

                                                                      image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                      Problem assignment of intensity-level in the new 750 times 750 grid

                                                                      Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                      intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                      This technique has the tendency to produce undesirable effects like severe distortion of

                                                                      straight edges

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                                                                      Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                      where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                      be written using the 4 nearest neighbors of point (x y)

                                                                      Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                      modest increase in computational effort

                                                                      Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                      nearest neighbors of the point 3 3

                                                                      0 0

                                                                      ( ) i ji j

                                                                      i jv x y c x y

                                                                      The coefficients cij are obtained solving a 16x16 linear system

                                                                      intensity levels of the 16 nearest neighbors of 3 3

                                                                      0 0

                                                                      ( )i ji j

                                                                      i jc x y x y

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                                                                      Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                      bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                      programs such as Adobe Photoshop and Corel Photopaint

                                                                      Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                      the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                      then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                      neighbor interpolation was used (both for shrinking and zooming)

                                                                      Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                      interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                      from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                      Digital Image Processing

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                                                                      Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

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                                                                      Neighbors of a Pixel

                                                                      A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                      This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                      The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                      and are denoted ND(p)

                                                                      The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                      N8 (p)

                                                                      If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                      fall outside the image

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                                                                      Adjacency Connectivity Regions Boundaries

                                                                      Denote by V the set of intensity levels used to define adjacency

                                                                      - in a binary image V 01 (V=0 V=1)

                                                                      - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                      We consider 3 types of adjacency

                                                                      (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                      (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                      (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                      m-adjacent if

                                                                      4( )q N p or

                                                                      ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                      Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                      ambiguities that often arise when 8-adjacency is used Consider the example

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                                                                      binary image

                                                                      0 1 1 0 1 1 0 1 1

                                                                      1 0 1 0 0 1 0 0 1 0

                                                                      0 0 1 0 0 1 0 0 1

                                                                      V

                                                                      The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                      8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                      m-adjacency

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                                                                      A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                      is a sequence of distinct pixels with coordinates

                                                                      and are adjacent 0 0 1 1

                                                                      1 1

                                                                      ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                      n n

                                                                      i i i i

                                                                      x y x y x y x y s tx y x y i n

                                                                      The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                      Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                      Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                      in S if there exists a path between them consisting only of pixels from S

                                                                      S is a connected set if there is a path in S between any 2 pixels in S

                                                                      Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                      Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                      that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                      8-adjacency are considered

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                                                                      Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                      touches the image border

                                                                      the complement of 1

                                                                      ( )K

                                                                      cu k u u

                                                                      k

                                                                      R R R R

                                                                      We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                      background of the image

                                                                      The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                      points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                      region that have at least one background neighbor This definition is referred to as the

                                                                      inner border to distinguish it from the notion of outer border which is the corresponding

                                                                      border in the background

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                                                                      Distance measures

                                                                      For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                      function or metric if

                                                                      (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                      (b) D(p q) = D(q p)

                                                                      (c) D(p z) le D(p q) + D(q z)

                                                                      The Euclidean distance between p and q is defined as 1

                                                                      2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                      The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                      centered at (x y)

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                                                                      The D4 distance (also called city-block distance) between p and q is defined as

                                                                      4( ) | | | |D p q x s y t

                                                                      The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                      4

                                                                      22 1 2

                                                                      2 2 1 0 1 22 1 2

                                                                      2

                                                                      D

                                                                      The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                      The D8 distance (called the chessboard distance) between p and q is defined as

                                                                      8( ) max| | | |D p q x s y t

                                                                      The pixels q for which 8( )D p q r form a square centered at (x y)

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                                                                      8

                                                                      2 2 2 2 22 1 1 1 2

                                                                      2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                      D

                                                                      The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                      D4 and D8 distances are independent of any paths that might exist between p and q

                                                                      because these distances involve only the coordinates of the point

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                                                                      Array versus Matrix Operations

                                                                      An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                      11 12 11 12

                                                                      21 22 21 22

                                                                      a a b ba a b b

                                                                      Array product

                                                                      11 12 11 12 11 11 12 12

                                                                      21 22 21 22 21 21 22 21

                                                                      a a b b a b a ba a b b a b a b

                                                                      Matrix product

                                                                      11 12 11 12 11 11 12 21 11 12 12 21

                                                                      21 22 21 22 21 11 22 21 21 12 22 22

                                                                      a a b b a b a b a b a ba a b b a b a b a b a b

                                                                      We assume array operations unless stated otherwise

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                                                                      Linear versus Nonlinear Operations

                                                                      One of the most important classifications of image-processing methods is whether it is

                                                                      linear or nonlinear

                                                                      ( ) ( )H f x y g x y

                                                                      H is said to be a linear operator if

                                                                      images1 2 1 2

                                                                      1 2

                                                                      ( ) ( ) ( ) ( )

                                                                      H a f x y b f x y a H f x y b H f x y

                                                                      a b f f

                                                                      Example of nonlinear operator

                                                                      the maximum value of the pixels of image max ( )H f f x y f

                                                                      1 2

                                                                      0 2 6 5 1 1

                                                                      2 3 4 7f f a b

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                                                                      1 2

                                                                      0 2 6 5 6 3max max 1 ( 1) max 2

                                                                      2 3 4 7 2 4a f b f

                                                                      0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                      2 3 4 7

                                                                      Arithmetic Operations in Image Processing

                                                                      Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                      f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                      For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                      value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                      The two random variables are uncorrelated when their covariance is 0

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                                                                      Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                      used in image enhancement)

                                                                      1

                                                                      1( ) ( )K

                                                                      ii

                                                                      g x y g x yK

                                                                      If the noise satisfies the properties stated above we have

                                                                      2 2( ) ( )

                                                                      1( ) ( ) g x y x yE g x y f x yK

                                                                      ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                      and g respectively The standard deviation (square root of the variance) at any point in

                                                                      the average image is

                                                                      ( ) ( )1

                                                                      g x y x yK

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                                                                      As K increases the variability (as measured by the variance or the standard deviation) of

                                                                      the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                      means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                      averaging process increases

                                                                      An important application of image averaging is in the field of astronomy where imaging

                                                                      under very low light levels frequently causes sensor noise to render single images

                                                                      virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                      was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                      64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                      images respectively

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                                                                      Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                      100 noisy images

                                                                      a b c d e f

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                                                                      A frequent application of image subtraction is in the enhancement of differences between

                                                                      images

                                                                      (a) (b) (c)

                                                                      Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                      significant bit of each pixel (c) the difference between the two images

                                                                      Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                      Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                      images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                      difference between images (a) and (b)

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                                                                      Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                      h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                      intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                      source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                      bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                      anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                      images after injection of the contrast medium

                                                                      In g(x y) we can find the differences between h and f as enhanced detail

                                                                      Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                      propagates through the various arteries in the area being observed

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                                                                      a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

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                                                                      An important application of image multiplication (and division) is shading correction

                                                                      Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                      f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                      When the shading function is known

                                                                      ( )( )( )

                                                                      g x yf x yh x y

                                                                      h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                      approximation to the shading function by imaging a target of constant intensity When the

                                                                      sensor is not available often the shading pattern can be estimated from the image

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                                                                      Week 1

                                                                      (a) (b) (c)

                                                                      Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                      Digital Image Processing

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                                                                      Another use of image multiplication is in masking also called region of interest (ROI)

                                                                      operations The process consists of multiplying a given image by a mask image that has

                                                                      1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                      image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                      (a) (b) (c)

                                                                      Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

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                                                                      Week 1

                                                                      In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                      min( )mf f f

                                                                      0 ( 255)max( )

                                                                      ms

                                                                      m

                                                                      ff K K K

                                                                      f

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Spatial Operations

                                                                      - are performed directly on the pixels of a given image

                                                                      There are three categories of spatial operations

                                                                      single-pixel operations

                                                                      neighborhood operations

                                                                      geometric spatial transformations

                                                                      Single-pixel operations

                                                                      - change the values of intensity for the individual pixels ( )s T z

                                                                      where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                      corresponding pixel in the processed image

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Neighborhood operations

                                                                      Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                      in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                      based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                      rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                      intensity by computing the average value of the pixels in Sxy

                                                                      ( )

                                                                      1( ) ( )xyr c S

                                                                      g x y f r cm n

                                                                      The net effect is to perform local blurring in the original image This type of process is

                                                                      used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                      largest region of an image

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Geometric spatial transformations and image registration

                                                                      - modify the spatial relationship between pixels in an image

                                                                      - these transformations are often called rubber-sheet transformations (analogous to

                                                                      printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                      predefined set of rules

                                                                      A geometric transformation consists of 2 basic operations

                                                                      1 a spatial transformation of coordinates

                                                                      2 intensity interpolation that assign intensity values to the spatial transformed

                                                                      pixels

                                                                      The coordinate system transformation ( ) [( )]x y T v w

                                                                      (v w) ndash pixel coordinates in the original image

                                                                      (x y) ndash pixel coordinates in the transformed image

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                                                                      [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                      Affine transform

                                                                      11 1211 21 31

                                                                      21 2212 22 33

                                                                      31 32

                                                                      0[ 1] [ 1] [ 1] 0

                                                                      1

                                                                      t tx t v t w t

                                                                      x y v w T v w t ty t v t w t

                                                                      t t

                                                                      (AT)

                                                                      This transform can scale rotate translate or shear a set of coordinate points depending

                                                                      on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                      result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                      scaling rotation and translation matrices from Table 1

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Affine transformations

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                                                                      The preceding transformations relocate pixels on an image to new locations To complete

                                                                      the process we have to assign intensity values to those locations This task is done by

                                                                      using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                      In practice we can use equation (AT) in two basic ways

                                                                      forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                      location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                      Problems

                                                                      - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                      the same location in the output image

                                                                      - some output locations have no correspondent in the original image (no intensity

                                                                      assignment)

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                                                                      Week 1

                                                                      inverse mapping scans the output pixel locations and at each location (x y)

                                                                      computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                      It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                      pixel value

                                                                      Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                      numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                      Digital Image Processing

                                                                      Week 1

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                                                                      Image registration ndash align two or more images of the same scene

                                                                      In image registration we have available the input and output images but the specific

                                                                      transformation that produced the output image from the input is generally unknown

                                                                      The problem is to estimate the transformation function and then use it to register the two

                                                                      images

                                                                      - it may be of interest to align (register) two or more image taken at approximately the

                                                                      same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                      - align images of a given location taken by the same instrument at different moments

                                                                      of time (satellite images)

                                                                      Solving the problem using tie points (also called control points) which are

                                                                      corresponding points whose locations are known precisely in the input and reference

                                                                      image

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      How to select tie points

                                                                      - interactively selecting them

                                                                      - use of algorithms that try to detect these points

                                                                      - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                      the imaging sensors These objects produce a set of known points (called reseau

                                                                      marks) directly on all images captured by the system which can be used as guides

                                                                      for establishing tie points

                                                                      The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                      of 4 tie points both on the input image and the reference image A simple model based on

                                                                      a bilinear approximation is given by

                                                                      1 2 3 4

                                                                      5 6 7 8

                                                                      x c v c w c v w cy c v c w c v w c

                                                                      (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                      frequently is to select a larger number of tie points and using this new set of tie points

                                                                      subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                      subregions marked by 4 tie points we applied the transformation model described above

                                                                      The number of tie points and the sophistication of the model required to solve the register

                                                                      problem depend on the severity of the geometrical distortion

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Probabilistic Methods

                                                                      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                      p(zk) = the probability that the intensity level zk occurs in the given image

                                                                      ( ) kk

                                                                      np zM N

                                                                      nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                      pixels in the image) 1

                                                                      0( ) 1

                                                                      L

                                                                      kk

                                                                      p z

                                                                      The mean (average) intensity of an image is given by 1

                                                                      0( )

                                                                      L

                                                                      k kk

                                                                      m z p z

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      The variance of the intensities is 1

                                                                      2 2

                                                                      0( ) ( )

                                                                      L

                                                                      k kk

                                                                      z m p z

                                                                      The variance is a measure of the spread of the values of z about the mean so it is a

                                                                      measure of image contrast Usually for measuring image contrast the standard deviation

                                                                      ( ) is used

                                                                      The n-th moment of a random variable z about the mean is defined as 1

                                                                      0( ) ( ) ( )

                                                                      Ln

                                                                      n k kk

                                                                      z z m p z

                                                                      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                      3( ) 0z the intensities are biased to values higher than the mean

                                                                      ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                      mean

                                                                      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                      Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                      Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                      Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Intensity Transformations and Spatial Filtering

                                                                      ( ) ( )g x y T f x y

                                                                      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                      neighborhood of (x y)

                                                                      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                      and much smaller in size than the image

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                      called spatial filter (spatial mask kernel template or window)

                                                                      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                      ( )s T r

                                                                      s and r are denoting respectively the intensity of g and f at (x y)

                                                                      Figure 2 left - T produces an output image of higher contrast than the original by

                                                                      darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                      is called contrast stretching

                                                                      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                      thresholding function

                                                                      Some Basic Intensity Transformation Functions

                                                                      Image Negatives

                                                                      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                      - equivalent of a photographic negative

                                                                      - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                      image

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Original Negative image

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                      Some basic intensity transformation functions

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      This transformation maps a narrow range of low intensity values in the input into a wider

                                                                      range An operator of this type is used to expand the values of dark pixels in an image

                                                                      while compressing the higher-level values The opposite is true for the inverse log

                                                                      transformation The log functions compress the dynamic range of images with large

                                                                      variations in pixel values

                                                                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Power-Law (Gamma) Transformations

                                                                      - positive constants( ) ( ( ) )s T r c r c s c r

                                                                      Plots of gamma transformation for different values of γ (c=1)

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                      of output values with the opposite being true for higher values of input values The

                                                                      curves with 1 have the opposite effect of those generated with values of 1

                                                                      1c - identity transformation

                                                                      A variety of devices used for image capture printing and display respond according to a

                                                                      power law The process used to correct these power-law response phenomena is called

                                                                      gamma correction

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Piecewise-Linear Transformations Functions

                                                                      Contrast stretching

                                                                      - a process that expands the range of intensity levels in an image so it spans the full

                                                                      intensity range of the recording tool or display device

                                                                      a b c d Fig5

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      11

                                                                      1

                                                                      2 1 1 21 2

                                                                      2 1 2 1

                                                                      22

                                                                      2

                                                                      [0 ]

                                                                      ( ) ( )( ) [ ]( ) ( )

                                                                      ( 1 ) [ 1]( 1 )

                                                                      s r r rrs r r s r rT r r r r

                                                                      r r r rs L r r r L

                                                                      L r

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      1 1 2 2r s r s identity transformation (no change)

                                                                      1 2 1 2 0 1r r s s L thresholding function

                                                                      Figure 5(b) shows an 8-bit image with low contrast

                                                                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                      in the image respectively Thus the transformation function stretched the levels linearly

                                                                      from their original range to the full range [0 L-1]

                                                                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                      2 2 1r s m L where m is the mean gray level in the image

                                                                      The original image on which these results are based is a scanning electron microscope

                                                                      image of pollen magnified approximately 700 times

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Intensity-level slicing

                                                                      - highlighting a specific range of intensities in an image

                                                                      There are two approaches for intensity-level slicing

                                                                      1 display in one value (white for example) all the values in the range of interest and in

                                                                      another (say black) all other intensities (Figure 311 (a))

                                                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                      intensities in the image (Figure 311 (b))

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                      the top of the scale of intensities This type of enhancement produces a binary image

                                                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                      Highlights range [A B] and preserves all other intensities

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                      blockageshellip)

                                                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                      image around the mean intensity was set to black the other intensities remain unchanged

                                                                      Fig 6 - Aortic angiogram and intensity sliced versions

                                                                      Digital Image Processing

                                                                      Week 1

                                                                      Bit-plane slicing

                                                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                      This technique highlights the contribution made to the whole image appearances by each

                                                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

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                                                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                      • DIP 1 2017
                                                                      • DIP 02 (2017)

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        Images of the Crab Pulsar covering the electromagnetic spectrum

                                                                        Gamma X-ray Optical Infrared Radio

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        Other Imaging Modalities

                                                                        acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                                                        Imaging using sound geological explorations industry medicine

                                                                        Mineral and oil exploration

                                                                        For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

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                                                                        Biometry - iris

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                                                                        Biometry - fingerprint

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                                                                        Face detection and recognition

                                                                        Digital Image ProcessingDigital Image Processing

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                                                                        Gender identification

                                                                        Digital Image ProcessingDigital Image Processing

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                                                                        Image morphing

                                                                        Digital Image ProcessingDigital Image Processing

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                                                                        Fundamental Steps in DIP

                                                                        methods whose input and output are images

                                                                        methods whose inputs are images but whose outputs are attributes extracted from those images

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        Outputs are images

                                                                        bull image acquisition

                                                                        bull image filtering and enhancement

                                                                        bull image restoration

                                                                        bull color image processing

                                                                        bull wavelets and multiresolution processing

                                                                        bull compression

                                                                        bull morphological processing

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                                                                        Outputs are attributes

                                                                        bull morphological processing

                                                                        bull segmentation

                                                                        bull representation and description

                                                                        bull object recognition

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                                                                        Week 1Week 1

                                                                        Image acquisition - may involve preprocessing such as scaling

                                                                        Image enhancement

                                                                        bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                        bull enhancement is problem oriented

                                                                        bull there is no general sbquotheoryrsquo of image enhancement

                                                                        bull enhancement use subjective methods for image emprovement

                                                                        bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

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                                                                        Week 1Week 1

                                                                        Image restoration

                                                                        bull improving the appearance of an image

                                                                        bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                        Color image processing

                                                                        bull fundamental concept in color models

                                                                        bull basic color processing in a digital domain

                                                                        Wavelets and multiresolution processing

                                                                        representing images in various degree of resolution

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                                                                        Week 1Week 1

                                                                        Compression

                                                                        reducing the storage required to save an image or the bandwidth required to transmit it

                                                                        Morphological processing

                                                                        bull tools for extracting image components that are useful in the representation and description of shape

                                                                        bull a transition from processes that output images to processes that outputimage attributes

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        Segmentation

                                                                        bull partitioning an image into its constituents parts or objects

                                                                        bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                        bull the more accurate the segmentation the more likley recognition is to succeed

                                                                        Representation and description (almost always follows segmentation)

                                                                        bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                        bull converting the data produced by segmentation to a form suitable for computer processing

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                        bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                        bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                        Object recognition

                                                                        the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                        Knowledge database

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                                                                        Week 1Week 1

                                                                        Simplified diagramof a cross sectionof the human eye

                                                                        Digital Image ProcessingDigital Image Processing

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                                                                        Week 1Week 1

                                                                        Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                        The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                        Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                        The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                        The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                        Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                        Fovea = the place where the image of the object of interest falls on

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                                                                        Week 1Week 1

                                                                        Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                        Blind spot region without receptors

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        Image formation in the eye

                                                                        Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                        Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                        distance between lens and retina along visual axix = 17 mm

                                                                        range of focal length = 14 mm to 17 mm

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

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                                                                        Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        Optical illusions

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                        gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                        quantities that describe the quality of a chromatic light source radiance

                                                                        the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                        measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                        brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        the physical meaning is determined by the source of the image

                                                                        ( )f D f x y

                                                                        Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                        f(xy) ndash characterized by two components

                                                                        i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                        r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                        ( ) ( ) ( )

                                                                        0 ( ) 0 ( ) 1

                                                                        f x y i x y r x y

                                                                        i x y r x y

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                        i(xy) ndash determined by the illumination source

                                                                        r(xy) ndash determined by the characteristics of the imaged objects

                                                                        is called gray (or intensity) scale

                                                                        In practice

                                                                        min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                        indoor values without additional illuminationmin max10 1000L L

                                                                        black whitemin max0 1 0 1 0 1L L L L l l L

                                                                        min maxL L

                                                                        Digital Image ProcessingDigital Image Processing

                                                                        Week 1Week 1

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Image Sampling and Quantization

                                                                        - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                        scene

                                                                        converting a continuous image f to digital form

                                                                        - digitizing (x y) is called sampling

                                                                        - digitizing f(x y) is called quantization

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Digital Image Processing

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                                                                        Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                        (00) (01) (0 1)(10) (11) (1 1)

                                                                        ( )

                                                                        ( 10) ( 11) ( 1 1)

                                                                        f f f Nf f f N

                                                                        f x y

                                                                        f M f M f M N

                                                                        image element pixel

                                                                        00 01 0 1

                                                                        10 11 1 1

                                                                        10 11 1 1

                                                                        ( ) ( )

                                                                        N

                                                                        i jN M N

                                                                        i j

                                                                        M M M N

                                                                        a a aa f x i y j f i ja a a

                                                                        Aa

                                                                        a a a

                                                                        f(00) ndash the upper left corner of the image

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        M N ge 0 L=2k

                                                                        [0 1]i j i ja a L

                                                                        Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Number of bits required to store a digitized image

                                                                        for 2 b M N k M N b N k

                                                                        When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                        Measures line pairs per unit distance dots (pixels) per unit distance

                                                                        Image resolution = the largest number of discernible line pairs per unit distance

                                                                        (eg 100 line pairs per mm)

                                                                        Dots per unit distance are commonly used in printing and publishing

                                                                        In US the measure is expressed in dots per inch (dpi)

                                                                        (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                        Intensity resolution ndash the smallest discernible change in intensity level

                                                                        The number of intensity levels (L) is determined by hardware considerations

                                                                        L=2k ndash most common k = 8

                                                                        Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                        150 dpi (lower left) 72 dpi (lower right)

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Reducing the number of gray levels 256 128 64 32

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Reducing the number of gray levels 16 8 4 2

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                        Shrinking zooming ndash image resizing ndash image resampling methods

                                                                        Interpolation is the process of using known data to estimate values at unknown locations

                                                                        Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                        750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                        same spacing as the original and then shrink it so that it fits exactly over the original

                                                                        image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                        Problem assignment of intensity-level in the new 750 times 750 grid

                                                                        Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                        intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                        This technique has the tendency to produce undesirable effects like severe distortion of

                                                                        straight edges

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                        where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                        be written using the 4 nearest neighbors of point (x y)

                                                                        Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                        modest increase in computational effort

                                                                        Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                        nearest neighbors of the point 3 3

                                                                        0 0

                                                                        ( ) i ji j

                                                                        i jv x y c x y

                                                                        The coefficients cij are obtained solving a 16x16 linear system

                                                                        intensity levels of the 16 nearest neighbors of 3 3

                                                                        0 0

                                                                        ( )i ji j

                                                                        i jc x y x y

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                        bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                        programs such as Adobe Photoshop and Corel Photopaint

                                                                        Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                        the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                        then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                        neighbor interpolation was used (both for shrinking and zooming)

                                                                        Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                        interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                        from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Neighbors of a Pixel

                                                                        A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                        This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                        The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                        and are denoted ND(p)

                                                                        The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                        N8 (p)

                                                                        If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                        fall outside the image

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Adjacency Connectivity Regions Boundaries

                                                                        Denote by V the set of intensity levels used to define adjacency

                                                                        - in a binary image V 01 (V=0 V=1)

                                                                        - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                        We consider 3 types of adjacency

                                                                        (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                        (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                        (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                        m-adjacent if

                                                                        4( )q N p or

                                                                        ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                        Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                        ambiguities that often arise when 8-adjacency is used Consider the example

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        binary image

                                                                        0 1 1 0 1 1 0 1 1

                                                                        1 0 1 0 0 1 0 0 1 0

                                                                        0 0 1 0 0 1 0 0 1

                                                                        V

                                                                        The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                        8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                        m-adjacency

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                        is a sequence of distinct pixels with coordinates

                                                                        and are adjacent 0 0 1 1

                                                                        1 1

                                                                        ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                        n n

                                                                        i i i i

                                                                        x y x y x y x y s tx y x y i n

                                                                        The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                        Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                        Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                        in S if there exists a path between them consisting only of pixels from S

                                                                        S is a connected set if there is a path in S between any 2 pixels in S

                                                                        Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                        Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                        that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                        8-adjacency are considered

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                        touches the image border

                                                                        the complement of 1

                                                                        ( )K

                                                                        cu k u u

                                                                        k

                                                                        R R R R

                                                                        We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                        background of the image

                                                                        The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                        points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                        region that have at least one background neighbor This definition is referred to as the

                                                                        inner border to distinguish it from the notion of outer border which is the corresponding

                                                                        border in the background

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Distance measures

                                                                        For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                        function or metric if

                                                                        (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                        (b) D(p q) = D(q p)

                                                                        (c) D(p z) le D(p q) + D(q z)

                                                                        The Euclidean distance between p and q is defined as 1

                                                                        2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                        The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                        centered at (x y)

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        The D4 distance (also called city-block distance) between p and q is defined as

                                                                        4( ) | | | |D p q x s y t

                                                                        The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                        4

                                                                        22 1 2

                                                                        2 2 1 0 1 22 1 2

                                                                        2

                                                                        D

                                                                        The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                        The D8 distance (called the chessboard distance) between p and q is defined as

                                                                        8( ) max| | | |D p q x s y t

                                                                        The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        8

                                                                        2 2 2 2 22 1 1 1 2

                                                                        2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                        D

                                                                        The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                        D4 and D8 distances are independent of any paths that might exist between p and q

                                                                        because these distances involve only the coordinates of the point

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Array versus Matrix Operations

                                                                        An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                        11 12 11 12

                                                                        21 22 21 22

                                                                        a a b ba a b b

                                                                        Array product

                                                                        11 12 11 12 11 11 12 12

                                                                        21 22 21 22 21 21 22 21

                                                                        a a b b a b a ba a b b a b a b

                                                                        Matrix product

                                                                        11 12 11 12 11 11 12 21 11 12 12 21

                                                                        21 22 21 22 21 11 22 21 21 12 22 22

                                                                        a a b b a b a b a b a ba a b b a b a b a b a b

                                                                        We assume array operations unless stated otherwise

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Linear versus Nonlinear Operations

                                                                        One of the most important classifications of image-processing methods is whether it is

                                                                        linear or nonlinear

                                                                        ( ) ( )H f x y g x y

                                                                        H is said to be a linear operator if

                                                                        images1 2 1 2

                                                                        1 2

                                                                        ( ) ( ) ( ) ( )

                                                                        H a f x y b f x y a H f x y b H f x y

                                                                        a b f f

                                                                        Example of nonlinear operator

                                                                        the maximum value of the pixels of image max ( )H f f x y f

                                                                        1 2

                                                                        0 2 6 5 1 1

                                                                        2 3 4 7f f a b

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        1 2

                                                                        0 2 6 5 6 3max max 1 ( 1) max 2

                                                                        2 3 4 7 2 4a f b f

                                                                        0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                        2 3 4 7

                                                                        Arithmetic Operations in Image Processing

                                                                        Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                        f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                        For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                        value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                        The two random variables are uncorrelated when their covariance is 0

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                        used in image enhancement)

                                                                        1

                                                                        1( ) ( )K

                                                                        ii

                                                                        g x y g x yK

                                                                        If the noise satisfies the properties stated above we have

                                                                        2 2( ) ( )

                                                                        1( ) ( ) g x y x yE g x y f x yK

                                                                        ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                        and g respectively The standard deviation (square root of the variance) at any point in

                                                                        the average image is

                                                                        ( ) ( )1

                                                                        g x y x yK

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        As K increases the variability (as measured by the variance or the standard deviation) of

                                                                        the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                        means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                        averaging process increases

                                                                        An important application of image averaging is in the field of astronomy where imaging

                                                                        under very low light levels frequently causes sensor noise to render single images

                                                                        virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                        was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                        64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                        images respectively

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                        100 noisy images

                                                                        a b c d e f

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        A frequent application of image subtraction is in the enhancement of differences between

                                                                        images

                                                                        (a) (b) (c)

                                                                        Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                        significant bit of each pixel (c) the difference between the two images

                                                                        Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                        Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                        images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                        difference between images (a) and (b)

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                        h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                        intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                        source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                        bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                        anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                        images after injection of the contrast medium

                                                                        In g(x y) we can find the differences between h and f as enhanced detail

                                                                        Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                        propagates through the various arteries in the area being observed

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        An important application of image multiplication (and division) is shading correction

                                                                        Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                        f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                        When the shading function is known

                                                                        ( )( )( )

                                                                        g x yf x yh x y

                                                                        h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                        approximation to the shading function by imaging a target of constant intensity When the

                                                                        sensor is not available often the shading pattern can be estimated from the image

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        (a) (b) (c)

                                                                        Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Another use of image multiplication is in masking also called region of interest (ROI)

                                                                        operations The process consists of multiplying a given image by a mask image that has

                                                                        1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                        image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                        (a) (b) (c)

                                                                        Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                        min( )mf f f

                                                                        0 ( 255)max( )

                                                                        ms

                                                                        m

                                                                        ff K K K

                                                                        f

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Spatial Operations

                                                                        - are performed directly on the pixels of a given image

                                                                        There are three categories of spatial operations

                                                                        single-pixel operations

                                                                        neighborhood operations

                                                                        geometric spatial transformations

                                                                        Single-pixel operations

                                                                        - change the values of intensity for the individual pixels ( )s T z

                                                                        where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                        corresponding pixel in the processed image

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Neighborhood operations

                                                                        Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                        in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                        based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                        rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                        intensity by computing the average value of the pixels in Sxy

                                                                        ( )

                                                                        1( ) ( )xyr c S

                                                                        g x y f r cm n

                                                                        The net effect is to perform local blurring in the original image This type of process is

                                                                        used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                        largest region of an image

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Geometric spatial transformations and image registration

                                                                        - modify the spatial relationship between pixels in an image

                                                                        - these transformations are often called rubber-sheet transformations (analogous to

                                                                        printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                        predefined set of rules

                                                                        A geometric transformation consists of 2 basic operations

                                                                        1 a spatial transformation of coordinates

                                                                        2 intensity interpolation that assign intensity values to the spatial transformed

                                                                        pixels

                                                                        The coordinate system transformation ( ) [( )]x y T v w

                                                                        (v w) ndash pixel coordinates in the original image

                                                                        (x y) ndash pixel coordinates in the transformed image

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                        Affine transform

                                                                        11 1211 21 31

                                                                        21 2212 22 33

                                                                        31 32

                                                                        0[ 1] [ 1] [ 1] 0

                                                                        1

                                                                        t tx t v t w t

                                                                        x y v w T v w t ty t v t w t

                                                                        t t

                                                                        (AT)

                                                                        This transform can scale rotate translate or shear a set of coordinate points depending

                                                                        on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                        result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                        scaling rotation and translation matrices from Table 1

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Affine transformations

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        The preceding transformations relocate pixels on an image to new locations To complete

                                                                        the process we have to assign intensity values to those locations This task is done by

                                                                        using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                        In practice we can use equation (AT) in two basic ways

                                                                        forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                        location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                        Problems

                                                                        - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                        the same location in the output image

                                                                        - some output locations have no correspondent in the original image (no intensity

                                                                        assignment)

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        inverse mapping scans the output pixel locations and at each location (x y)

                                                                        computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                        It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                        pixel value

                                                                        Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                        numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Image registration ndash align two or more images of the same scene

                                                                        In image registration we have available the input and output images but the specific

                                                                        transformation that produced the output image from the input is generally unknown

                                                                        The problem is to estimate the transformation function and then use it to register the two

                                                                        images

                                                                        - it may be of interest to align (register) two or more image taken at approximately the

                                                                        same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                        - align images of a given location taken by the same instrument at different moments

                                                                        of time (satellite images)

                                                                        Solving the problem using tie points (also called control points) which are

                                                                        corresponding points whose locations are known precisely in the input and reference

                                                                        image

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        How to select tie points

                                                                        - interactively selecting them

                                                                        - use of algorithms that try to detect these points

                                                                        - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                        the imaging sensors These objects produce a set of known points (called reseau

                                                                        marks) directly on all images captured by the system which can be used as guides

                                                                        for establishing tie points

                                                                        The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                        of 4 tie points both on the input image and the reference image A simple model based on

                                                                        a bilinear approximation is given by

                                                                        1 2 3 4

                                                                        5 6 7 8

                                                                        x c v c w c v w cy c v c w c v w c

                                                                        (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                        frequently is to select a larger number of tie points and using this new set of tie points

                                                                        subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                        subregions marked by 4 tie points we applied the transformation model described above

                                                                        The number of tie points and the sophistication of the model required to solve the register

                                                                        problem depend on the severity of the geometrical distortion

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Probabilistic Methods

                                                                        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                        p(zk) = the probability that the intensity level zk occurs in the given image

                                                                        ( ) kk

                                                                        np zM N

                                                                        nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                        pixels in the image) 1

                                                                        0( ) 1

                                                                        L

                                                                        kk

                                                                        p z

                                                                        The mean (average) intensity of an image is given by 1

                                                                        0( )

                                                                        L

                                                                        k kk

                                                                        m z p z

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        The variance of the intensities is 1

                                                                        2 2

                                                                        0( ) ( )

                                                                        L

                                                                        k kk

                                                                        z m p z

                                                                        The variance is a measure of the spread of the values of z about the mean so it is a

                                                                        measure of image contrast Usually for measuring image contrast the standard deviation

                                                                        ( ) is used

                                                                        The n-th moment of a random variable z about the mean is defined as 1

                                                                        0( ) ( ) ( )

                                                                        Ln

                                                                        n k kk

                                                                        z z m p z

                                                                        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                        3( ) 0z the intensities are biased to values higher than the mean

                                                                        ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                        mean

                                                                        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                        Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                        Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                        Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Intensity Transformations and Spatial Filtering

                                                                        ( ) ( )g x y T f x y

                                                                        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                        neighborhood of (x y)

                                                                        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                        and much smaller in size than the image

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                        called spatial filter (spatial mask kernel template or window)

                                                                        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                        ( )s T r

                                                                        s and r are denoting respectively the intensity of g and f at (x y)

                                                                        Figure 2 left - T produces an output image of higher contrast than the original by

                                                                        darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                        is called contrast stretching

                                                                        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                        thresholding function

                                                                        Some Basic Intensity Transformation Functions

                                                                        Image Negatives

                                                                        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                        - equivalent of a photographic negative

                                                                        - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                        image

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Original Negative image

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                        Some basic intensity transformation functions

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        This transformation maps a narrow range of low intensity values in the input into a wider

                                                                        range An operator of this type is used to expand the values of dark pixels in an image

                                                                        while compressing the higher-level values The opposite is true for the inverse log

                                                                        transformation The log functions compress the dynamic range of images with large

                                                                        variations in pixel values

                                                                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Power-Law (Gamma) Transformations

                                                                        - positive constants( ) ( ( ) )s T r c r c s c r

                                                                        Plots of gamma transformation for different values of γ (c=1)

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                        of output values with the opposite being true for higher values of input values The

                                                                        curves with 1 have the opposite effect of those generated with values of 1

                                                                        1c - identity transformation

                                                                        A variety of devices used for image capture printing and display respond according to a

                                                                        power law The process used to correct these power-law response phenomena is called

                                                                        gamma correction

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Piecewise-Linear Transformations Functions

                                                                        Contrast stretching

                                                                        - a process that expands the range of intensity levels in an image so it spans the full

                                                                        intensity range of the recording tool or display device

                                                                        a b c d Fig5

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        11

                                                                        1

                                                                        2 1 1 21 2

                                                                        2 1 2 1

                                                                        22

                                                                        2

                                                                        [0 ]

                                                                        ( ) ( )( ) [ ]( ) ( )

                                                                        ( 1 ) [ 1]( 1 )

                                                                        s r r rrs r r s r rT r r r r

                                                                        r r r rs L r r r L

                                                                        L r

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        1 1 2 2r s r s identity transformation (no change)

                                                                        1 2 1 2 0 1r r s s L thresholding function

                                                                        Figure 5(b) shows an 8-bit image with low contrast

                                                                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                        in the image respectively Thus the transformation function stretched the levels linearly

                                                                        from their original range to the full range [0 L-1]

                                                                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                        2 2 1r s m L where m is the mean gray level in the image

                                                                        The original image on which these results are based is a scanning electron microscope

                                                                        image of pollen magnified approximately 700 times

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Intensity-level slicing

                                                                        - highlighting a specific range of intensities in an image

                                                                        There are two approaches for intensity-level slicing

                                                                        1 display in one value (white for example) all the values in the range of interest and in

                                                                        another (say black) all other intensities (Figure 311 (a))

                                                                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                        intensities in the image (Figure 311 (b))

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                        the top of the scale of intensities This type of enhancement produces a binary image

                                                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                        Highlights range [A B] and preserves all other intensities

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                        blockageshellip)

                                                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                        image around the mean intensity was set to black the other intensities remain unchanged

                                                                        Fig 6 - Aortic angiogram and intensity sliced versions

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Bit-plane slicing

                                                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                        This technique highlights the contribution made to the whole image appearances by each

                                                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        Digital Image Processing

                                                                        Week 1

                                                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                        • DIP 1 2017
                                                                        • DIP 02 (2017)

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Other Imaging Modalities

                                                                          acoustic imaging electron microscopy synthetic (computer-generated) imaging

                                                                          Imaging using sound geological explorations industry medicine

                                                                          Mineral and oil exploration

                                                                          For image acquisition over land one of the main approaches is to use a large truck an a large flat steel plate The plate is pressed on the ground by the truck and the truck is vibratedthrough a frequency spectrum up to 100 Hz The strength and the speed of the returning sound waves are determined by the composition of the Earth below the surface These are analysed by a computer and images are generated from the resulting analysis

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Biometry - iris

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Biometry - fingerprint

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Face detection and recognition

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Gender identification

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Image morphing

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Fundamental Steps in DIP

                                                                          methods whose input and output are images

                                                                          methods whose inputs are images but whose outputs are attributes extracted from those images

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Outputs are images

                                                                          bull image acquisition

                                                                          bull image filtering and enhancement

                                                                          bull image restoration

                                                                          bull color image processing

                                                                          bull wavelets and multiresolution processing

                                                                          bull compression

                                                                          bull morphological processing

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Outputs are attributes

                                                                          bull morphological processing

                                                                          bull segmentation

                                                                          bull representation and description

                                                                          bull object recognition

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Image acquisition - may involve preprocessing such as scaling

                                                                          Image enhancement

                                                                          bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                          bull enhancement is problem oriented

                                                                          bull there is no general sbquotheoryrsquo of image enhancement

                                                                          bull enhancement use subjective methods for image emprovement

                                                                          bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

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                                                                          Image restoration

                                                                          bull improving the appearance of an image

                                                                          bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                          Color image processing

                                                                          bull fundamental concept in color models

                                                                          bull basic color processing in a digital domain

                                                                          Wavelets and multiresolution processing

                                                                          representing images in various degree of resolution

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                                                                          Week 1Week 1

                                                                          Compression

                                                                          reducing the storage required to save an image or the bandwidth required to transmit it

                                                                          Morphological processing

                                                                          bull tools for extracting image components that are useful in the representation and description of shape

                                                                          bull a transition from processes that output images to processes that outputimage attributes

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                                                                          Week 1Week 1

                                                                          Segmentation

                                                                          bull partitioning an image into its constituents parts or objects

                                                                          bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                          bull the more accurate the segmentation the more likley recognition is to succeed

                                                                          Representation and description (almost always follows segmentation)

                                                                          bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                          bull converting the data produced by segmentation to a form suitable for computer processing

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                                                                          Week 1Week 1

                                                                          bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                          bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                          bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                          Object recognition

                                                                          the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                          Knowledge database

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Simplified diagramof a cross sectionof the human eye

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

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                                                                          Week 1Week 1

                                                                          Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                          The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                          Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                          The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                          The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                          Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                          Fovea = the place where the image of the object of interest falls on

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                                                                          Week 1Week 1

                                                                          Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                          Blind spot region without receptors

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Image formation in the eye

                                                                          Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                          Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                          distance between lens and retina along visual axix = 17 mm

                                                                          range of focal length = 14 mm to 17 mm

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Digital Image ProcessingDigital Image Processing

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                                                                          Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Optical illusions

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                          gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                          quantities that describe the quality of a chromatic light source radiance

                                                                          the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                          measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                          brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          the physical meaning is determined by the source of the image

                                                                          ( )f D f x y

                                                                          Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                          f(xy) ndash characterized by two components

                                                                          i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                          r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                          ( ) ( ) ( )

                                                                          0 ( ) 0 ( ) 1

                                                                          f x y i x y r x y

                                                                          i x y r x y

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                                                                          Week 1Week 1

                                                                          r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                          i(xy) ndash determined by the illumination source

                                                                          r(xy) ndash determined by the characteristics of the imaged objects

                                                                          is called gray (or intensity) scale

                                                                          In practice

                                                                          min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                          indoor values without additional illuminationmin max10 1000L L

                                                                          black whitemin max0 1 0 1 0 1L L L L l l L

                                                                          min maxL L

                                                                          Digital Image ProcessingDigital Image Processing

                                                                          Week 1Week 1

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Image Sampling and Quantization

                                                                          - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                          scene

                                                                          converting a continuous image f to digital form

                                                                          - digitizing (x y) is called sampling

                                                                          - digitizing f(x y) is called quantization

                                                                          Digital Image Processing

                                                                          Week 1

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                                                                          Week 1

                                                                          Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                          (00) (01) (0 1)(10) (11) (1 1)

                                                                          ( )

                                                                          ( 10) ( 11) ( 1 1)

                                                                          f f f Nf f f N

                                                                          f x y

                                                                          f M f M f M N

                                                                          image element pixel

                                                                          00 01 0 1

                                                                          10 11 1 1

                                                                          10 11 1 1

                                                                          ( ) ( )

                                                                          N

                                                                          i jN M N

                                                                          i j

                                                                          M M M N

                                                                          a a aa f x i y j f i ja a a

                                                                          Aa

                                                                          a a a

                                                                          f(00) ndash the upper left corner of the image

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          M N ge 0 L=2k

                                                                          [0 1]i j i ja a L

                                                                          Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                          Digital Image Processing

                                                                          Week 1

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                                                                          Week 1

                                                                          Number of bits required to store a digitized image

                                                                          for 2 b M N k M N b N k

                                                                          When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                          Measures line pairs per unit distance dots (pixels) per unit distance

                                                                          Image resolution = the largest number of discernible line pairs per unit distance

                                                                          (eg 100 line pairs per mm)

                                                                          Dots per unit distance are commonly used in printing and publishing

                                                                          In US the measure is expressed in dots per inch (dpi)

                                                                          (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                          Intensity resolution ndash the smallest discernible change in intensity level

                                                                          The number of intensity levels (L) is determined by hardware considerations

                                                                          L=2k ndash most common k = 8

                                                                          Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                          150 dpi (lower left) 72 dpi (lower right)

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Reducing the number of gray levels 256 128 64 32

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Reducing the number of gray levels 16 8 4 2

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                          Shrinking zooming ndash image resizing ndash image resampling methods

                                                                          Interpolation is the process of using known data to estimate values at unknown locations

                                                                          Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                          750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                          same spacing as the original and then shrink it so that it fits exactly over the original

                                                                          image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                          Problem assignment of intensity-level in the new 750 times 750 grid

                                                                          Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                          intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                          This technique has the tendency to produce undesirable effects like severe distortion of

                                                                          straight edges

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                          where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                          be written using the 4 nearest neighbors of point (x y)

                                                                          Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                          modest increase in computational effort

                                                                          Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                          nearest neighbors of the point 3 3

                                                                          0 0

                                                                          ( ) i ji j

                                                                          i jv x y c x y

                                                                          The coefficients cij are obtained solving a 16x16 linear system

                                                                          intensity levels of the 16 nearest neighbors of 3 3

                                                                          0 0

                                                                          ( )i ji j

                                                                          i jc x y x y

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                          bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                          programs such as Adobe Photoshop and Corel Photopaint

                                                                          Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                          the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                          then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                          neighbor interpolation was used (both for shrinking and zooming)

                                                                          Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                          interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                          from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Neighbors of a Pixel

                                                                          A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                          This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                          The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                          and are denoted ND(p)

                                                                          The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                          N8 (p)

                                                                          If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                          fall outside the image

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Adjacency Connectivity Regions Boundaries

                                                                          Denote by V the set of intensity levels used to define adjacency

                                                                          - in a binary image V 01 (V=0 V=1)

                                                                          - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                          We consider 3 types of adjacency

                                                                          (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                          (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                          (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                          m-adjacent if

                                                                          4( )q N p or

                                                                          ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                          Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                          ambiguities that often arise when 8-adjacency is used Consider the example

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          binary image

                                                                          0 1 1 0 1 1 0 1 1

                                                                          1 0 1 0 0 1 0 0 1 0

                                                                          0 0 1 0 0 1 0 0 1

                                                                          V

                                                                          The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                          8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                          m-adjacency

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                          is a sequence of distinct pixels with coordinates

                                                                          and are adjacent 0 0 1 1

                                                                          1 1

                                                                          ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                          n n

                                                                          i i i i

                                                                          x y x y x y x y s tx y x y i n

                                                                          The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                          Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                          Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                          in S if there exists a path between them consisting only of pixels from S

                                                                          S is a connected set if there is a path in S between any 2 pixels in S

                                                                          Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                          Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                          that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                          8-adjacency are considered

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                          touches the image border

                                                                          the complement of 1

                                                                          ( )K

                                                                          cu k u u

                                                                          k

                                                                          R R R R

                                                                          We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                          background of the image

                                                                          The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                          points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                          region that have at least one background neighbor This definition is referred to as the

                                                                          inner border to distinguish it from the notion of outer border which is the corresponding

                                                                          border in the background

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Distance measures

                                                                          For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                          function or metric if

                                                                          (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                          (b) D(p q) = D(q p)

                                                                          (c) D(p z) le D(p q) + D(q z)

                                                                          The Euclidean distance between p and q is defined as 1

                                                                          2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                          The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                          centered at (x y)

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          The D4 distance (also called city-block distance) between p and q is defined as

                                                                          4( ) | | | |D p q x s y t

                                                                          The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                          4

                                                                          22 1 2

                                                                          2 2 1 0 1 22 1 2

                                                                          2

                                                                          D

                                                                          The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                          The D8 distance (called the chessboard distance) between p and q is defined as

                                                                          8( ) max| | | |D p q x s y t

                                                                          The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          8

                                                                          2 2 2 2 22 1 1 1 2

                                                                          2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                          D

                                                                          The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                          D4 and D8 distances are independent of any paths that might exist between p and q

                                                                          because these distances involve only the coordinates of the point

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Array versus Matrix Operations

                                                                          An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                          11 12 11 12

                                                                          21 22 21 22

                                                                          a a b ba a b b

                                                                          Array product

                                                                          11 12 11 12 11 11 12 12

                                                                          21 22 21 22 21 21 22 21

                                                                          a a b b a b a ba a b b a b a b

                                                                          Matrix product

                                                                          11 12 11 12 11 11 12 21 11 12 12 21

                                                                          21 22 21 22 21 11 22 21 21 12 22 22

                                                                          a a b b a b a b a b a ba a b b a b a b a b a b

                                                                          We assume array operations unless stated otherwise

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Linear versus Nonlinear Operations

                                                                          One of the most important classifications of image-processing methods is whether it is

                                                                          linear or nonlinear

                                                                          ( ) ( )H f x y g x y

                                                                          H is said to be a linear operator if

                                                                          images1 2 1 2

                                                                          1 2

                                                                          ( ) ( ) ( ) ( )

                                                                          H a f x y b f x y a H f x y b H f x y

                                                                          a b f f

                                                                          Example of nonlinear operator

                                                                          the maximum value of the pixels of image max ( )H f f x y f

                                                                          1 2

                                                                          0 2 6 5 1 1

                                                                          2 3 4 7f f a b

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          1 2

                                                                          0 2 6 5 6 3max max 1 ( 1) max 2

                                                                          2 3 4 7 2 4a f b f

                                                                          0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                          2 3 4 7

                                                                          Arithmetic Operations in Image Processing

                                                                          Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                          f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                          For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                          value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                          The two random variables are uncorrelated when their covariance is 0

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                          used in image enhancement)

                                                                          1

                                                                          1( ) ( )K

                                                                          ii

                                                                          g x y g x yK

                                                                          If the noise satisfies the properties stated above we have

                                                                          2 2( ) ( )

                                                                          1( ) ( ) g x y x yE g x y f x yK

                                                                          ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                          and g respectively The standard deviation (square root of the variance) at any point in

                                                                          the average image is

                                                                          ( ) ( )1

                                                                          g x y x yK

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          As K increases the variability (as measured by the variance or the standard deviation) of

                                                                          the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                          means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                          averaging process increases

                                                                          An important application of image averaging is in the field of astronomy where imaging

                                                                          under very low light levels frequently causes sensor noise to render single images

                                                                          virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                          was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                          64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                          images respectively

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                          100 noisy images

                                                                          a b c d e f

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          A frequent application of image subtraction is in the enhancement of differences between

                                                                          images

                                                                          (a) (b) (c)

                                                                          Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                          significant bit of each pixel (c) the difference between the two images

                                                                          Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                          Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                          images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                          difference between images (a) and (b)

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                          h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                          intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                          source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                          bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                          anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                          images after injection of the contrast medium

                                                                          In g(x y) we can find the differences between h and f as enhanced detail

                                                                          Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                          propagates through the various arteries in the area being observed

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          An important application of image multiplication (and division) is shading correction

                                                                          Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                          f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                          When the shading function is known

                                                                          ( )( )( )

                                                                          g x yf x yh x y

                                                                          h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                          approximation to the shading function by imaging a target of constant intensity When the

                                                                          sensor is not available often the shading pattern can be estimated from the image

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          (a) (b) (c)

                                                                          Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Another use of image multiplication is in masking also called region of interest (ROI)

                                                                          operations The process consists of multiplying a given image by a mask image that has

                                                                          1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                          image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                          (a) (b) (c)

                                                                          Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                          min( )mf f f

                                                                          0 ( 255)max( )

                                                                          ms

                                                                          m

                                                                          ff K K K

                                                                          f

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Spatial Operations

                                                                          - are performed directly on the pixels of a given image

                                                                          There are three categories of spatial operations

                                                                          single-pixel operations

                                                                          neighborhood operations

                                                                          geometric spatial transformations

                                                                          Single-pixel operations

                                                                          - change the values of intensity for the individual pixels ( )s T z

                                                                          where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                          corresponding pixel in the processed image

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Neighborhood operations

                                                                          Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                          in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                          based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                          rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                          intensity by computing the average value of the pixels in Sxy

                                                                          ( )

                                                                          1( ) ( )xyr c S

                                                                          g x y f r cm n

                                                                          The net effect is to perform local blurring in the original image This type of process is

                                                                          used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                          largest region of an image

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Geometric spatial transformations and image registration

                                                                          - modify the spatial relationship between pixels in an image

                                                                          - these transformations are often called rubber-sheet transformations (analogous to

                                                                          printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                          predefined set of rules

                                                                          A geometric transformation consists of 2 basic operations

                                                                          1 a spatial transformation of coordinates

                                                                          2 intensity interpolation that assign intensity values to the spatial transformed

                                                                          pixels

                                                                          The coordinate system transformation ( ) [( )]x y T v w

                                                                          (v w) ndash pixel coordinates in the original image

                                                                          (x y) ndash pixel coordinates in the transformed image

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                          Affine transform

                                                                          11 1211 21 31

                                                                          21 2212 22 33

                                                                          31 32

                                                                          0[ 1] [ 1] [ 1] 0

                                                                          1

                                                                          t tx t v t w t

                                                                          x y v w T v w t ty t v t w t

                                                                          t t

                                                                          (AT)

                                                                          This transform can scale rotate translate or shear a set of coordinate points depending

                                                                          on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                          result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                          scaling rotation and translation matrices from Table 1

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Affine transformations

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          The preceding transformations relocate pixels on an image to new locations To complete

                                                                          the process we have to assign intensity values to those locations This task is done by

                                                                          using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                          In practice we can use equation (AT) in two basic ways

                                                                          forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                          location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                          Problems

                                                                          - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                          the same location in the output image

                                                                          - some output locations have no correspondent in the original image (no intensity

                                                                          assignment)

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          inverse mapping scans the output pixel locations and at each location (x y)

                                                                          computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                          It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                          pixel value

                                                                          Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                          numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Image registration ndash align two or more images of the same scene

                                                                          In image registration we have available the input and output images but the specific

                                                                          transformation that produced the output image from the input is generally unknown

                                                                          The problem is to estimate the transformation function and then use it to register the two

                                                                          images

                                                                          - it may be of interest to align (register) two or more image taken at approximately the

                                                                          same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                          - align images of a given location taken by the same instrument at different moments

                                                                          of time (satellite images)

                                                                          Solving the problem using tie points (also called control points) which are

                                                                          corresponding points whose locations are known precisely in the input and reference

                                                                          image

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          How to select tie points

                                                                          - interactively selecting them

                                                                          - use of algorithms that try to detect these points

                                                                          - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                          the imaging sensors These objects produce a set of known points (called reseau

                                                                          marks) directly on all images captured by the system which can be used as guides

                                                                          for establishing tie points

                                                                          The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                          of 4 tie points both on the input image and the reference image A simple model based on

                                                                          a bilinear approximation is given by

                                                                          1 2 3 4

                                                                          5 6 7 8

                                                                          x c v c w c v w cy c v c w c v w c

                                                                          (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                          frequently is to select a larger number of tie points and using this new set of tie points

                                                                          subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                          subregions marked by 4 tie points we applied the transformation model described above

                                                                          The number of tie points and the sophistication of the model required to solve the register

                                                                          problem depend on the severity of the geometrical distortion

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Probabilistic Methods

                                                                          zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                          p(zk) = the probability that the intensity level zk occurs in the given image

                                                                          ( ) kk

                                                                          np zM N

                                                                          nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                          pixels in the image) 1

                                                                          0( ) 1

                                                                          L

                                                                          kk

                                                                          p z

                                                                          The mean (average) intensity of an image is given by 1

                                                                          0( )

                                                                          L

                                                                          k kk

                                                                          m z p z

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          The variance of the intensities is 1

                                                                          2 2

                                                                          0( ) ( )

                                                                          L

                                                                          k kk

                                                                          z m p z

                                                                          The variance is a measure of the spread of the values of z about the mean so it is a

                                                                          measure of image contrast Usually for measuring image contrast the standard deviation

                                                                          ( ) is used

                                                                          The n-th moment of a random variable z about the mean is defined as 1

                                                                          0( ) ( ) ( )

                                                                          Ln

                                                                          n k kk

                                                                          z z m p z

                                                                          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                          3( ) 0z the intensities are biased to values higher than the mean

                                                                          ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                          mean

                                                                          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                          Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                          Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                          Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Intensity Transformations and Spatial Filtering

                                                                          ( ) ( )g x y T f x y

                                                                          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                          neighborhood of (x y)

                                                                          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                          and much smaller in size than the image

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                          called spatial filter (spatial mask kernel template or window)

                                                                          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                          ( )s T r

                                                                          s and r are denoting respectively the intensity of g and f at (x y)

                                                                          Figure 2 left - T produces an output image of higher contrast than the original by

                                                                          darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                          is called contrast stretching

                                                                          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                          thresholding function

                                                                          Some Basic Intensity Transformation Functions

                                                                          Image Negatives

                                                                          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                          - equivalent of a photographic negative

                                                                          - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                          image

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Original Negative image

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                          Some basic intensity transformation functions

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          This transformation maps a narrow range of low intensity values in the input into a wider

                                                                          range An operator of this type is used to expand the values of dark pixels in an image

                                                                          while compressing the higher-level values The opposite is true for the inverse log

                                                                          transformation The log functions compress the dynamic range of images with large

                                                                          variations in pixel values

                                                                          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Power-Law (Gamma) Transformations

                                                                          - positive constants( ) ( ( ) )s T r c r c s c r

                                                                          Plots of gamma transformation for different values of γ (c=1)

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                          of output values with the opposite being true for higher values of input values The

                                                                          curves with 1 have the opposite effect of those generated with values of 1

                                                                          1c - identity transformation

                                                                          A variety of devices used for image capture printing and display respond according to a

                                                                          power law The process used to correct these power-law response phenomena is called

                                                                          gamma correction

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Piecewise-Linear Transformations Functions

                                                                          Contrast stretching

                                                                          - a process that expands the range of intensity levels in an image so it spans the full

                                                                          intensity range of the recording tool or display device

                                                                          a b c d Fig5

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          11

                                                                          1

                                                                          2 1 1 21 2

                                                                          2 1 2 1

                                                                          22

                                                                          2

                                                                          [0 ]

                                                                          ( ) ( )( ) [ ]( ) ( )

                                                                          ( 1 ) [ 1]( 1 )

                                                                          s r r rrs r r s r rT r r r r

                                                                          r r r rs L r r r L

                                                                          L r

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          1 1 2 2r s r s identity transformation (no change)

                                                                          1 2 1 2 0 1r r s s L thresholding function

                                                                          Figure 5(b) shows an 8-bit image with low contrast

                                                                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                          in the image respectively Thus the transformation function stretched the levels linearly

                                                                          from their original range to the full range [0 L-1]

                                                                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                          2 2 1r s m L where m is the mean gray level in the image

                                                                          The original image on which these results are based is a scanning electron microscope

                                                                          image of pollen magnified approximately 700 times

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Intensity-level slicing

                                                                          - highlighting a specific range of intensities in an image

                                                                          There are two approaches for intensity-level slicing

                                                                          1 display in one value (white for example) all the values in the range of interest and in

                                                                          another (say black) all other intensities (Figure 311 (a))

                                                                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                          intensities in the image (Figure 311 (b))

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                          the top of the scale of intensities This type of enhancement produces a binary image

                                                                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                          Highlights range [A B] and preserves all other intensities

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                          blockageshellip)

                                                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                          image around the mean intensity was set to black the other intensities remain unchanged

                                                                          Fig 6 - Aortic angiogram and intensity sliced versions

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Bit-plane slicing

                                                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                          This technique highlights the contribution made to the whole image appearances by each

                                                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          Digital Image Processing

                                                                          Week 1

                                                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                          • DIP 1 2017
                                                                          • DIP 02 (2017)

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Biometry - iris

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Biometry - fingerprint

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Face detection and recognition

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Gender identification

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Image morphing

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Fundamental Steps in DIP

                                                                            methods whose input and output are images

                                                                            methods whose inputs are images but whose outputs are attributes extracted from those images

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Outputs are images

                                                                            bull image acquisition

                                                                            bull image filtering and enhancement

                                                                            bull image restoration

                                                                            bull color image processing

                                                                            bull wavelets and multiresolution processing

                                                                            bull compression

                                                                            bull morphological processing

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Outputs are attributes

                                                                            bull morphological processing

                                                                            bull segmentation

                                                                            bull representation and description

                                                                            bull object recognition

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Image acquisition - may involve preprocessing such as scaling

                                                                            Image enhancement

                                                                            bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                            bull enhancement is problem oriented

                                                                            bull there is no general sbquotheoryrsquo of image enhancement

                                                                            bull enhancement use subjective methods for image emprovement

                                                                            bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Image restoration

                                                                            bull improving the appearance of an image

                                                                            bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                            Color image processing

                                                                            bull fundamental concept in color models

                                                                            bull basic color processing in a digital domain

                                                                            Wavelets and multiresolution processing

                                                                            representing images in various degree of resolution

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Compression

                                                                            reducing the storage required to save an image or the bandwidth required to transmit it

                                                                            Morphological processing

                                                                            bull tools for extracting image components that are useful in the representation and description of shape

                                                                            bull a transition from processes that output images to processes that outputimage attributes

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Segmentation

                                                                            bull partitioning an image into its constituents parts or objects

                                                                            bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                            bull the more accurate the segmentation the more likley recognition is to succeed

                                                                            Representation and description (almost always follows segmentation)

                                                                            bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                            bull converting the data produced by segmentation to a form suitable for computer processing

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                            bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                            bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                            Object recognition

                                                                            the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                            Knowledge database

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Simplified diagramof a cross sectionof the human eye

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                            The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                            Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                            The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                            The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                            Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                            Fovea = the place where the image of the object of interest falls on

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                            Blind spot region without receptors

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Image formation in the eye

                                                                            Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                            Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                            distance between lens and retina along visual axix = 17 mm

                                                                            range of focal length = 14 mm to 17 mm

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Optical illusions

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                            gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                            quantities that describe the quality of a chromatic light source radiance

                                                                            the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                            measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                            brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            the physical meaning is determined by the source of the image

                                                                            ( )f D f x y

                                                                            Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                            f(xy) ndash characterized by two components

                                                                            i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                            r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                            ( ) ( ) ( )

                                                                            0 ( ) 0 ( ) 1

                                                                            f x y i x y r x y

                                                                            i x y r x y

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                            i(xy) ndash determined by the illumination source

                                                                            r(xy) ndash determined by the characteristics of the imaged objects

                                                                            is called gray (or intensity) scale

                                                                            In practice

                                                                            min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                            indoor values without additional illuminationmin max10 1000L L

                                                                            black whitemin max0 1 0 1 0 1L L L L l l L

                                                                            min maxL L

                                                                            Digital Image ProcessingDigital Image Processing

                                                                            Week 1Week 1

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Image Sampling and Quantization

                                                                            - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                            scene

                                                                            converting a continuous image f to digital form

                                                                            - digitizing (x y) is called sampling

                                                                            - digitizing f(x y) is called quantization

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                            (00) (01) (0 1)(10) (11) (1 1)

                                                                            ( )

                                                                            ( 10) ( 11) ( 1 1)

                                                                            f f f Nf f f N

                                                                            f x y

                                                                            f M f M f M N

                                                                            image element pixel

                                                                            00 01 0 1

                                                                            10 11 1 1

                                                                            10 11 1 1

                                                                            ( ) ( )

                                                                            N

                                                                            i jN M N

                                                                            i j

                                                                            M M M N

                                                                            a a aa f x i y j f i ja a a

                                                                            Aa

                                                                            a a a

                                                                            f(00) ndash the upper left corner of the image

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            M N ge 0 L=2k

                                                                            [0 1]i j i ja a L

                                                                            Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Number of bits required to store a digitized image

                                                                            for 2 b M N k M N b N k

                                                                            When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                            Measures line pairs per unit distance dots (pixels) per unit distance

                                                                            Image resolution = the largest number of discernible line pairs per unit distance

                                                                            (eg 100 line pairs per mm)

                                                                            Dots per unit distance are commonly used in printing and publishing

                                                                            In US the measure is expressed in dots per inch (dpi)

                                                                            (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                            Intensity resolution ndash the smallest discernible change in intensity level

                                                                            The number of intensity levels (L) is determined by hardware considerations

                                                                            L=2k ndash most common k = 8

                                                                            Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                            150 dpi (lower left) 72 dpi (lower right)

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Reducing the number of gray levels 256 128 64 32

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Reducing the number of gray levels 16 8 4 2

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                            Shrinking zooming ndash image resizing ndash image resampling methods

                                                                            Interpolation is the process of using known data to estimate values at unknown locations

                                                                            Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                            750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                            same spacing as the original and then shrink it so that it fits exactly over the original

                                                                            image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                            Problem assignment of intensity-level in the new 750 times 750 grid

                                                                            Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                            intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                            This technique has the tendency to produce undesirable effects like severe distortion of

                                                                            straight edges

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                            where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                            be written using the 4 nearest neighbors of point (x y)

                                                                            Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                            modest increase in computational effort

                                                                            Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                            nearest neighbors of the point 3 3

                                                                            0 0

                                                                            ( ) i ji j

                                                                            i jv x y c x y

                                                                            The coefficients cij are obtained solving a 16x16 linear system

                                                                            intensity levels of the 16 nearest neighbors of 3 3

                                                                            0 0

                                                                            ( )i ji j

                                                                            i jc x y x y

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                            bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                            programs such as Adobe Photoshop and Corel Photopaint

                                                                            Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                            the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                            then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                            neighbor interpolation was used (both for shrinking and zooming)

                                                                            Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                            interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                            from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Neighbors of a Pixel

                                                                            A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                            This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                            The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                            and are denoted ND(p)

                                                                            The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                            N8 (p)

                                                                            If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                            fall outside the image

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Adjacency Connectivity Regions Boundaries

                                                                            Denote by V the set of intensity levels used to define adjacency

                                                                            - in a binary image V 01 (V=0 V=1)

                                                                            - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                            We consider 3 types of adjacency

                                                                            (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                            (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                            (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                            m-adjacent if

                                                                            4( )q N p or

                                                                            ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                            Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                            ambiguities that often arise when 8-adjacency is used Consider the example

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            binary image

                                                                            0 1 1 0 1 1 0 1 1

                                                                            1 0 1 0 0 1 0 0 1 0

                                                                            0 0 1 0 0 1 0 0 1

                                                                            V

                                                                            The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                            8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                            m-adjacency

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                            is a sequence of distinct pixels with coordinates

                                                                            and are adjacent 0 0 1 1

                                                                            1 1

                                                                            ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                            n n

                                                                            i i i i

                                                                            x y x y x y x y s tx y x y i n

                                                                            The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                            Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                            Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                            in S if there exists a path between them consisting only of pixels from S

                                                                            S is a connected set if there is a path in S between any 2 pixels in S

                                                                            Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                            Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                            that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                            8-adjacency are considered

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                            touches the image border

                                                                            the complement of 1

                                                                            ( )K

                                                                            cu k u u

                                                                            k

                                                                            R R R R

                                                                            We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                            background of the image

                                                                            The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                            points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                            region that have at least one background neighbor This definition is referred to as the

                                                                            inner border to distinguish it from the notion of outer border which is the corresponding

                                                                            border in the background

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Distance measures

                                                                            For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                            function or metric if

                                                                            (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                            (b) D(p q) = D(q p)

                                                                            (c) D(p z) le D(p q) + D(q z)

                                                                            The Euclidean distance between p and q is defined as 1

                                                                            2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                            The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                            centered at (x y)

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            The D4 distance (also called city-block distance) between p and q is defined as

                                                                            4( ) | | | |D p q x s y t

                                                                            The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                            4

                                                                            22 1 2

                                                                            2 2 1 0 1 22 1 2

                                                                            2

                                                                            D

                                                                            The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                            The D8 distance (called the chessboard distance) between p and q is defined as

                                                                            8( ) max| | | |D p q x s y t

                                                                            The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            8

                                                                            2 2 2 2 22 1 1 1 2

                                                                            2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                            D

                                                                            The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                            D4 and D8 distances are independent of any paths that might exist between p and q

                                                                            because these distances involve only the coordinates of the point

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Array versus Matrix Operations

                                                                            An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                            11 12 11 12

                                                                            21 22 21 22

                                                                            a a b ba a b b

                                                                            Array product

                                                                            11 12 11 12 11 11 12 12

                                                                            21 22 21 22 21 21 22 21

                                                                            a a b b a b a ba a b b a b a b

                                                                            Matrix product

                                                                            11 12 11 12 11 11 12 21 11 12 12 21

                                                                            21 22 21 22 21 11 22 21 21 12 22 22

                                                                            a a b b a b a b a b a ba a b b a b a b a b a b

                                                                            We assume array operations unless stated otherwise

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Linear versus Nonlinear Operations

                                                                            One of the most important classifications of image-processing methods is whether it is

                                                                            linear or nonlinear

                                                                            ( ) ( )H f x y g x y

                                                                            H is said to be a linear operator if

                                                                            images1 2 1 2

                                                                            1 2

                                                                            ( ) ( ) ( ) ( )

                                                                            H a f x y b f x y a H f x y b H f x y

                                                                            a b f f

                                                                            Example of nonlinear operator

                                                                            the maximum value of the pixels of image max ( )H f f x y f

                                                                            1 2

                                                                            0 2 6 5 1 1

                                                                            2 3 4 7f f a b

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            1 2

                                                                            0 2 6 5 6 3max max 1 ( 1) max 2

                                                                            2 3 4 7 2 4a f b f

                                                                            0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                            2 3 4 7

                                                                            Arithmetic Operations in Image Processing

                                                                            Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                            f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                            For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                            value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                            The two random variables are uncorrelated when their covariance is 0

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                            used in image enhancement)

                                                                            1

                                                                            1( ) ( )K

                                                                            ii

                                                                            g x y g x yK

                                                                            If the noise satisfies the properties stated above we have

                                                                            2 2( ) ( )

                                                                            1( ) ( ) g x y x yE g x y f x yK

                                                                            ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                            and g respectively The standard deviation (square root of the variance) at any point in

                                                                            the average image is

                                                                            ( ) ( )1

                                                                            g x y x yK

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            As K increases the variability (as measured by the variance or the standard deviation) of

                                                                            the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                            means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                            averaging process increases

                                                                            An important application of image averaging is in the field of astronomy where imaging

                                                                            under very low light levels frequently causes sensor noise to render single images

                                                                            virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                            was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                            64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                            images respectively

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                            100 noisy images

                                                                            a b c d e f

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            A frequent application of image subtraction is in the enhancement of differences between

                                                                            images

                                                                            (a) (b) (c)

                                                                            Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                            significant bit of each pixel (c) the difference between the two images

                                                                            Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                            Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                            images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                            difference between images (a) and (b)

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                            h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                            intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                            source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                            bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                            anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                            images after injection of the contrast medium

                                                                            In g(x y) we can find the differences between h and f as enhanced detail

                                                                            Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                            propagates through the various arteries in the area being observed

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            An important application of image multiplication (and division) is shading correction

                                                                            Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                            f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                            When the shading function is known

                                                                            ( )( )( )

                                                                            g x yf x yh x y

                                                                            h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                            approximation to the shading function by imaging a target of constant intensity When the

                                                                            sensor is not available often the shading pattern can be estimated from the image

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            (a) (b) (c)

                                                                            Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Another use of image multiplication is in masking also called region of interest (ROI)

                                                                            operations The process consists of multiplying a given image by a mask image that has

                                                                            1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                            image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                            (a) (b) (c)

                                                                            Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                            min( )mf f f

                                                                            0 ( 255)max( )

                                                                            ms

                                                                            m

                                                                            ff K K K

                                                                            f

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Spatial Operations

                                                                            - are performed directly on the pixels of a given image

                                                                            There are three categories of spatial operations

                                                                            single-pixel operations

                                                                            neighborhood operations

                                                                            geometric spatial transformations

                                                                            Single-pixel operations

                                                                            - change the values of intensity for the individual pixels ( )s T z

                                                                            where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                            corresponding pixel in the processed image

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Neighborhood operations

                                                                            Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                            in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                            based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                            rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                            intensity by computing the average value of the pixels in Sxy

                                                                            ( )

                                                                            1( ) ( )xyr c S

                                                                            g x y f r cm n

                                                                            The net effect is to perform local blurring in the original image This type of process is

                                                                            used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                            largest region of an image

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Geometric spatial transformations and image registration

                                                                            - modify the spatial relationship between pixels in an image

                                                                            - these transformations are often called rubber-sheet transformations (analogous to

                                                                            printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                            predefined set of rules

                                                                            A geometric transformation consists of 2 basic operations

                                                                            1 a spatial transformation of coordinates

                                                                            2 intensity interpolation that assign intensity values to the spatial transformed

                                                                            pixels

                                                                            The coordinate system transformation ( ) [( )]x y T v w

                                                                            (v w) ndash pixel coordinates in the original image

                                                                            (x y) ndash pixel coordinates in the transformed image

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                            Affine transform

                                                                            11 1211 21 31

                                                                            21 2212 22 33

                                                                            31 32

                                                                            0[ 1] [ 1] [ 1] 0

                                                                            1

                                                                            t tx t v t w t

                                                                            x y v w T v w t ty t v t w t

                                                                            t t

                                                                            (AT)

                                                                            This transform can scale rotate translate or shear a set of coordinate points depending

                                                                            on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                            result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                            scaling rotation and translation matrices from Table 1

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Affine transformations

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            The preceding transformations relocate pixels on an image to new locations To complete

                                                                            the process we have to assign intensity values to those locations This task is done by

                                                                            using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                            In practice we can use equation (AT) in two basic ways

                                                                            forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                            location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                            Problems

                                                                            - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                            the same location in the output image

                                                                            - some output locations have no correspondent in the original image (no intensity

                                                                            assignment)

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            inverse mapping scans the output pixel locations and at each location (x y)

                                                                            computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                            It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                            pixel value

                                                                            Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                            numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Image registration ndash align two or more images of the same scene

                                                                            In image registration we have available the input and output images but the specific

                                                                            transformation that produced the output image from the input is generally unknown

                                                                            The problem is to estimate the transformation function and then use it to register the two

                                                                            images

                                                                            - it may be of interest to align (register) two or more image taken at approximately the

                                                                            same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                            - align images of a given location taken by the same instrument at different moments

                                                                            of time (satellite images)

                                                                            Solving the problem using tie points (also called control points) which are

                                                                            corresponding points whose locations are known precisely in the input and reference

                                                                            image

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            How to select tie points

                                                                            - interactively selecting them

                                                                            - use of algorithms that try to detect these points

                                                                            - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                            the imaging sensors These objects produce a set of known points (called reseau

                                                                            marks) directly on all images captured by the system which can be used as guides

                                                                            for establishing tie points

                                                                            The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                            of 4 tie points both on the input image and the reference image A simple model based on

                                                                            a bilinear approximation is given by

                                                                            1 2 3 4

                                                                            5 6 7 8

                                                                            x c v c w c v w cy c v c w c v w c

                                                                            (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                            frequently is to select a larger number of tie points and using this new set of tie points

                                                                            subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                            subregions marked by 4 tie points we applied the transformation model described above

                                                                            The number of tie points and the sophistication of the model required to solve the register

                                                                            problem depend on the severity of the geometrical distortion

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Probabilistic Methods

                                                                            zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                            p(zk) = the probability that the intensity level zk occurs in the given image

                                                                            ( ) kk

                                                                            np zM N

                                                                            nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                            pixels in the image) 1

                                                                            0( ) 1

                                                                            L

                                                                            kk

                                                                            p z

                                                                            The mean (average) intensity of an image is given by 1

                                                                            0( )

                                                                            L

                                                                            k kk

                                                                            m z p z

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            The variance of the intensities is 1

                                                                            2 2

                                                                            0( ) ( )

                                                                            L

                                                                            k kk

                                                                            z m p z

                                                                            The variance is a measure of the spread of the values of z about the mean so it is a

                                                                            measure of image contrast Usually for measuring image contrast the standard deviation

                                                                            ( ) is used

                                                                            The n-th moment of a random variable z about the mean is defined as 1

                                                                            0( ) ( ) ( )

                                                                            Ln

                                                                            n k kk

                                                                            z z m p z

                                                                            ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                            3( ) 0z the intensities are biased to values higher than the mean

                                                                            ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                            mean

                                                                            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                            Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                            Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                            Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Intensity Transformations and Spatial Filtering

                                                                            ( ) ( )g x y T f x y

                                                                            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                            neighborhood of (x y)

                                                                            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                            and much smaller in size than the image

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                            called spatial filter (spatial mask kernel template or window)

                                                                            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                            ( )s T r

                                                                            s and r are denoting respectively the intensity of g and f at (x y)

                                                                            Figure 2 left - T produces an output image of higher contrast than the original by

                                                                            darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                            is called contrast stretching

                                                                            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                            thresholding function

                                                                            Some Basic Intensity Transformation Functions

                                                                            Image Negatives

                                                                            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                            - equivalent of a photographic negative

                                                                            - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                            image

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Original Negative image

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                            Some basic intensity transformation functions

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            This transformation maps a narrow range of low intensity values in the input into a wider

                                                                            range An operator of this type is used to expand the values of dark pixels in an image

                                                                            while compressing the higher-level values The opposite is true for the inverse log

                                                                            transformation The log functions compress the dynamic range of images with large

                                                                            variations in pixel values

                                                                            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Power-Law (Gamma) Transformations

                                                                            - positive constants( ) ( ( ) )s T r c r c s c r

                                                                            Plots of gamma transformation for different values of γ (c=1)

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                            of output values with the opposite being true for higher values of input values The

                                                                            curves with 1 have the opposite effect of those generated with values of 1

                                                                            1c - identity transformation

                                                                            A variety of devices used for image capture printing and display respond according to a

                                                                            power law The process used to correct these power-law response phenomena is called

                                                                            gamma correction

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Piecewise-Linear Transformations Functions

                                                                            Contrast stretching

                                                                            - a process that expands the range of intensity levels in an image so it spans the full

                                                                            intensity range of the recording tool or display device

                                                                            a b c d Fig5

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            11

                                                                            1

                                                                            2 1 1 21 2

                                                                            2 1 2 1

                                                                            22

                                                                            2

                                                                            [0 ]

                                                                            ( ) ( )( ) [ ]( ) ( )

                                                                            ( 1 ) [ 1]( 1 )

                                                                            s r r rrs r r s r rT r r r r

                                                                            r r r rs L r r r L

                                                                            L r

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            1 1 2 2r s r s identity transformation (no change)

                                                                            1 2 1 2 0 1r r s s L thresholding function

                                                                            Figure 5(b) shows an 8-bit image with low contrast

                                                                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                            in the image respectively Thus the transformation function stretched the levels linearly

                                                                            from their original range to the full range [0 L-1]

                                                                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                            2 2 1r s m L where m is the mean gray level in the image

                                                                            The original image on which these results are based is a scanning electron microscope

                                                                            image of pollen magnified approximately 700 times

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Intensity-level slicing

                                                                            - highlighting a specific range of intensities in an image

                                                                            There are two approaches for intensity-level slicing

                                                                            1 display in one value (white for example) all the values in the range of interest and in

                                                                            another (say black) all other intensities (Figure 311 (a))

                                                                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                            intensities in the image (Figure 311 (b))

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                            the top of the scale of intensities This type of enhancement produces a binary image

                                                                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                            Highlights range [A B] and preserves all other intensities

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                            blockageshellip)

                                                                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                            image around the mean intensity was set to black the other intensities remain unchanged

                                                                            Fig 6 - Aortic angiogram and intensity sliced versions

                                                                            Digital Image Processing

                                                                            Week 1

                                                                            Bit-plane slicing

                                                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                            This technique highlights the contribution made to the whole image appearances by each

                                                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

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                                                                            Week 1

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                                                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                            • DIP 1 2017
                                                                            • DIP 02 (2017)

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              Biometry - iris

                                                                              Digital Image ProcessingDigital Image Processing

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                                                                              Biometry - fingerprint

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                                                                              Week 1Week 1

                                                                              Face detection and recognition

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              Gender identification

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                                                                              Week 1Week 1

                                                                              Image morphing

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                                                                              Fundamental Steps in DIP

                                                                              methods whose input and output are images

                                                                              methods whose inputs are images but whose outputs are attributes extracted from those images

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                                                                              Week 1Week 1

                                                                              Outputs are images

                                                                              bull image acquisition

                                                                              bull image filtering and enhancement

                                                                              bull image restoration

                                                                              bull color image processing

                                                                              bull wavelets and multiresolution processing

                                                                              bull compression

                                                                              bull morphological processing

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                                                                              Week 1Week 1

                                                                              Outputs are attributes

                                                                              bull morphological processing

                                                                              bull segmentation

                                                                              bull representation and description

                                                                              bull object recognition

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                                                                              Week 1Week 1

                                                                              Image acquisition - may involve preprocessing such as scaling

                                                                              Image enhancement

                                                                              bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                              bull enhancement is problem oriented

                                                                              bull there is no general sbquotheoryrsquo of image enhancement

                                                                              bull enhancement use subjective methods for image emprovement

                                                                              bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

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                                                                              Week 1Week 1

                                                                              Image restoration

                                                                              bull improving the appearance of an image

                                                                              bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                              Color image processing

                                                                              bull fundamental concept in color models

                                                                              bull basic color processing in a digital domain

                                                                              Wavelets and multiresolution processing

                                                                              representing images in various degree of resolution

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              Compression

                                                                              reducing the storage required to save an image or the bandwidth required to transmit it

                                                                              Morphological processing

                                                                              bull tools for extracting image components that are useful in the representation and description of shape

                                                                              bull a transition from processes that output images to processes that outputimage attributes

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                                                                              Week 1Week 1

                                                                              Segmentation

                                                                              bull partitioning an image into its constituents parts or objects

                                                                              bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                              bull the more accurate the segmentation the more likley recognition is to succeed

                                                                              Representation and description (almost always follows segmentation)

                                                                              bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                              bull converting the data produced by segmentation to a form suitable for computer processing

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                              bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                              bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                              Object recognition

                                                                              the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                              Knowledge database

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                                                                              Week 1Week 1

                                                                              Simplified diagramof a cross sectionof the human eye

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                                                                              Week 1Week 1

                                                                              Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                              The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                              Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                              The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                              The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                              Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                              Fovea = the place where the image of the object of interest falls on

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                              Blind spot region without receptors

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              Image formation in the eye

                                                                              Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                              Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                              distance between lens and retina along visual axix = 17 mm

                                                                              range of focal length = 14 mm to 17 mm

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              Digital Image ProcessingDigital Image Processing

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                                                                              Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              Optical illusions

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                              gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                              quantities that describe the quality of a chromatic light source radiance

                                                                              the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                              measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                              brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              the physical meaning is determined by the source of the image

                                                                              ( )f D f x y

                                                                              Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                              f(xy) ndash characterized by two components

                                                                              i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                              r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                              ( ) ( ) ( )

                                                                              0 ( ) 0 ( ) 1

                                                                              f x y i x y r x y

                                                                              i x y r x y

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                              i(xy) ndash determined by the illumination source

                                                                              r(xy) ndash determined by the characteristics of the imaged objects

                                                                              is called gray (or intensity) scale

                                                                              In practice

                                                                              min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                              indoor values without additional illuminationmin max10 1000L L

                                                                              black whitemin max0 1 0 1 0 1L L L L l l L

                                                                              min maxL L

                                                                              Digital Image ProcessingDigital Image Processing

                                                                              Week 1Week 1

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Image Sampling and Quantization

                                                                              - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                              scene

                                                                              converting a continuous image f to digital form

                                                                              - digitizing (x y) is called sampling

                                                                              - digitizing f(x y) is called quantization

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                              (00) (01) (0 1)(10) (11) (1 1)

                                                                              ( )

                                                                              ( 10) ( 11) ( 1 1)

                                                                              f f f Nf f f N

                                                                              f x y

                                                                              f M f M f M N

                                                                              image element pixel

                                                                              00 01 0 1

                                                                              10 11 1 1

                                                                              10 11 1 1

                                                                              ( ) ( )

                                                                              N

                                                                              i jN M N

                                                                              i j

                                                                              M M M N

                                                                              a a aa f x i y j f i ja a a

                                                                              Aa

                                                                              a a a

                                                                              f(00) ndash the upper left corner of the image

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              M N ge 0 L=2k

                                                                              [0 1]i j i ja a L

                                                                              Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Number of bits required to store a digitized image

                                                                              for 2 b M N k M N b N k

                                                                              When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                              Measures line pairs per unit distance dots (pixels) per unit distance

                                                                              Image resolution = the largest number of discernible line pairs per unit distance

                                                                              (eg 100 line pairs per mm)

                                                                              Dots per unit distance are commonly used in printing and publishing

                                                                              In US the measure is expressed in dots per inch (dpi)

                                                                              (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                              Intensity resolution ndash the smallest discernible change in intensity level

                                                                              The number of intensity levels (L) is determined by hardware considerations

                                                                              L=2k ndash most common k = 8

                                                                              Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                              150 dpi (lower left) 72 dpi (lower right)

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Reducing the number of gray levels 256 128 64 32

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Reducing the number of gray levels 16 8 4 2

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                              Shrinking zooming ndash image resizing ndash image resampling methods

                                                                              Interpolation is the process of using known data to estimate values at unknown locations

                                                                              Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                              750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                              same spacing as the original and then shrink it so that it fits exactly over the original

                                                                              image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                              Problem assignment of intensity-level in the new 750 times 750 grid

                                                                              Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                              intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                              This technique has the tendency to produce undesirable effects like severe distortion of

                                                                              straight edges

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                              where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                              be written using the 4 nearest neighbors of point (x y)

                                                                              Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                              modest increase in computational effort

                                                                              Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                              nearest neighbors of the point 3 3

                                                                              0 0

                                                                              ( ) i ji j

                                                                              i jv x y c x y

                                                                              The coefficients cij are obtained solving a 16x16 linear system

                                                                              intensity levels of the 16 nearest neighbors of 3 3

                                                                              0 0

                                                                              ( )i ji j

                                                                              i jc x y x y

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                              bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                              programs such as Adobe Photoshop and Corel Photopaint

                                                                              Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                              the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                              then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                              neighbor interpolation was used (both for shrinking and zooming)

                                                                              Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                              interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                              from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Neighbors of a Pixel

                                                                              A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                              This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                              The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                              and are denoted ND(p)

                                                                              The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                              N8 (p)

                                                                              If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                              fall outside the image

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Adjacency Connectivity Regions Boundaries

                                                                              Denote by V the set of intensity levels used to define adjacency

                                                                              - in a binary image V 01 (V=0 V=1)

                                                                              - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                              We consider 3 types of adjacency

                                                                              (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                              (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                              (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                              m-adjacent if

                                                                              4( )q N p or

                                                                              ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                              Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                              ambiguities that often arise when 8-adjacency is used Consider the example

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              binary image

                                                                              0 1 1 0 1 1 0 1 1

                                                                              1 0 1 0 0 1 0 0 1 0

                                                                              0 0 1 0 0 1 0 0 1

                                                                              V

                                                                              The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                              8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                              m-adjacency

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                              is a sequence of distinct pixels with coordinates

                                                                              and are adjacent 0 0 1 1

                                                                              1 1

                                                                              ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                              n n

                                                                              i i i i

                                                                              x y x y x y x y s tx y x y i n

                                                                              The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                              Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                              Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                              in S if there exists a path between them consisting only of pixels from S

                                                                              S is a connected set if there is a path in S between any 2 pixels in S

                                                                              Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                              Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                              that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                              8-adjacency are considered

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                              touches the image border

                                                                              the complement of 1

                                                                              ( )K

                                                                              cu k u u

                                                                              k

                                                                              R R R R

                                                                              We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                              background of the image

                                                                              The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                              points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                              region that have at least one background neighbor This definition is referred to as the

                                                                              inner border to distinguish it from the notion of outer border which is the corresponding

                                                                              border in the background

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Distance measures

                                                                              For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                              function or metric if

                                                                              (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                              (b) D(p q) = D(q p)

                                                                              (c) D(p z) le D(p q) + D(q z)

                                                                              The Euclidean distance between p and q is defined as 1

                                                                              2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                              The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                              centered at (x y)

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              The D4 distance (also called city-block distance) between p and q is defined as

                                                                              4( ) | | | |D p q x s y t

                                                                              The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                              4

                                                                              22 1 2

                                                                              2 2 1 0 1 22 1 2

                                                                              2

                                                                              D

                                                                              The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                              The D8 distance (called the chessboard distance) between p and q is defined as

                                                                              8( ) max| | | |D p q x s y t

                                                                              The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              8

                                                                              2 2 2 2 22 1 1 1 2

                                                                              2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                              D

                                                                              The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                              D4 and D8 distances are independent of any paths that might exist between p and q

                                                                              because these distances involve only the coordinates of the point

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Array versus Matrix Operations

                                                                              An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                              11 12 11 12

                                                                              21 22 21 22

                                                                              a a b ba a b b

                                                                              Array product

                                                                              11 12 11 12 11 11 12 12

                                                                              21 22 21 22 21 21 22 21

                                                                              a a b b a b a ba a b b a b a b

                                                                              Matrix product

                                                                              11 12 11 12 11 11 12 21 11 12 12 21

                                                                              21 22 21 22 21 11 22 21 21 12 22 22

                                                                              a a b b a b a b a b a ba a b b a b a b a b a b

                                                                              We assume array operations unless stated otherwise

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Linear versus Nonlinear Operations

                                                                              One of the most important classifications of image-processing methods is whether it is

                                                                              linear or nonlinear

                                                                              ( ) ( )H f x y g x y

                                                                              H is said to be a linear operator if

                                                                              images1 2 1 2

                                                                              1 2

                                                                              ( ) ( ) ( ) ( )

                                                                              H a f x y b f x y a H f x y b H f x y

                                                                              a b f f

                                                                              Example of nonlinear operator

                                                                              the maximum value of the pixels of image max ( )H f f x y f

                                                                              1 2

                                                                              0 2 6 5 1 1

                                                                              2 3 4 7f f a b

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              1 2

                                                                              0 2 6 5 6 3max max 1 ( 1) max 2

                                                                              2 3 4 7 2 4a f b f

                                                                              0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                              2 3 4 7

                                                                              Arithmetic Operations in Image Processing

                                                                              Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                              f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                              For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                              value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                              The two random variables are uncorrelated when their covariance is 0

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                              used in image enhancement)

                                                                              1

                                                                              1( ) ( )K

                                                                              ii

                                                                              g x y g x yK

                                                                              If the noise satisfies the properties stated above we have

                                                                              2 2( ) ( )

                                                                              1( ) ( ) g x y x yE g x y f x yK

                                                                              ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                              and g respectively The standard deviation (square root of the variance) at any point in

                                                                              the average image is

                                                                              ( ) ( )1

                                                                              g x y x yK

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              As K increases the variability (as measured by the variance or the standard deviation) of

                                                                              the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                              means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                              averaging process increases

                                                                              An important application of image averaging is in the field of astronomy where imaging

                                                                              under very low light levels frequently causes sensor noise to render single images

                                                                              virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                              was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                              64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                              images respectively

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                              100 noisy images

                                                                              a b c d e f

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              A frequent application of image subtraction is in the enhancement of differences between

                                                                              images

                                                                              (a) (b) (c)

                                                                              Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                              significant bit of each pixel (c) the difference between the two images

                                                                              Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                              Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                              images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                              difference between images (a) and (b)

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                              h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                              intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                              source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                              bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                              anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                              images after injection of the contrast medium

                                                                              In g(x y) we can find the differences between h and f as enhanced detail

                                                                              Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                              propagates through the various arteries in the area being observed

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              An important application of image multiplication (and division) is shading correction

                                                                              Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                              f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                              When the shading function is known

                                                                              ( )( )( )

                                                                              g x yf x yh x y

                                                                              h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                              approximation to the shading function by imaging a target of constant intensity When the

                                                                              sensor is not available often the shading pattern can be estimated from the image

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              (a) (b) (c)

                                                                              Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Another use of image multiplication is in masking also called region of interest (ROI)

                                                                              operations The process consists of multiplying a given image by a mask image that has

                                                                              1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                              image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                              (a) (b) (c)

                                                                              Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                              min( )mf f f

                                                                              0 ( 255)max( )

                                                                              ms

                                                                              m

                                                                              ff K K K

                                                                              f

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Spatial Operations

                                                                              - are performed directly on the pixels of a given image

                                                                              There are three categories of spatial operations

                                                                              single-pixel operations

                                                                              neighborhood operations

                                                                              geometric spatial transformations

                                                                              Single-pixel operations

                                                                              - change the values of intensity for the individual pixels ( )s T z

                                                                              where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                              corresponding pixel in the processed image

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Neighborhood operations

                                                                              Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                              in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                              based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                              rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                              intensity by computing the average value of the pixels in Sxy

                                                                              ( )

                                                                              1( ) ( )xyr c S

                                                                              g x y f r cm n

                                                                              The net effect is to perform local blurring in the original image This type of process is

                                                                              used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                              largest region of an image

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Geometric spatial transformations and image registration

                                                                              - modify the spatial relationship between pixels in an image

                                                                              - these transformations are often called rubber-sheet transformations (analogous to

                                                                              printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                              predefined set of rules

                                                                              A geometric transformation consists of 2 basic operations

                                                                              1 a spatial transformation of coordinates

                                                                              2 intensity interpolation that assign intensity values to the spatial transformed

                                                                              pixels

                                                                              The coordinate system transformation ( ) [( )]x y T v w

                                                                              (v w) ndash pixel coordinates in the original image

                                                                              (x y) ndash pixel coordinates in the transformed image

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                              Affine transform

                                                                              11 1211 21 31

                                                                              21 2212 22 33

                                                                              31 32

                                                                              0[ 1] [ 1] [ 1] 0

                                                                              1

                                                                              t tx t v t w t

                                                                              x y v w T v w t ty t v t w t

                                                                              t t

                                                                              (AT)

                                                                              This transform can scale rotate translate or shear a set of coordinate points depending

                                                                              on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                              result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                              scaling rotation and translation matrices from Table 1

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Affine transformations

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              The preceding transformations relocate pixels on an image to new locations To complete

                                                                              the process we have to assign intensity values to those locations This task is done by

                                                                              using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                              In practice we can use equation (AT) in two basic ways

                                                                              forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                              location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                              Problems

                                                                              - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                              the same location in the output image

                                                                              - some output locations have no correspondent in the original image (no intensity

                                                                              assignment)

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              inverse mapping scans the output pixel locations and at each location (x y)

                                                                              computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                              It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                              pixel value

                                                                              Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                              numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Image registration ndash align two or more images of the same scene

                                                                              In image registration we have available the input and output images but the specific

                                                                              transformation that produced the output image from the input is generally unknown

                                                                              The problem is to estimate the transformation function and then use it to register the two

                                                                              images

                                                                              - it may be of interest to align (register) two or more image taken at approximately the

                                                                              same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                              - align images of a given location taken by the same instrument at different moments

                                                                              of time (satellite images)

                                                                              Solving the problem using tie points (also called control points) which are

                                                                              corresponding points whose locations are known precisely in the input and reference

                                                                              image

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              How to select tie points

                                                                              - interactively selecting them

                                                                              - use of algorithms that try to detect these points

                                                                              - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                              the imaging sensors These objects produce a set of known points (called reseau

                                                                              marks) directly on all images captured by the system which can be used as guides

                                                                              for establishing tie points

                                                                              The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                              of 4 tie points both on the input image and the reference image A simple model based on

                                                                              a bilinear approximation is given by

                                                                              1 2 3 4

                                                                              5 6 7 8

                                                                              x c v c w c v w cy c v c w c v w c

                                                                              (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                              frequently is to select a larger number of tie points and using this new set of tie points

                                                                              subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                              subregions marked by 4 tie points we applied the transformation model described above

                                                                              The number of tie points and the sophistication of the model required to solve the register

                                                                              problem depend on the severity of the geometrical distortion

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Probabilistic Methods

                                                                              zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                              p(zk) = the probability that the intensity level zk occurs in the given image

                                                                              ( ) kk

                                                                              np zM N

                                                                              nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                              pixels in the image) 1

                                                                              0( ) 1

                                                                              L

                                                                              kk

                                                                              p z

                                                                              The mean (average) intensity of an image is given by 1

                                                                              0( )

                                                                              L

                                                                              k kk

                                                                              m z p z

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              The variance of the intensities is 1

                                                                              2 2

                                                                              0( ) ( )

                                                                              L

                                                                              k kk

                                                                              z m p z

                                                                              The variance is a measure of the spread of the values of z about the mean so it is a

                                                                              measure of image contrast Usually for measuring image contrast the standard deviation

                                                                              ( ) is used

                                                                              The n-th moment of a random variable z about the mean is defined as 1

                                                                              0( ) ( ) ( )

                                                                              Ln

                                                                              n k kk

                                                                              z z m p z

                                                                              ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                              3( ) 0z the intensities are biased to values higher than the mean

                                                                              ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                              mean

                                                                              Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                              Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                              Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                              Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Intensity Transformations and Spatial Filtering

                                                                              ( ) ( )g x y T f x y

                                                                              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                              neighborhood of (x y)

                                                                              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                              and much smaller in size than the image

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                              called spatial filter (spatial mask kernel template or window)

                                                                              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                              ( )s T r

                                                                              s and r are denoting respectively the intensity of g and f at (x y)

                                                                              Figure 2 left - T produces an output image of higher contrast than the original by

                                                                              darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                              is called contrast stretching

                                                                              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                              thresholding function

                                                                              Some Basic Intensity Transformation Functions

                                                                              Image Negatives

                                                                              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                              - equivalent of a photographic negative

                                                                              - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                              image

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Original Negative image

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                              Some basic intensity transformation functions

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              This transformation maps a narrow range of low intensity values in the input into a wider

                                                                              range An operator of this type is used to expand the values of dark pixels in an image

                                                                              while compressing the higher-level values The opposite is true for the inverse log

                                                                              transformation The log functions compress the dynamic range of images with large

                                                                              variations in pixel values

                                                                              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Power-Law (Gamma) Transformations

                                                                              - positive constants( ) ( ( ) )s T r c r c s c r

                                                                              Plots of gamma transformation for different values of γ (c=1)

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                              of output values with the opposite being true for higher values of input values The

                                                                              curves with 1 have the opposite effect of those generated with values of 1

                                                                              1c - identity transformation

                                                                              A variety of devices used for image capture printing and display respond according to a

                                                                              power law The process used to correct these power-law response phenomena is called

                                                                              gamma correction

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Piecewise-Linear Transformations Functions

                                                                              Contrast stretching

                                                                              - a process that expands the range of intensity levels in an image so it spans the full

                                                                              intensity range of the recording tool or display device

                                                                              a b c d Fig5

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              11

                                                                              1

                                                                              2 1 1 21 2

                                                                              2 1 2 1

                                                                              22

                                                                              2

                                                                              [0 ]

                                                                              ( ) ( )( ) [ ]( ) ( )

                                                                              ( 1 ) [ 1]( 1 )

                                                                              s r r rrs r r s r rT r r r r

                                                                              r r r rs L r r r L

                                                                              L r

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              1 1 2 2r s r s identity transformation (no change)

                                                                              1 2 1 2 0 1r r s s L thresholding function

                                                                              Figure 5(b) shows an 8-bit image with low contrast

                                                                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                              in the image respectively Thus the transformation function stretched the levels linearly

                                                                              from their original range to the full range [0 L-1]

                                                                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                              2 2 1r s m L where m is the mean gray level in the image

                                                                              The original image on which these results are based is a scanning electron microscope

                                                                              image of pollen magnified approximately 700 times

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Intensity-level slicing

                                                                              - highlighting a specific range of intensities in an image

                                                                              There are two approaches for intensity-level slicing

                                                                              1 display in one value (white for example) all the values in the range of interest and in

                                                                              another (say black) all other intensities (Figure 311 (a))

                                                                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                              intensities in the image (Figure 311 (b))

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                              the top of the scale of intensities This type of enhancement produces a binary image

                                                                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                              Highlights range [A B] and preserves all other intensities

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                              blockageshellip)

                                                                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                              image around the mean intensity was set to black the other intensities remain unchanged

                                                                              Fig 6 - Aortic angiogram and intensity sliced versions

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Bit-plane slicing

                                                                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                              This technique highlights the contribution made to the whole image appearances by each

                                                                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              Digital Image Processing

                                                                              Week 1

                                                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                              • DIP 1 2017
                                                                              • DIP 02 (2017)

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                Biometry - fingerprint

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                Face detection and recognition

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                Gender identification

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                Image morphing

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                Fundamental Steps in DIP

                                                                                methods whose input and output are images

                                                                                methods whose inputs are images but whose outputs are attributes extracted from those images

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                Outputs are images

                                                                                bull image acquisition

                                                                                bull image filtering and enhancement

                                                                                bull image restoration

                                                                                bull color image processing

                                                                                bull wavelets and multiresolution processing

                                                                                bull compression

                                                                                bull morphological processing

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                Outputs are attributes

                                                                                bull morphological processing

                                                                                bull segmentation

                                                                                bull representation and description

                                                                                bull object recognition

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                                                                                Week 1Week 1

                                                                                Image acquisition - may involve preprocessing such as scaling

                                                                                Image enhancement

                                                                                bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                                bull enhancement is problem oriented

                                                                                bull there is no general sbquotheoryrsquo of image enhancement

                                                                                bull enhancement use subjective methods for image emprovement

                                                                                bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

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                                                                                Week 1Week 1

                                                                                Image restoration

                                                                                bull improving the appearance of an image

                                                                                bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                                Color image processing

                                                                                bull fundamental concept in color models

                                                                                bull basic color processing in a digital domain

                                                                                Wavelets and multiresolution processing

                                                                                representing images in various degree of resolution

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                                                                                Week 1Week 1

                                                                                Compression

                                                                                reducing the storage required to save an image or the bandwidth required to transmit it

                                                                                Morphological processing

                                                                                bull tools for extracting image components that are useful in the representation and description of shape

                                                                                bull a transition from processes that output images to processes that outputimage attributes

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                                                                                Week 1Week 1

                                                                                Segmentation

                                                                                bull partitioning an image into its constituents parts or objects

                                                                                bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                                bull the more accurate the segmentation the more likley recognition is to succeed

                                                                                Representation and description (almost always follows segmentation)

                                                                                bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                                bull converting the data produced by segmentation to a form suitable for computer processing

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                                                                                Week 1Week 1

                                                                                bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                                bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                                bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                                Object recognition

                                                                                the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                                Knowledge database

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                                                                                Week 1Week 1

                                                                                Simplified diagramof a cross sectionof the human eye

                                                                                Digital Image ProcessingDigital Image Processing

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                                                                                Week 1Week 1

                                                                                Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                Fovea = the place where the image of the object of interest falls on

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                                                                                Week 1Week 1

                                                                                Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                Blind spot region without receptors

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                Image formation in the eye

                                                                                Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                distance between lens and retina along visual axix = 17 mm

                                                                                range of focal length = 14 mm to 17 mm

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

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                                                                                Week 1Week 1

                                                                                Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                Optical illusions

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                quantities that describe the quality of a chromatic light source radiance

                                                                                the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                the physical meaning is determined by the source of the image

                                                                                ( )f D f x y

                                                                                Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                f(xy) ndash characterized by two components

                                                                                i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                ( ) ( ) ( )

                                                                                0 ( ) 0 ( ) 1

                                                                                f x y i x y r x y

                                                                                i x y r x y

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                                                                                Week 1Week 1

                                                                                r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                i(xy) ndash determined by the illumination source

                                                                                r(xy) ndash determined by the characteristics of the imaged objects

                                                                                is called gray (or intensity) scale

                                                                                In practice

                                                                                min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                indoor values without additional illuminationmin max10 1000L L

                                                                                black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                min maxL L

                                                                                Digital Image ProcessingDigital Image Processing

                                                                                Week 1Week 1

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Image Sampling and Quantization

                                                                                - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                scene

                                                                                converting a continuous image f to digital form

                                                                                - digitizing (x y) is called sampling

                                                                                - digitizing f(x y) is called quantization

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                (00) (01) (0 1)(10) (11) (1 1)

                                                                                ( )

                                                                                ( 10) ( 11) ( 1 1)

                                                                                f f f Nf f f N

                                                                                f x y

                                                                                f M f M f M N

                                                                                image element pixel

                                                                                00 01 0 1

                                                                                10 11 1 1

                                                                                10 11 1 1

                                                                                ( ) ( )

                                                                                N

                                                                                i jN M N

                                                                                i j

                                                                                M M M N

                                                                                a a aa f x i y j f i ja a a

                                                                                Aa

                                                                                a a a

                                                                                f(00) ndash the upper left corner of the image

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                M N ge 0 L=2k

                                                                                [0 1]i j i ja a L

                                                                                Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Number of bits required to store a digitized image

                                                                                for 2 b M N k M N b N k

                                                                                When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                Image resolution = the largest number of discernible line pairs per unit distance

                                                                                (eg 100 line pairs per mm)

                                                                                Dots per unit distance are commonly used in printing and publishing

                                                                                In US the measure is expressed in dots per inch (dpi)

                                                                                (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                Intensity resolution ndash the smallest discernible change in intensity level

                                                                                The number of intensity levels (L) is determined by hardware considerations

                                                                                L=2k ndash most common k = 8

                                                                                Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                150 dpi (lower left) 72 dpi (lower right)

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Reducing the number of gray levels 256 128 64 32

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Reducing the number of gray levels 16 8 4 2

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                Interpolation is the process of using known data to estimate values at unknown locations

                                                                                Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                straight edges

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                be written using the 4 nearest neighbors of point (x y)

                                                                                Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                modest increase in computational effort

                                                                                Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                nearest neighbors of the point 3 3

                                                                                0 0

                                                                                ( ) i ji j

                                                                                i jv x y c x y

                                                                                The coefficients cij are obtained solving a 16x16 linear system

                                                                                intensity levels of the 16 nearest neighbors of 3 3

                                                                                0 0

                                                                                ( )i ji j

                                                                                i jc x y x y

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                programs such as Adobe Photoshop and Corel Photopaint

                                                                                Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                neighbor interpolation was used (both for shrinking and zooming)

                                                                                Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Neighbors of a Pixel

                                                                                A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                and are denoted ND(p)

                                                                                The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                N8 (p)

                                                                                If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                fall outside the image

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Adjacency Connectivity Regions Boundaries

                                                                                Denote by V the set of intensity levels used to define adjacency

                                                                                - in a binary image V 01 (V=0 V=1)

                                                                                - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                We consider 3 types of adjacency

                                                                                (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                m-adjacent if

                                                                                4( )q N p or

                                                                                ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                binary image

                                                                                0 1 1 0 1 1 0 1 1

                                                                                1 0 1 0 0 1 0 0 1 0

                                                                                0 0 1 0 0 1 0 0 1

                                                                                V

                                                                                The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                m-adjacency

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                is a sequence of distinct pixels with coordinates

                                                                                and are adjacent 0 0 1 1

                                                                                1 1

                                                                                ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                n n

                                                                                i i i i

                                                                                x y x y x y x y s tx y x y i n

                                                                                The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                in S if there exists a path between them consisting only of pixels from S

                                                                                S is a connected set if there is a path in S between any 2 pixels in S

                                                                                Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                8-adjacency are considered

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                touches the image border

                                                                                the complement of 1

                                                                                ( )K

                                                                                cu k u u

                                                                                k

                                                                                R R R R

                                                                                We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                background of the image

                                                                                The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                region that have at least one background neighbor This definition is referred to as the

                                                                                inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                border in the background

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Distance measures

                                                                                For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                function or metric if

                                                                                (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                (b) D(p q) = D(q p)

                                                                                (c) D(p z) le D(p q) + D(q z)

                                                                                The Euclidean distance between p and q is defined as 1

                                                                                2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                centered at (x y)

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                The D4 distance (also called city-block distance) between p and q is defined as

                                                                                4( ) | | | |D p q x s y t

                                                                                The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                4

                                                                                22 1 2

                                                                                2 2 1 0 1 22 1 2

                                                                                2

                                                                                D

                                                                                The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                8( ) max| | | |D p q x s y t

                                                                                The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                8

                                                                                2 2 2 2 22 1 1 1 2

                                                                                2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                D

                                                                                The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                because these distances involve only the coordinates of the point

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Array versus Matrix Operations

                                                                                An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                11 12 11 12

                                                                                21 22 21 22

                                                                                a a b ba a b b

                                                                                Array product

                                                                                11 12 11 12 11 11 12 12

                                                                                21 22 21 22 21 21 22 21

                                                                                a a b b a b a ba a b b a b a b

                                                                                Matrix product

                                                                                11 12 11 12 11 11 12 21 11 12 12 21

                                                                                21 22 21 22 21 11 22 21 21 12 22 22

                                                                                a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                We assume array operations unless stated otherwise

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Linear versus Nonlinear Operations

                                                                                One of the most important classifications of image-processing methods is whether it is

                                                                                linear or nonlinear

                                                                                ( ) ( )H f x y g x y

                                                                                H is said to be a linear operator if

                                                                                images1 2 1 2

                                                                                1 2

                                                                                ( ) ( ) ( ) ( )

                                                                                H a f x y b f x y a H f x y b H f x y

                                                                                a b f f

                                                                                Example of nonlinear operator

                                                                                the maximum value of the pixels of image max ( )H f f x y f

                                                                                1 2

                                                                                0 2 6 5 1 1

                                                                                2 3 4 7f f a b

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                1 2

                                                                                0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                2 3 4 7 2 4a f b f

                                                                                0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                2 3 4 7

                                                                                Arithmetic Operations in Image Processing

                                                                                Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                The two random variables are uncorrelated when their covariance is 0

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                used in image enhancement)

                                                                                1

                                                                                1( ) ( )K

                                                                                ii

                                                                                g x y g x yK

                                                                                If the noise satisfies the properties stated above we have

                                                                                2 2( ) ( )

                                                                                1( ) ( ) g x y x yE g x y f x yK

                                                                                ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                and g respectively The standard deviation (square root of the variance) at any point in

                                                                                the average image is

                                                                                ( ) ( )1

                                                                                g x y x yK

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                averaging process increases

                                                                                An important application of image averaging is in the field of astronomy where imaging

                                                                                under very low light levels frequently causes sensor noise to render single images

                                                                                virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                images respectively

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                100 noisy images

                                                                                a b c d e f

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                A frequent application of image subtraction is in the enhancement of differences between

                                                                                images

                                                                                (a) (b) (c)

                                                                                Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                significant bit of each pixel (c) the difference between the two images

                                                                                Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                difference between images (a) and (b)

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                images after injection of the contrast medium

                                                                                In g(x y) we can find the differences between h and f as enhanced detail

                                                                                Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                propagates through the various arteries in the area being observed

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                An important application of image multiplication (and division) is shading correction

                                                                                Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                When the shading function is known

                                                                                ( )( )( )

                                                                                g x yf x yh x y

                                                                                h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                approximation to the shading function by imaging a target of constant intensity When the

                                                                                sensor is not available often the shading pattern can be estimated from the image

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                (a) (b) (c)

                                                                                Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                operations The process consists of multiplying a given image by a mask image that has

                                                                                1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                (a) (b) (c)

                                                                                Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                min( )mf f f

                                                                                0 ( 255)max( )

                                                                                ms

                                                                                m

                                                                                ff K K K

                                                                                f

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Spatial Operations

                                                                                - are performed directly on the pixels of a given image

                                                                                There are three categories of spatial operations

                                                                                single-pixel operations

                                                                                neighborhood operations

                                                                                geometric spatial transformations

                                                                                Single-pixel operations

                                                                                - change the values of intensity for the individual pixels ( )s T z

                                                                                where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                corresponding pixel in the processed image

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Neighborhood operations

                                                                                Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                intensity by computing the average value of the pixels in Sxy

                                                                                ( )

                                                                                1( ) ( )xyr c S

                                                                                g x y f r cm n

                                                                                The net effect is to perform local blurring in the original image This type of process is

                                                                                used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                largest region of an image

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Geometric spatial transformations and image registration

                                                                                - modify the spatial relationship between pixels in an image

                                                                                - these transformations are often called rubber-sheet transformations (analogous to

                                                                                printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                predefined set of rules

                                                                                A geometric transformation consists of 2 basic operations

                                                                                1 a spatial transformation of coordinates

                                                                                2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                pixels

                                                                                The coordinate system transformation ( ) [( )]x y T v w

                                                                                (v w) ndash pixel coordinates in the original image

                                                                                (x y) ndash pixel coordinates in the transformed image

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                Affine transform

                                                                                11 1211 21 31

                                                                                21 2212 22 33

                                                                                31 32

                                                                                0[ 1] [ 1] [ 1] 0

                                                                                1

                                                                                t tx t v t w t

                                                                                x y v w T v w t ty t v t w t

                                                                                t t

                                                                                (AT)

                                                                                This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                scaling rotation and translation matrices from Table 1

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Affine transformations

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                The preceding transformations relocate pixels on an image to new locations To complete

                                                                                the process we have to assign intensity values to those locations This task is done by

                                                                                using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                In practice we can use equation (AT) in two basic ways

                                                                                forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                Problems

                                                                                - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                the same location in the output image

                                                                                - some output locations have no correspondent in the original image (no intensity

                                                                                assignment)

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                inverse mapping scans the output pixel locations and at each location (x y)

                                                                                computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                pixel value

                                                                                Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Image registration ndash align two or more images of the same scene

                                                                                In image registration we have available the input and output images but the specific

                                                                                transformation that produced the output image from the input is generally unknown

                                                                                The problem is to estimate the transformation function and then use it to register the two

                                                                                images

                                                                                - it may be of interest to align (register) two or more image taken at approximately the

                                                                                same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                - align images of a given location taken by the same instrument at different moments

                                                                                of time (satellite images)

                                                                                Solving the problem using tie points (also called control points) which are

                                                                                corresponding points whose locations are known precisely in the input and reference

                                                                                image

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                How to select tie points

                                                                                - interactively selecting them

                                                                                - use of algorithms that try to detect these points

                                                                                - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                the imaging sensors These objects produce a set of known points (called reseau

                                                                                marks) directly on all images captured by the system which can be used as guides

                                                                                for establishing tie points

                                                                                The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                of 4 tie points both on the input image and the reference image A simple model based on

                                                                                a bilinear approximation is given by

                                                                                1 2 3 4

                                                                                5 6 7 8

                                                                                x c v c w c v w cy c v c w c v w c

                                                                                (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                frequently is to select a larger number of tie points and using this new set of tie points

                                                                                subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                subregions marked by 4 tie points we applied the transformation model described above

                                                                                The number of tie points and the sophistication of the model required to solve the register

                                                                                problem depend on the severity of the geometrical distortion

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Probabilistic Methods

                                                                                zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                ( ) kk

                                                                                np zM N

                                                                                nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                pixels in the image) 1

                                                                                0( ) 1

                                                                                L

                                                                                kk

                                                                                p z

                                                                                The mean (average) intensity of an image is given by 1

                                                                                0( )

                                                                                L

                                                                                k kk

                                                                                m z p z

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                The variance of the intensities is 1

                                                                                2 2

                                                                                0( ) ( )

                                                                                L

                                                                                k kk

                                                                                z m p z

                                                                                The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                ( ) is used

                                                                                The n-th moment of a random variable z about the mean is defined as 1

                                                                                0( ) ( ) ( )

                                                                                Ln

                                                                                n k kk

                                                                                z z m p z

                                                                                ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                3( ) 0z the intensities are biased to values higher than the mean

                                                                                ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                mean

                                                                                Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Intensity Transformations and Spatial Filtering

                                                                                ( ) ( )g x y T f x y

                                                                                f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                neighborhood of (x y)

                                                                                - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                and much smaller in size than the image

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                called spatial filter (spatial mask kernel template or window)

                                                                                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                ( )s T r

                                                                                s and r are denoting respectively the intensity of g and f at (x y)

                                                                                Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                is called contrast stretching

                                                                                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                thresholding function

                                                                                Some Basic Intensity Transformation Functions

                                                                                Image Negatives

                                                                                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                - equivalent of a photographic negative

                                                                                - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                image

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Original Negative image

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                Some basic intensity transformation functions

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                range An operator of this type is used to expand the values of dark pixels in an image

                                                                                while compressing the higher-level values The opposite is true for the inverse log

                                                                                transformation The log functions compress the dynamic range of images with large

                                                                                variations in pixel values

                                                                                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Power-Law (Gamma) Transformations

                                                                                - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                Plots of gamma transformation for different values of γ (c=1)

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                of output values with the opposite being true for higher values of input values The

                                                                                curves with 1 have the opposite effect of those generated with values of 1

                                                                                1c - identity transformation

                                                                                A variety of devices used for image capture printing and display respond according to a

                                                                                power law The process used to correct these power-law response phenomena is called

                                                                                gamma correction

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Piecewise-Linear Transformations Functions

                                                                                Contrast stretching

                                                                                - a process that expands the range of intensity levels in an image so it spans the full

                                                                                intensity range of the recording tool or display device

                                                                                a b c d Fig5

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                11

                                                                                1

                                                                                2 1 1 21 2

                                                                                2 1 2 1

                                                                                22

                                                                                2

                                                                                [0 ]

                                                                                ( ) ( )( ) [ ]( ) ( )

                                                                                ( 1 ) [ 1]( 1 )

                                                                                s r r rrs r r s r rT r r r r

                                                                                r r r rs L r r r L

                                                                                L r

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                1 1 2 2r s r s identity transformation (no change)

                                                                                1 2 1 2 0 1r r s s L thresholding function

                                                                                Figure 5(b) shows an 8-bit image with low contrast

                                                                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                in the image respectively Thus the transformation function stretched the levels linearly

                                                                                from their original range to the full range [0 L-1]

                                                                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                2 2 1r s m L where m is the mean gray level in the image

                                                                                The original image on which these results are based is a scanning electron microscope

                                                                                image of pollen magnified approximately 700 times

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Intensity-level slicing

                                                                                - highlighting a specific range of intensities in an image

                                                                                There are two approaches for intensity-level slicing

                                                                                1 display in one value (white for example) all the values in the range of interest and in

                                                                                another (say black) all other intensities (Figure 311 (a))

                                                                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                intensities in the image (Figure 311 (b))

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                the top of the scale of intensities This type of enhancement produces a binary image

                                                                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                Highlights range [A B] and preserves all other intensities

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                blockageshellip)

                                                                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                image around the mean intensity was set to black the other intensities remain unchanged

                                                                                Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Bit-plane slicing

                                                                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                This technique highlights the contribution made to the whole image appearances by each

                                                                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                Digital Image Processing

                                                                                Week 1

                                                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                • DIP 1 2017
                                                                                • DIP 02 (2017)

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Face detection and recognition

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Gender identification

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Image morphing

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Fundamental Steps in DIP

                                                                                  methods whose input and output are images

                                                                                  methods whose inputs are images but whose outputs are attributes extracted from those images

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Outputs are images

                                                                                  bull image acquisition

                                                                                  bull image filtering and enhancement

                                                                                  bull image restoration

                                                                                  bull color image processing

                                                                                  bull wavelets and multiresolution processing

                                                                                  bull compression

                                                                                  bull morphological processing

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Outputs are attributes

                                                                                  bull morphological processing

                                                                                  bull segmentation

                                                                                  bull representation and description

                                                                                  bull object recognition

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Image acquisition - may involve preprocessing such as scaling

                                                                                  Image enhancement

                                                                                  bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                                  bull enhancement is problem oriented

                                                                                  bull there is no general sbquotheoryrsquo of image enhancement

                                                                                  bull enhancement use subjective methods for image emprovement

                                                                                  bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Image restoration

                                                                                  bull improving the appearance of an image

                                                                                  bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                                  Color image processing

                                                                                  bull fundamental concept in color models

                                                                                  bull basic color processing in a digital domain

                                                                                  Wavelets and multiresolution processing

                                                                                  representing images in various degree of resolution

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Compression

                                                                                  reducing the storage required to save an image or the bandwidth required to transmit it

                                                                                  Morphological processing

                                                                                  bull tools for extracting image components that are useful in the representation and description of shape

                                                                                  bull a transition from processes that output images to processes that outputimage attributes

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Segmentation

                                                                                  bull partitioning an image into its constituents parts or objects

                                                                                  bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                                  bull the more accurate the segmentation the more likley recognition is to succeed

                                                                                  Representation and description (almost always follows segmentation)

                                                                                  bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                                  bull converting the data produced by segmentation to a form suitable for computer processing

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                                  bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                                  bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                                  Object recognition

                                                                                  the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                                  Knowledge database

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Simplified diagramof a cross sectionof the human eye

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                  The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                  Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                  The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                  The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                  Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                  Fovea = the place where the image of the object of interest falls on

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                  Blind spot region without receptors

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Image formation in the eye

                                                                                  Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                  Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                  distance between lens and retina along visual axix = 17 mm

                                                                                  range of focal length = 14 mm to 17 mm

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Optical illusions

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                  gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                  quantities that describe the quality of a chromatic light source radiance

                                                                                  the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                  measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                  brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  the physical meaning is determined by the source of the image

                                                                                  ( )f D f x y

                                                                                  Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                  f(xy) ndash characterized by two components

                                                                                  i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                  r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                  ( ) ( ) ( )

                                                                                  0 ( ) 0 ( ) 1

                                                                                  f x y i x y r x y

                                                                                  i x y r x y

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                  i(xy) ndash determined by the illumination source

                                                                                  r(xy) ndash determined by the characteristics of the imaged objects

                                                                                  is called gray (or intensity) scale

                                                                                  In practice

                                                                                  min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                  indoor values without additional illuminationmin max10 1000L L

                                                                                  black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                  min maxL L

                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                  Week 1Week 1

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Image Sampling and Quantization

                                                                                  - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                  scene

                                                                                  converting a continuous image f to digital form

                                                                                  - digitizing (x y) is called sampling

                                                                                  - digitizing f(x y) is called quantization

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                  (00) (01) (0 1)(10) (11) (1 1)

                                                                                  ( )

                                                                                  ( 10) ( 11) ( 1 1)

                                                                                  f f f Nf f f N

                                                                                  f x y

                                                                                  f M f M f M N

                                                                                  image element pixel

                                                                                  00 01 0 1

                                                                                  10 11 1 1

                                                                                  10 11 1 1

                                                                                  ( ) ( )

                                                                                  N

                                                                                  i jN M N

                                                                                  i j

                                                                                  M M M N

                                                                                  a a aa f x i y j f i ja a a

                                                                                  Aa

                                                                                  a a a

                                                                                  f(00) ndash the upper left corner of the image

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  M N ge 0 L=2k

                                                                                  [0 1]i j i ja a L

                                                                                  Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Number of bits required to store a digitized image

                                                                                  for 2 b M N k M N b N k

                                                                                  When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                  Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                  Image resolution = the largest number of discernible line pairs per unit distance

                                                                                  (eg 100 line pairs per mm)

                                                                                  Dots per unit distance are commonly used in printing and publishing

                                                                                  In US the measure is expressed in dots per inch (dpi)

                                                                                  (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                  Intensity resolution ndash the smallest discernible change in intensity level

                                                                                  The number of intensity levels (L) is determined by hardware considerations

                                                                                  L=2k ndash most common k = 8

                                                                                  Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                  150 dpi (lower left) 72 dpi (lower right)

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Reducing the number of gray levels 256 128 64 32

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Reducing the number of gray levels 16 8 4 2

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                  Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                  Interpolation is the process of using known data to estimate values at unknown locations

                                                                                  Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                  750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                  same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                  image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                  Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                  Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                  intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                  This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                  straight edges

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                  where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                  be written using the 4 nearest neighbors of point (x y)

                                                                                  Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                  modest increase in computational effort

                                                                                  Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                  nearest neighbors of the point 3 3

                                                                                  0 0

                                                                                  ( ) i ji j

                                                                                  i jv x y c x y

                                                                                  The coefficients cij are obtained solving a 16x16 linear system

                                                                                  intensity levels of the 16 nearest neighbors of 3 3

                                                                                  0 0

                                                                                  ( )i ji j

                                                                                  i jc x y x y

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                  bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                  programs such as Adobe Photoshop and Corel Photopaint

                                                                                  Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                  the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                  then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                  neighbor interpolation was used (both for shrinking and zooming)

                                                                                  Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                  interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                  from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Neighbors of a Pixel

                                                                                  A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                  This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                  The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                  and are denoted ND(p)

                                                                                  The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                  N8 (p)

                                                                                  If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                  fall outside the image

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Adjacency Connectivity Regions Boundaries

                                                                                  Denote by V the set of intensity levels used to define adjacency

                                                                                  - in a binary image V 01 (V=0 V=1)

                                                                                  - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                  We consider 3 types of adjacency

                                                                                  (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                  (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                  (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                  m-adjacent if

                                                                                  4( )q N p or

                                                                                  ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                  Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                  ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  binary image

                                                                                  0 1 1 0 1 1 0 1 1

                                                                                  1 0 1 0 0 1 0 0 1 0

                                                                                  0 0 1 0 0 1 0 0 1

                                                                                  V

                                                                                  The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                  8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                  m-adjacency

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                  is a sequence of distinct pixels with coordinates

                                                                                  and are adjacent 0 0 1 1

                                                                                  1 1

                                                                                  ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                  n n

                                                                                  i i i i

                                                                                  x y x y x y x y s tx y x y i n

                                                                                  The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                  Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                  Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                  in S if there exists a path between them consisting only of pixels from S

                                                                                  S is a connected set if there is a path in S between any 2 pixels in S

                                                                                  Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                  Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                  that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                  8-adjacency are considered

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                  touches the image border

                                                                                  the complement of 1

                                                                                  ( )K

                                                                                  cu k u u

                                                                                  k

                                                                                  R R R R

                                                                                  We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                  background of the image

                                                                                  The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                  points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                  region that have at least one background neighbor This definition is referred to as the

                                                                                  inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                  border in the background

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Distance measures

                                                                                  For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                  function or metric if

                                                                                  (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                  (b) D(p q) = D(q p)

                                                                                  (c) D(p z) le D(p q) + D(q z)

                                                                                  The Euclidean distance between p and q is defined as 1

                                                                                  2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                  The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                  centered at (x y)

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  The D4 distance (also called city-block distance) between p and q is defined as

                                                                                  4( ) | | | |D p q x s y t

                                                                                  The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                  4

                                                                                  22 1 2

                                                                                  2 2 1 0 1 22 1 2

                                                                                  2

                                                                                  D

                                                                                  The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                  The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                  8( ) max| | | |D p q x s y t

                                                                                  The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  8

                                                                                  2 2 2 2 22 1 1 1 2

                                                                                  2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                  D

                                                                                  The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                  D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                  because these distances involve only the coordinates of the point

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Array versus Matrix Operations

                                                                                  An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                  11 12 11 12

                                                                                  21 22 21 22

                                                                                  a a b ba a b b

                                                                                  Array product

                                                                                  11 12 11 12 11 11 12 12

                                                                                  21 22 21 22 21 21 22 21

                                                                                  a a b b a b a ba a b b a b a b

                                                                                  Matrix product

                                                                                  11 12 11 12 11 11 12 21 11 12 12 21

                                                                                  21 22 21 22 21 11 22 21 21 12 22 22

                                                                                  a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                  We assume array operations unless stated otherwise

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Linear versus Nonlinear Operations

                                                                                  One of the most important classifications of image-processing methods is whether it is

                                                                                  linear or nonlinear

                                                                                  ( ) ( )H f x y g x y

                                                                                  H is said to be a linear operator if

                                                                                  images1 2 1 2

                                                                                  1 2

                                                                                  ( ) ( ) ( ) ( )

                                                                                  H a f x y b f x y a H f x y b H f x y

                                                                                  a b f f

                                                                                  Example of nonlinear operator

                                                                                  the maximum value of the pixels of image max ( )H f f x y f

                                                                                  1 2

                                                                                  0 2 6 5 1 1

                                                                                  2 3 4 7f f a b

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  1 2

                                                                                  0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                  2 3 4 7 2 4a f b f

                                                                                  0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                  2 3 4 7

                                                                                  Arithmetic Operations in Image Processing

                                                                                  Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                  f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                  For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                  value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                  The two random variables are uncorrelated when their covariance is 0

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                  used in image enhancement)

                                                                                  1

                                                                                  1( ) ( )K

                                                                                  ii

                                                                                  g x y g x yK

                                                                                  If the noise satisfies the properties stated above we have

                                                                                  2 2( ) ( )

                                                                                  1( ) ( ) g x y x yE g x y f x yK

                                                                                  ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                  and g respectively The standard deviation (square root of the variance) at any point in

                                                                                  the average image is

                                                                                  ( ) ( )1

                                                                                  g x y x yK

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                  the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                  means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                  averaging process increases

                                                                                  An important application of image averaging is in the field of astronomy where imaging

                                                                                  under very low light levels frequently causes sensor noise to render single images

                                                                                  virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                  was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                  64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                  images respectively

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                  100 noisy images

                                                                                  a b c d e f

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  A frequent application of image subtraction is in the enhancement of differences between

                                                                                  images

                                                                                  (a) (b) (c)

                                                                                  Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                  significant bit of each pixel (c) the difference between the two images

                                                                                  Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                  Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                  images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                  difference between images (a) and (b)

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                  h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                  intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                  source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                  bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                  anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                  images after injection of the contrast medium

                                                                                  In g(x y) we can find the differences between h and f as enhanced detail

                                                                                  Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                  propagates through the various arteries in the area being observed

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  An important application of image multiplication (and division) is shading correction

                                                                                  Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                  f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                  When the shading function is known

                                                                                  ( )( )( )

                                                                                  g x yf x yh x y

                                                                                  h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                  approximation to the shading function by imaging a target of constant intensity When the

                                                                                  sensor is not available often the shading pattern can be estimated from the image

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  (a) (b) (c)

                                                                                  Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                  operations The process consists of multiplying a given image by a mask image that has

                                                                                  1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                  image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                  (a) (b) (c)

                                                                                  Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                  min( )mf f f

                                                                                  0 ( 255)max( )

                                                                                  ms

                                                                                  m

                                                                                  ff K K K

                                                                                  f

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Spatial Operations

                                                                                  - are performed directly on the pixels of a given image

                                                                                  There are three categories of spatial operations

                                                                                  single-pixel operations

                                                                                  neighborhood operations

                                                                                  geometric spatial transformations

                                                                                  Single-pixel operations

                                                                                  - change the values of intensity for the individual pixels ( )s T z

                                                                                  where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                  corresponding pixel in the processed image

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Neighborhood operations

                                                                                  Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                  in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                  based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                  rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                  intensity by computing the average value of the pixels in Sxy

                                                                                  ( )

                                                                                  1( ) ( )xyr c S

                                                                                  g x y f r cm n

                                                                                  The net effect is to perform local blurring in the original image This type of process is

                                                                                  used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                  largest region of an image

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Geometric spatial transformations and image registration

                                                                                  - modify the spatial relationship between pixels in an image

                                                                                  - these transformations are often called rubber-sheet transformations (analogous to

                                                                                  printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                  predefined set of rules

                                                                                  A geometric transformation consists of 2 basic operations

                                                                                  1 a spatial transformation of coordinates

                                                                                  2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                  pixels

                                                                                  The coordinate system transformation ( ) [( )]x y T v w

                                                                                  (v w) ndash pixel coordinates in the original image

                                                                                  (x y) ndash pixel coordinates in the transformed image

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                  Affine transform

                                                                                  11 1211 21 31

                                                                                  21 2212 22 33

                                                                                  31 32

                                                                                  0[ 1] [ 1] [ 1] 0

                                                                                  1

                                                                                  t tx t v t w t

                                                                                  x y v w T v w t ty t v t w t

                                                                                  t t

                                                                                  (AT)

                                                                                  This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                  on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                  result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                  scaling rotation and translation matrices from Table 1

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Affine transformations

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  The preceding transformations relocate pixels on an image to new locations To complete

                                                                                  the process we have to assign intensity values to those locations This task is done by

                                                                                  using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                  In practice we can use equation (AT) in two basic ways

                                                                                  forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                  location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                  Problems

                                                                                  - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                  the same location in the output image

                                                                                  - some output locations have no correspondent in the original image (no intensity

                                                                                  assignment)

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  inverse mapping scans the output pixel locations and at each location (x y)

                                                                                  computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                  It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                  pixel value

                                                                                  Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                  numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Image registration ndash align two or more images of the same scene

                                                                                  In image registration we have available the input and output images but the specific

                                                                                  transformation that produced the output image from the input is generally unknown

                                                                                  The problem is to estimate the transformation function and then use it to register the two

                                                                                  images

                                                                                  - it may be of interest to align (register) two or more image taken at approximately the

                                                                                  same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                  - align images of a given location taken by the same instrument at different moments

                                                                                  of time (satellite images)

                                                                                  Solving the problem using tie points (also called control points) which are

                                                                                  corresponding points whose locations are known precisely in the input and reference

                                                                                  image

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  How to select tie points

                                                                                  - interactively selecting them

                                                                                  - use of algorithms that try to detect these points

                                                                                  - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                  the imaging sensors These objects produce a set of known points (called reseau

                                                                                  marks) directly on all images captured by the system which can be used as guides

                                                                                  for establishing tie points

                                                                                  The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                  of 4 tie points both on the input image and the reference image A simple model based on

                                                                                  a bilinear approximation is given by

                                                                                  1 2 3 4

                                                                                  5 6 7 8

                                                                                  x c v c w c v w cy c v c w c v w c

                                                                                  (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                  frequently is to select a larger number of tie points and using this new set of tie points

                                                                                  subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                  subregions marked by 4 tie points we applied the transformation model described above

                                                                                  The number of tie points and the sophistication of the model required to solve the register

                                                                                  problem depend on the severity of the geometrical distortion

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Probabilistic Methods

                                                                                  zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                  p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                  ( ) kk

                                                                                  np zM N

                                                                                  nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                  pixels in the image) 1

                                                                                  0( ) 1

                                                                                  L

                                                                                  kk

                                                                                  p z

                                                                                  The mean (average) intensity of an image is given by 1

                                                                                  0( )

                                                                                  L

                                                                                  k kk

                                                                                  m z p z

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  The variance of the intensities is 1

                                                                                  2 2

                                                                                  0( ) ( )

                                                                                  L

                                                                                  k kk

                                                                                  z m p z

                                                                                  The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                  measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                  ( ) is used

                                                                                  The n-th moment of a random variable z about the mean is defined as 1

                                                                                  0( ) ( ) ( )

                                                                                  Ln

                                                                                  n k kk

                                                                                  z z m p z

                                                                                  ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                  3( ) 0z the intensities are biased to values higher than the mean

                                                                                  ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                  mean

                                                                                  Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                  Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                  Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                  Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Intensity Transformations and Spatial Filtering

                                                                                  ( ) ( )g x y T f x y

                                                                                  f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                  neighborhood of (x y)

                                                                                  - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                  and much smaller in size than the image

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                  called spatial filter (spatial mask kernel template or window)

                                                                                  ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                  ( )s T r

                                                                                  s and r are denoting respectively the intensity of g and f at (x y)

                                                                                  Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                  darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                  is called contrast stretching

                                                                                  Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                  thresholding function

                                                                                  Some Basic Intensity Transformation Functions

                                                                                  Image Negatives

                                                                                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                  - equivalent of a photographic negative

                                                                                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                  image

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Original Negative image

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                  Some basic intensity transformation functions

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                  range An operator of this type is used to expand the values of dark pixels in an image

                                                                                  while compressing the higher-level values The opposite is true for the inverse log

                                                                                  transformation The log functions compress the dynamic range of images with large

                                                                                  variations in pixel values

                                                                                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Power-Law (Gamma) Transformations

                                                                                  - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                  Plots of gamma transformation for different values of γ (c=1)

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                  of output values with the opposite being true for higher values of input values The

                                                                                  curves with 1 have the opposite effect of those generated with values of 1

                                                                                  1c - identity transformation

                                                                                  A variety of devices used for image capture printing and display respond according to a

                                                                                  power law The process used to correct these power-law response phenomena is called

                                                                                  gamma correction

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Piecewise-Linear Transformations Functions

                                                                                  Contrast stretching

                                                                                  - a process that expands the range of intensity levels in an image so it spans the full

                                                                                  intensity range of the recording tool or display device

                                                                                  a b c d Fig5

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  11

                                                                                  1

                                                                                  2 1 1 21 2

                                                                                  2 1 2 1

                                                                                  22

                                                                                  2

                                                                                  [0 ]

                                                                                  ( ) ( )( ) [ ]( ) ( )

                                                                                  ( 1 ) [ 1]( 1 )

                                                                                  s r r rrs r r s r rT r r r r

                                                                                  r r r rs L r r r L

                                                                                  L r

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  1 1 2 2r s r s identity transformation (no change)

                                                                                  1 2 1 2 0 1r r s s L thresholding function

                                                                                  Figure 5(b) shows an 8-bit image with low contrast

                                                                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                  in the image respectively Thus the transformation function stretched the levels linearly

                                                                                  from their original range to the full range [0 L-1]

                                                                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                  2 2 1r s m L where m is the mean gray level in the image

                                                                                  The original image on which these results are based is a scanning electron microscope

                                                                                  image of pollen magnified approximately 700 times

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Intensity-level slicing

                                                                                  - highlighting a specific range of intensities in an image

                                                                                  There are two approaches for intensity-level slicing

                                                                                  1 display in one value (white for example) all the values in the range of interest and in

                                                                                  another (say black) all other intensities (Figure 311 (a))

                                                                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                  intensities in the image (Figure 311 (b))

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                                                                                  Week 1

                                                                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                  the top of the scale of intensities This type of enhancement produces a binary image

                                                                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                  Highlights range [A B] and preserves all other intensities

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                  blockageshellip)

                                                                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                  image around the mean intensity was set to black the other intensities remain unchanged

                                                                                  Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                  Digital Image Processing

                                                                                  Week 1

                                                                                  Bit-plane slicing

                                                                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                  This technique highlights the contribution made to the whole image appearances by each

                                                                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

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                                                                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                  • DIP 1 2017
                                                                                  • DIP 02 (2017)

                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                    Week 1Week 1

                                                                                    Gender identification

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                                                                                    Image morphing

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                                                                                    Fundamental Steps in DIP

                                                                                    methods whose input and output are images

                                                                                    methods whose inputs are images but whose outputs are attributes extracted from those images

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                                                                                    Week 1Week 1

                                                                                    Outputs are images

                                                                                    bull image acquisition

                                                                                    bull image filtering and enhancement

                                                                                    bull image restoration

                                                                                    bull color image processing

                                                                                    bull wavelets and multiresolution processing

                                                                                    bull compression

                                                                                    bull morphological processing

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                                                                                    Week 1Week 1

                                                                                    Outputs are attributes

                                                                                    bull morphological processing

                                                                                    bull segmentation

                                                                                    bull representation and description

                                                                                    bull object recognition

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                                                                                    Week 1Week 1

                                                                                    Image acquisition - may involve preprocessing such as scaling

                                                                                    Image enhancement

                                                                                    bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                                    bull enhancement is problem oriented

                                                                                    bull there is no general sbquotheoryrsquo of image enhancement

                                                                                    bull enhancement use subjective methods for image emprovement

                                                                                    bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

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                                                                                    Image restoration

                                                                                    bull improving the appearance of an image

                                                                                    bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                                    Color image processing

                                                                                    bull fundamental concept in color models

                                                                                    bull basic color processing in a digital domain

                                                                                    Wavelets and multiresolution processing

                                                                                    representing images in various degree of resolution

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                                                                                    Week 1Week 1

                                                                                    Compression

                                                                                    reducing the storage required to save an image or the bandwidth required to transmit it

                                                                                    Morphological processing

                                                                                    bull tools for extracting image components that are useful in the representation and description of shape

                                                                                    bull a transition from processes that output images to processes that outputimage attributes

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                                                                                    Week 1Week 1

                                                                                    Segmentation

                                                                                    bull partitioning an image into its constituents parts or objects

                                                                                    bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                                    bull the more accurate the segmentation the more likley recognition is to succeed

                                                                                    Representation and description (almost always follows segmentation)

                                                                                    bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                                    bull converting the data produced by segmentation to a form suitable for computer processing

                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                    Week 1Week 1

                                                                                    bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                                    bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                                    bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                                    Object recognition

                                                                                    the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                                    Knowledge database

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                                                                                    Week 1Week 1

                                                                                    Simplified diagramof a cross sectionof the human eye

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                                                                                    Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                    The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                    Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                    The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                    Week 1Week 1

                                                                                    The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                    The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                    Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                    Fovea = the place where the image of the object of interest falls on

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                                                                                    Week 1Week 1

                                                                                    Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                    Blind spot region without receptors

                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                    Week 1Week 1

                                                                                    Image formation in the eye

                                                                                    Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                    Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                    distance between lens and retina along visual axix = 17 mm

                                                                                    range of focal length = 14 mm to 17 mm

                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                    Week 1Week 1

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                                                                                    Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                    Week 1Week 1

                                                                                    All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                    Week 1Week 1

                                                                                    Optical illusions

                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                    Week 1Week 1

                                                                                    ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                    Week 1Week 1

                                                                                    Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                    gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                    quantities that describe the quality of a chromatic light source radiance

                                                                                    the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                    measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                    Week 1Week 1

                                                                                    For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                    brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                    Week 1Week 1

                                                                                    the physical meaning is determined by the source of the image

                                                                                    ( )f D f x y

                                                                                    Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                    f(xy) ndash characterized by two components

                                                                                    i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                    r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                    ( ) ( ) ( )

                                                                                    0 ( ) 0 ( ) 1

                                                                                    f x y i x y r x y

                                                                                    i x y r x y

                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                    Week 1Week 1

                                                                                    r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                    i(xy) ndash determined by the illumination source

                                                                                    r(xy) ndash determined by the characteristics of the imaged objects

                                                                                    is called gray (or intensity) scale

                                                                                    In practice

                                                                                    min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                    indoor values without additional illuminationmin max10 1000L L

                                                                                    black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                    min maxL L

                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                    Week 1Week 1

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Image Sampling and Quantization

                                                                                    - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                    scene

                                                                                    converting a continuous image f to digital form

                                                                                    - digitizing (x y) is called sampling

                                                                                    - digitizing f(x y) is called quantization

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                    (00) (01) (0 1)(10) (11) (1 1)

                                                                                    ( )

                                                                                    ( 10) ( 11) ( 1 1)

                                                                                    f f f Nf f f N

                                                                                    f x y

                                                                                    f M f M f M N

                                                                                    image element pixel

                                                                                    00 01 0 1

                                                                                    10 11 1 1

                                                                                    10 11 1 1

                                                                                    ( ) ( )

                                                                                    N

                                                                                    i jN M N

                                                                                    i j

                                                                                    M M M N

                                                                                    a a aa f x i y j f i ja a a

                                                                                    Aa

                                                                                    a a a

                                                                                    f(00) ndash the upper left corner of the image

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    M N ge 0 L=2k

                                                                                    [0 1]i j i ja a L

                                                                                    Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Number of bits required to store a digitized image

                                                                                    for 2 b M N k M N b N k

                                                                                    When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                    Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                    Image resolution = the largest number of discernible line pairs per unit distance

                                                                                    (eg 100 line pairs per mm)

                                                                                    Dots per unit distance are commonly used in printing and publishing

                                                                                    In US the measure is expressed in dots per inch (dpi)

                                                                                    (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                    Intensity resolution ndash the smallest discernible change in intensity level

                                                                                    The number of intensity levels (L) is determined by hardware considerations

                                                                                    L=2k ndash most common k = 8

                                                                                    Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                    150 dpi (lower left) 72 dpi (lower right)

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Reducing the number of gray levels 256 128 64 32

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Reducing the number of gray levels 16 8 4 2

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                    Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                    Interpolation is the process of using known data to estimate values at unknown locations

                                                                                    Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                    750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                    same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                    image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                    Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                    Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                    intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                    This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                    straight edges

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                    where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                    be written using the 4 nearest neighbors of point (x y)

                                                                                    Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                    modest increase in computational effort

                                                                                    Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                    nearest neighbors of the point 3 3

                                                                                    0 0

                                                                                    ( ) i ji j

                                                                                    i jv x y c x y

                                                                                    The coefficients cij are obtained solving a 16x16 linear system

                                                                                    intensity levels of the 16 nearest neighbors of 3 3

                                                                                    0 0

                                                                                    ( )i ji j

                                                                                    i jc x y x y

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                    bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                    programs such as Adobe Photoshop and Corel Photopaint

                                                                                    Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                    the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                    then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                    neighbor interpolation was used (both for shrinking and zooming)

                                                                                    Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                    interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                    from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Neighbors of a Pixel

                                                                                    A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                    This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                    The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                    and are denoted ND(p)

                                                                                    The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                    N8 (p)

                                                                                    If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                    fall outside the image

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Adjacency Connectivity Regions Boundaries

                                                                                    Denote by V the set of intensity levels used to define adjacency

                                                                                    - in a binary image V 01 (V=0 V=1)

                                                                                    - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                    We consider 3 types of adjacency

                                                                                    (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                    (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                    (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                    m-adjacent if

                                                                                    4( )q N p or

                                                                                    ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                    Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                    ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    binary image

                                                                                    0 1 1 0 1 1 0 1 1

                                                                                    1 0 1 0 0 1 0 0 1 0

                                                                                    0 0 1 0 0 1 0 0 1

                                                                                    V

                                                                                    The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                    8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                    m-adjacency

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                    is a sequence of distinct pixels with coordinates

                                                                                    and are adjacent 0 0 1 1

                                                                                    1 1

                                                                                    ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                    n n

                                                                                    i i i i

                                                                                    x y x y x y x y s tx y x y i n

                                                                                    The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                    Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                    Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                    in S if there exists a path between them consisting only of pixels from S

                                                                                    S is a connected set if there is a path in S between any 2 pixels in S

                                                                                    Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                    Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                    that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                    8-adjacency are considered

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                    touches the image border

                                                                                    the complement of 1

                                                                                    ( )K

                                                                                    cu k u u

                                                                                    k

                                                                                    R R R R

                                                                                    We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                    background of the image

                                                                                    The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                    points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                    region that have at least one background neighbor This definition is referred to as the

                                                                                    inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                    border in the background

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Distance measures

                                                                                    For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                    function or metric if

                                                                                    (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                    (b) D(p q) = D(q p)

                                                                                    (c) D(p z) le D(p q) + D(q z)

                                                                                    The Euclidean distance between p and q is defined as 1

                                                                                    2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                    The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                    centered at (x y)

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    The D4 distance (also called city-block distance) between p and q is defined as

                                                                                    4( ) | | | |D p q x s y t

                                                                                    The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                    4

                                                                                    22 1 2

                                                                                    2 2 1 0 1 22 1 2

                                                                                    2

                                                                                    D

                                                                                    The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                    The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                    8( ) max| | | |D p q x s y t

                                                                                    The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    8

                                                                                    2 2 2 2 22 1 1 1 2

                                                                                    2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                    D

                                                                                    The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                    D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                    because these distances involve only the coordinates of the point

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Array versus Matrix Operations

                                                                                    An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                    11 12 11 12

                                                                                    21 22 21 22

                                                                                    a a b ba a b b

                                                                                    Array product

                                                                                    11 12 11 12 11 11 12 12

                                                                                    21 22 21 22 21 21 22 21

                                                                                    a a b b a b a ba a b b a b a b

                                                                                    Matrix product

                                                                                    11 12 11 12 11 11 12 21 11 12 12 21

                                                                                    21 22 21 22 21 11 22 21 21 12 22 22

                                                                                    a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                    We assume array operations unless stated otherwise

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Linear versus Nonlinear Operations

                                                                                    One of the most important classifications of image-processing methods is whether it is

                                                                                    linear or nonlinear

                                                                                    ( ) ( )H f x y g x y

                                                                                    H is said to be a linear operator if

                                                                                    images1 2 1 2

                                                                                    1 2

                                                                                    ( ) ( ) ( ) ( )

                                                                                    H a f x y b f x y a H f x y b H f x y

                                                                                    a b f f

                                                                                    Example of nonlinear operator

                                                                                    the maximum value of the pixels of image max ( )H f f x y f

                                                                                    1 2

                                                                                    0 2 6 5 1 1

                                                                                    2 3 4 7f f a b

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    1 2

                                                                                    0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                    2 3 4 7 2 4a f b f

                                                                                    0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                    2 3 4 7

                                                                                    Arithmetic Operations in Image Processing

                                                                                    Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                    f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                    For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                    value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                    The two random variables are uncorrelated when their covariance is 0

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                    used in image enhancement)

                                                                                    1

                                                                                    1( ) ( )K

                                                                                    ii

                                                                                    g x y g x yK

                                                                                    If the noise satisfies the properties stated above we have

                                                                                    2 2( ) ( )

                                                                                    1( ) ( ) g x y x yE g x y f x yK

                                                                                    ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                    and g respectively The standard deviation (square root of the variance) at any point in

                                                                                    the average image is

                                                                                    ( ) ( )1

                                                                                    g x y x yK

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                    the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                    means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                    averaging process increases

                                                                                    An important application of image averaging is in the field of astronomy where imaging

                                                                                    under very low light levels frequently causes sensor noise to render single images

                                                                                    virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                    was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                    64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                    images respectively

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                    100 noisy images

                                                                                    a b c d e f

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    A frequent application of image subtraction is in the enhancement of differences between

                                                                                    images

                                                                                    (a) (b) (c)

                                                                                    Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                    significant bit of each pixel (c) the difference between the two images

                                                                                    Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                    Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                    images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                    difference between images (a) and (b)

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                    h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                    intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                    source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                    bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                    anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                    images after injection of the contrast medium

                                                                                    In g(x y) we can find the differences between h and f as enhanced detail

                                                                                    Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                    propagates through the various arteries in the area being observed

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    An important application of image multiplication (and division) is shading correction

                                                                                    Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                    f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                    When the shading function is known

                                                                                    ( )( )( )

                                                                                    g x yf x yh x y

                                                                                    h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                    approximation to the shading function by imaging a target of constant intensity When the

                                                                                    sensor is not available often the shading pattern can be estimated from the image

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    (a) (b) (c)

                                                                                    Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                    operations The process consists of multiplying a given image by a mask image that has

                                                                                    1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                    image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                    (a) (b) (c)

                                                                                    Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                    min( )mf f f

                                                                                    0 ( 255)max( )

                                                                                    ms

                                                                                    m

                                                                                    ff K K K

                                                                                    f

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Spatial Operations

                                                                                    - are performed directly on the pixels of a given image

                                                                                    There are three categories of spatial operations

                                                                                    single-pixel operations

                                                                                    neighborhood operations

                                                                                    geometric spatial transformations

                                                                                    Single-pixel operations

                                                                                    - change the values of intensity for the individual pixels ( )s T z

                                                                                    where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                    corresponding pixel in the processed image

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Neighborhood operations

                                                                                    Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                    in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                    based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                    rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                    intensity by computing the average value of the pixels in Sxy

                                                                                    ( )

                                                                                    1( ) ( )xyr c S

                                                                                    g x y f r cm n

                                                                                    The net effect is to perform local blurring in the original image This type of process is

                                                                                    used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                    largest region of an image

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Geometric spatial transformations and image registration

                                                                                    - modify the spatial relationship between pixels in an image

                                                                                    - these transformations are often called rubber-sheet transformations (analogous to

                                                                                    printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                    predefined set of rules

                                                                                    A geometric transformation consists of 2 basic operations

                                                                                    1 a spatial transformation of coordinates

                                                                                    2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                    pixels

                                                                                    The coordinate system transformation ( ) [( )]x y T v w

                                                                                    (v w) ndash pixel coordinates in the original image

                                                                                    (x y) ndash pixel coordinates in the transformed image

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                    Affine transform

                                                                                    11 1211 21 31

                                                                                    21 2212 22 33

                                                                                    31 32

                                                                                    0[ 1] [ 1] [ 1] 0

                                                                                    1

                                                                                    t tx t v t w t

                                                                                    x y v w T v w t ty t v t w t

                                                                                    t t

                                                                                    (AT)

                                                                                    This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                    on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                    result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                    scaling rotation and translation matrices from Table 1

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Affine transformations

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    The preceding transformations relocate pixels on an image to new locations To complete

                                                                                    the process we have to assign intensity values to those locations This task is done by

                                                                                    using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                    In practice we can use equation (AT) in two basic ways

                                                                                    forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                    location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                    Problems

                                                                                    - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                    the same location in the output image

                                                                                    - some output locations have no correspondent in the original image (no intensity

                                                                                    assignment)

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    inverse mapping scans the output pixel locations and at each location (x y)

                                                                                    computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                    It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                    pixel value

                                                                                    Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                    numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Image registration ndash align two or more images of the same scene

                                                                                    In image registration we have available the input and output images but the specific

                                                                                    transformation that produced the output image from the input is generally unknown

                                                                                    The problem is to estimate the transformation function and then use it to register the two

                                                                                    images

                                                                                    - it may be of interest to align (register) two or more image taken at approximately the

                                                                                    same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                    - align images of a given location taken by the same instrument at different moments

                                                                                    of time (satellite images)

                                                                                    Solving the problem using tie points (also called control points) which are

                                                                                    corresponding points whose locations are known precisely in the input and reference

                                                                                    image

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    How to select tie points

                                                                                    - interactively selecting them

                                                                                    - use of algorithms that try to detect these points

                                                                                    - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                    the imaging sensors These objects produce a set of known points (called reseau

                                                                                    marks) directly on all images captured by the system which can be used as guides

                                                                                    for establishing tie points

                                                                                    The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                    of 4 tie points both on the input image and the reference image A simple model based on

                                                                                    a bilinear approximation is given by

                                                                                    1 2 3 4

                                                                                    5 6 7 8

                                                                                    x c v c w c v w cy c v c w c v w c

                                                                                    (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                    frequently is to select a larger number of tie points and using this new set of tie points

                                                                                    subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                    subregions marked by 4 tie points we applied the transformation model described above

                                                                                    The number of tie points and the sophistication of the model required to solve the register

                                                                                    problem depend on the severity of the geometrical distortion

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Probabilistic Methods

                                                                                    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                    p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                    ( ) kk

                                                                                    np zM N

                                                                                    nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                    pixels in the image) 1

                                                                                    0( ) 1

                                                                                    L

                                                                                    kk

                                                                                    p z

                                                                                    The mean (average) intensity of an image is given by 1

                                                                                    0( )

                                                                                    L

                                                                                    k kk

                                                                                    m z p z

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    The variance of the intensities is 1

                                                                                    2 2

                                                                                    0( ) ( )

                                                                                    L

                                                                                    k kk

                                                                                    z m p z

                                                                                    The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                    measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                    ( ) is used

                                                                                    The n-th moment of a random variable z about the mean is defined as 1

                                                                                    0( ) ( ) ( )

                                                                                    Ln

                                                                                    n k kk

                                                                                    z z m p z

                                                                                    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                    3( ) 0z the intensities are biased to values higher than the mean

                                                                                    ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                    mean

                                                                                    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                    Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                    Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                    Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Intensity Transformations and Spatial Filtering

                                                                                    ( ) ( )g x y T f x y

                                                                                    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                    neighborhood of (x y)

                                                                                    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                    and much smaller in size than the image

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                    called spatial filter (spatial mask kernel template or window)

                                                                                    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                    ( )s T r

                                                                                    s and r are denoting respectively the intensity of g and f at (x y)

                                                                                    Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                    darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                    is called contrast stretching

                                                                                    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                    thresholding function

                                                                                    Some Basic Intensity Transformation Functions

                                                                                    Image Negatives

                                                                                    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                    - equivalent of a photographic negative

                                                                                    - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                    image

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Original Negative image

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                    Some basic intensity transformation functions

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                    range An operator of this type is used to expand the values of dark pixels in an image

                                                                                    while compressing the higher-level values The opposite is true for the inverse log

                                                                                    transformation The log functions compress the dynamic range of images with large

                                                                                    variations in pixel values

                                                                                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Power-Law (Gamma) Transformations

                                                                                    - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                    Plots of gamma transformation for different values of γ (c=1)

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                    of output values with the opposite being true for higher values of input values The

                                                                                    curves with 1 have the opposite effect of those generated with values of 1

                                                                                    1c - identity transformation

                                                                                    A variety of devices used for image capture printing and display respond according to a

                                                                                    power law The process used to correct these power-law response phenomena is called

                                                                                    gamma correction

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Piecewise-Linear Transformations Functions

                                                                                    Contrast stretching

                                                                                    - a process that expands the range of intensity levels in an image so it spans the full

                                                                                    intensity range of the recording tool or display device

                                                                                    a b c d Fig5

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    11

                                                                                    1

                                                                                    2 1 1 21 2

                                                                                    2 1 2 1

                                                                                    22

                                                                                    2

                                                                                    [0 ]

                                                                                    ( ) ( )( ) [ ]( ) ( )

                                                                                    ( 1 ) [ 1]( 1 )

                                                                                    s r r rrs r r s r rT r r r r

                                                                                    r r r rs L r r r L

                                                                                    L r

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    1 1 2 2r s r s identity transformation (no change)

                                                                                    1 2 1 2 0 1r r s s L thresholding function

                                                                                    Figure 5(b) shows an 8-bit image with low contrast

                                                                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                    in the image respectively Thus the transformation function stretched the levels linearly

                                                                                    from their original range to the full range [0 L-1]

                                                                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                    2 2 1r s m L where m is the mean gray level in the image

                                                                                    The original image on which these results are based is a scanning electron microscope

                                                                                    image of pollen magnified approximately 700 times

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Intensity-level slicing

                                                                                    - highlighting a specific range of intensities in an image

                                                                                    There are two approaches for intensity-level slicing

                                                                                    1 display in one value (white for example) all the values in the range of interest and in

                                                                                    another (say black) all other intensities (Figure 311 (a))

                                                                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                    intensities in the image (Figure 311 (b))

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                    the top of the scale of intensities This type of enhancement produces a binary image

                                                                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                    Highlights range [A B] and preserves all other intensities

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                    blockageshellip)

                                                                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                    image around the mean intensity was set to black the other intensities remain unchanged

                                                                                    Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Bit-plane slicing

                                                                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                    This technique highlights the contribution made to the whole image appearances by each

                                                                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    Digital Image Processing

                                                                                    Week 1

                                                                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                    • DIP 1 2017
                                                                                    • DIP 02 (2017)

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Image morphing

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Fundamental Steps in DIP

                                                                                      methods whose input and output are images

                                                                                      methods whose inputs are images but whose outputs are attributes extracted from those images

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Outputs are images

                                                                                      bull image acquisition

                                                                                      bull image filtering and enhancement

                                                                                      bull image restoration

                                                                                      bull color image processing

                                                                                      bull wavelets and multiresolution processing

                                                                                      bull compression

                                                                                      bull morphological processing

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Outputs are attributes

                                                                                      bull morphological processing

                                                                                      bull segmentation

                                                                                      bull representation and description

                                                                                      bull object recognition

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Image acquisition - may involve preprocessing such as scaling

                                                                                      Image enhancement

                                                                                      bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                                      bull enhancement is problem oriented

                                                                                      bull there is no general sbquotheoryrsquo of image enhancement

                                                                                      bull enhancement use subjective methods for image emprovement

                                                                                      bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Image restoration

                                                                                      bull improving the appearance of an image

                                                                                      bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                                      Color image processing

                                                                                      bull fundamental concept in color models

                                                                                      bull basic color processing in a digital domain

                                                                                      Wavelets and multiresolution processing

                                                                                      representing images in various degree of resolution

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Compression

                                                                                      reducing the storage required to save an image or the bandwidth required to transmit it

                                                                                      Morphological processing

                                                                                      bull tools for extracting image components that are useful in the representation and description of shape

                                                                                      bull a transition from processes that output images to processes that outputimage attributes

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                                                                                      Week 1Week 1

                                                                                      Segmentation

                                                                                      bull partitioning an image into its constituents parts or objects

                                                                                      bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                                      bull the more accurate the segmentation the more likley recognition is to succeed

                                                                                      Representation and description (almost always follows segmentation)

                                                                                      bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                                      bull converting the data produced by segmentation to a form suitable for computer processing

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                                      bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                                      bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                                      Object recognition

                                                                                      the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                                      Knowledge database

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Simplified diagramof a cross sectionof the human eye

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                      The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                      Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                      The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                      The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                      Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                      Fovea = the place where the image of the object of interest falls on

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                                                                                      Week 1Week 1

                                                                                      Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                      Blind spot region without receptors

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Image formation in the eye

                                                                                      Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                      Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                      distance between lens and retina along visual axix = 17 mm

                                                                                      range of focal length = 14 mm to 17 mm

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Optical illusions

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                      gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                      quantities that describe the quality of a chromatic light source radiance

                                                                                      the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                      measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                      brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      the physical meaning is determined by the source of the image

                                                                                      ( )f D f x y

                                                                                      Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                      f(xy) ndash characterized by two components

                                                                                      i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                      r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                      ( ) ( ) ( )

                                                                                      0 ( ) 0 ( ) 1

                                                                                      f x y i x y r x y

                                                                                      i x y r x y

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                                                                                      Week 1Week 1

                                                                                      r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                      i(xy) ndash determined by the illumination source

                                                                                      r(xy) ndash determined by the characteristics of the imaged objects

                                                                                      is called gray (or intensity) scale

                                                                                      In practice

                                                                                      min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                      indoor values without additional illuminationmin max10 1000L L

                                                                                      black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                      min maxL L

                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                      Week 1Week 1

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Image Sampling and Quantization

                                                                                      - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                      scene

                                                                                      converting a continuous image f to digital form

                                                                                      - digitizing (x y) is called sampling

                                                                                      - digitizing f(x y) is called quantization

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                      (00) (01) (0 1)(10) (11) (1 1)

                                                                                      ( )

                                                                                      ( 10) ( 11) ( 1 1)

                                                                                      f f f Nf f f N

                                                                                      f x y

                                                                                      f M f M f M N

                                                                                      image element pixel

                                                                                      00 01 0 1

                                                                                      10 11 1 1

                                                                                      10 11 1 1

                                                                                      ( ) ( )

                                                                                      N

                                                                                      i jN M N

                                                                                      i j

                                                                                      M M M N

                                                                                      a a aa f x i y j f i ja a a

                                                                                      Aa

                                                                                      a a a

                                                                                      f(00) ndash the upper left corner of the image

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      M N ge 0 L=2k

                                                                                      [0 1]i j i ja a L

                                                                                      Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Number of bits required to store a digitized image

                                                                                      for 2 b M N k M N b N k

                                                                                      When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                      Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                      Image resolution = the largest number of discernible line pairs per unit distance

                                                                                      (eg 100 line pairs per mm)

                                                                                      Dots per unit distance are commonly used in printing and publishing

                                                                                      In US the measure is expressed in dots per inch (dpi)

                                                                                      (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                      Intensity resolution ndash the smallest discernible change in intensity level

                                                                                      The number of intensity levels (L) is determined by hardware considerations

                                                                                      L=2k ndash most common k = 8

                                                                                      Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                      150 dpi (lower left) 72 dpi (lower right)

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Reducing the number of gray levels 256 128 64 32

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Reducing the number of gray levels 16 8 4 2

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                      Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                      Interpolation is the process of using known data to estimate values at unknown locations

                                                                                      Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                      750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                      same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                      image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                      Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                      Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                      intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                      This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                      straight edges

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                      where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                      be written using the 4 nearest neighbors of point (x y)

                                                                                      Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                      modest increase in computational effort

                                                                                      Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                      nearest neighbors of the point 3 3

                                                                                      0 0

                                                                                      ( ) i ji j

                                                                                      i jv x y c x y

                                                                                      The coefficients cij are obtained solving a 16x16 linear system

                                                                                      intensity levels of the 16 nearest neighbors of 3 3

                                                                                      0 0

                                                                                      ( )i ji j

                                                                                      i jc x y x y

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                      bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                      programs such as Adobe Photoshop and Corel Photopaint

                                                                                      Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                      the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                      then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                      neighbor interpolation was used (both for shrinking and zooming)

                                                                                      Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                      interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                      from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Neighbors of a Pixel

                                                                                      A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                      This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                      The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                      and are denoted ND(p)

                                                                                      The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                      N8 (p)

                                                                                      If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                      fall outside the image

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Adjacency Connectivity Regions Boundaries

                                                                                      Denote by V the set of intensity levels used to define adjacency

                                                                                      - in a binary image V 01 (V=0 V=1)

                                                                                      - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                      We consider 3 types of adjacency

                                                                                      (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                      (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                      (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                      m-adjacent if

                                                                                      4( )q N p or

                                                                                      ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                      Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                      ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      binary image

                                                                                      0 1 1 0 1 1 0 1 1

                                                                                      1 0 1 0 0 1 0 0 1 0

                                                                                      0 0 1 0 0 1 0 0 1

                                                                                      V

                                                                                      The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                      8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                      m-adjacency

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                      is a sequence of distinct pixels with coordinates

                                                                                      and are adjacent 0 0 1 1

                                                                                      1 1

                                                                                      ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                      n n

                                                                                      i i i i

                                                                                      x y x y x y x y s tx y x y i n

                                                                                      The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                      Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                      Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                      in S if there exists a path between them consisting only of pixels from S

                                                                                      S is a connected set if there is a path in S between any 2 pixels in S

                                                                                      Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                      Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                      that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                      8-adjacency are considered

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                      touches the image border

                                                                                      the complement of 1

                                                                                      ( )K

                                                                                      cu k u u

                                                                                      k

                                                                                      R R R R

                                                                                      We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                      background of the image

                                                                                      The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                      points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                      region that have at least one background neighbor This definition is referred to as the

                                                                                      inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                      border in the background

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Distance measures

                                                                                      For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                      function or metric if

                                                                                      (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                      (b) D(p q) = D(q p)

                                                                                      (c) D(p z) le D(p q) + D(q z)

                                                                                      The Euclidean distance between p and q is defined as 1

                                                                                      2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                      The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                      centered at (x y)

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      The D4 distance (also called city-block distance) between p and q is defined as

                                                                                      4( ) | | | |D p q x s y t

                                                                                      The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                      4

                                                                                      22 1 2

                                                                                      2 2 1 0 1 22 1 2

                                                                                      2

                                                                                      D

                                                                                      The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                      The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                      8( ) max| | | |D p q x s y t

                                                                                      The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      8

                                                                                      2 2 2 2 22 1 1 1 2

                                                                                      2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                      D

                                                                                      The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                      D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                      because these distances involve only the coordinates of the point

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Array versus Matrix Operations

                                                                                      An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                      11 12 11 12

                                                                                      21 22 21 22

                                                                                      a a b ba a b b

                                                                                      Array product

                                                                                      11 12 11 12 11 11 12 12

                                                                                      21 22 21 22 21 21 22 21

                                                                                      a a b b a b a ba a b b a b a b

                                                                                      Matrix product

                                                                                      11 12 11 12 11 11 12 21 11 12 12 21

                                                                                      21 22 21 22 21 11 22 21 21 12 22 22

                                                                                      a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                      We assume array operations unless stated otherwise

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Linear versus Nonlinear Operations

                                                                                      One of the most important classifications of image-processing methods is whether it is

                                                                                      linear or nonlinear

                                                                                      ( ) ( )H f x y g x y

                                                                                      H is said to be a linear operator if

                                                                                      images1 2 1 2

                                                                                      1 2

                                                                                      ( ) ( ) ( ) ( )

                                                                                      H a f x y b f x y a H f x y b H f x y

                                                                                      a b f f

                                                                                      Example of nonlinear operator

                                                                                      the maximum value of the pixels of image max ( )H f f x y f

                                                                                      1 2

                                                                                      0 2 6 5 1 1

                                                                                      2 3 4 7f f a b

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      1 2

                                                                                      0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                      2 3 4 7 2 4a f b f

                                                                                      0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                      2 3 4 7

                                                                                      Arithmetic Operations in Image Processing

                                                                                      Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                      f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                      For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                      value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                      The two random variables are uncorrelated when their covariance is 0

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                      used in image enhancement)

                                                                                      1

                                                                                      1( ) ( )K

                                                                                      ii

                                                                                      g x y g x yK

                                                                                      If the noise satisfies the properties stated above we have

                                                                                      2 2( ) ( )

                                                                                      1( ) ( ) g x y x yE g x y f x yK

                                                                                      ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                      and g respectively The standard deviation (square root of the variance) at any point in

                                                                                      the average image is

                                                                                      ( ) ( )1

                                                                                      g x y x yK

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                      the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                      means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                      averaging process increases

                                                                                      An important application of image averaging is in the field of astronomy where imaging

                                                                                      under very low light levels frequently causes sensor noise to render single images

                                                                                      virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                      was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                      64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                      images respectively

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                      100 noisy images

                                                                                      a b c d e f

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      A frequent application of image subtraction is in the enhancement of differences between

                                                                                      images

                                                                                      (a) (b) (c)

                                                                                      Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                      significant bit of each pixel (c) the difference between the two images

                                                                                      Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                      Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                      images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                      difference between images (a) and (b)

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                      h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                      intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                      source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                      bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                      anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                      images after injection of the contrast medium

                                                                                      In g(x y) we can find the differences between h and f as enhanced detail

                                                                                      Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                      propagates through the various arteries in the area being observed

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      An important application of image multiplication (and division) is shading correction

                                                                                      Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                      f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                      When the shading function is known

                                                                                      ( )( )( )

                                                                                      g x yf x yh x y

                                                                                      h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                      approximation to the shading function by imaging a target of constant intensity When the

                                                                                      sensor is not available often the shading pattern can be estimated from the image

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      (a) (b) (c)

                                                                                      Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                      operations The process consists of multiplying a given image by a mask image that has

                                                                                      1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                      image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                      (a) (b) (c)

                                                                                      Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                      min( )mf f f

                                                                                      0 ( 255)max( )

                                                                                      ms

                                                                                      m

                                                                                      ff K K K

                                                                                      f

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Spatial Operations

                                                                                      - are performed directly on the pixels of a given image

                                                                                      There are three categories of spatial operations

                                                                                      single-pixel operations

                                                                                      neighborhood operations

                                                                                      geometric spatial transformations

                                                                                      Single-pixel operations

                                                                                      - change the values of intensity for the individual pixels ( )s T z

                                                                                      where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                      corresponding pixel in the processed image

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Neighborhood operations

                                                                                      Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                      in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                      based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                      rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                      intensity by computing the average value of the pixels in Sxy

                                                                                      ( )

                                                                                      1( ) ( )xyr c S

                                                                                      g x y f r cm n

                                                                                      The net effect is to perform local blurring in the original image This type of process is

                                                                                      used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                      largest region of an image

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Geometric spatial transformations and image registration

                                                                                      - modify the spatial relationship between pixels in an image

                                                                                      - these transformations are often called rubber-sheet transformations (analogous to

                                                                                      printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                      predefined set of rules

                                                                                      A geometric transformation consists of 2 basic operations

                                                                                      1 a spatial transformation of coordinates

                                                                                      2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                      pixels

                                                                                      The coordinate system transformation ( ) [( )]x y T v w

                                                                                      (v w) ndash pixel coordinates in the original image

                                                                                      (x y) ndash pixel coordinates in the transformed image

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                      Affine transform

                                                                                      11 1211 21 31

                                                                                      21 2212 22 33

                                                                                      31 32

                                                                                      0[ 1] [ 1] [ 1] 0

                                                                                      1

                                                                                      t tx t v t w t

                                                                                      x y v w T v w t ty t v t w t

                                                                                      t t

                                                                                      (AT)

                                                                                      This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                      on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                      result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                      scaling rotation and translation matrices from Table 1

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Affine transformations

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      The preceding transformations relocate pixels on an image to new locations To complete

                                                                                      the process we have to assign intensity values to those locations This task is done by

                                                                                      using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                      In practice we can use equation (AT) in two basic ways

                                                                                      forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                      location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                      Problems

                                                                                      - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                      the same location in the output image

                                                                                      - some output locations have no correspondent in the original image (no intensity

                                                                                      assignment)

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      inverse mapping scans the output pixel locations and at each location (x y)

                                                                                      computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                      It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                      pixel value

                                                                                      Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                      numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Image registration ndash align two or more images of the same scene

                                                                                      In image registration we have available the input and output images but the specific

                                                                                      transformation that produced the output image from the input is generally unknown

                                                                                      The problem is to estimate the transformation function and then use it to register the two

                                                                                      images

                                                                                      - it may be of interest to align (register) two or more image taken at approximately the

                                                                                      same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                      - align images of a given location taken by the same instrument at different moments

                                                                                      of time (satellite images)

                                                                                      Solving the problem using tie points (also called control points) which are

                                                                                      corresponding points whose locations are known precisely in the input and reference

                                                                                      image

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      How to select tie points

                                                                                      - interactively selecting them

                                                                                      - use of algorithms that try to detect these points

                                                                                      - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                      the imaging sensors These objects produce a set of known points (called reseau

                                                                                      marks) directly on all images captured by the system which can be used as guides

                                                                                      for establishing tie points

                                                                                      The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                      of 4 tie points both on the input image and the reference image A simple model based on

                                                                                      a bilinear approximation is given by

                                                                                      1 2 3 4

                                                                                      5 6 7 8

                                                                                      x c v c w c v w cy c v c w c v w c

                                                                                      (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                      frequently is to select a larger number of tie points and using this new set of tie points

                                                                                      subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                      subregions marked by 4 tie points we applied the transformation model described above

                                                                                      The number of tie points and the sophistication of the model required to solve the register

                                                                                      problem depend on the severity of the geometrical distortion

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Probabilistic Methods

                                                                                      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                      p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                      ( ) kk

                                                                                      np zM N

                                                                                      nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                      pixels in the image) 1

                                                                                      0( ) 1

                                                                                      L

                                                                                      kk

                                                                                      p z

                                                                                      The mean (average) intensity of an image is given by 1

                                                                                      0( )

                                                                                      L

                                                                                      k kk

                                                                                      m z p z

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      The variance of the intensities is 1

                                                                                      2 2

                                                                                      0( ) ( )

                                                                                      L

                                                                                      k kk

                                                                                      z m p z

                                                                                      The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                      measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                      ( ) is used

                                                                                      The n-th moment of a random variable z about the mean is defined as 1

                                                                                      0( ) ( ) ( )

                                                                                      Ln

                                                                                      n k kk

                                                                                      z z m p z

                                                                                      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                      3( ) 0z the intensities are biased to values higher than the mean

                                                                                      ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                      mean

                                                                                      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                      Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                      Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                      Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Intensity Transformations and Spatial Filtering

                                                                                      ( ) ( )g x y T f x y

                                                                                      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                      neighborhood of (x y)

                                                                                      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                      and much smaller in size than the image

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                      called spatial filter (spatial mask kernel template or window)

                                                                                      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                      ( )s T r

                                                                                      s and r are denoting respectively the intensity of g and f at (x y)

                                                                                      Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                      darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                      is called contrast stretching

                                                                                      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                      thresholding function

                                                                                      Some Basic Intensity Transformation Functions

                                                                                      Image Negatives

                                                                                      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                      - equivalent of a photographic negative

                                                                                      - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                      image

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Original Negative image

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                      Some basic intensity transformation functions

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                      range An operator of this type is used to expand the values of dark pixels in an image

                                                                                      while compressing the higher-level values The opposite is true for the inverse log

                                                                                      transformation The log functions compress the dynamic range of images with large

                                                                                      variations in pixel values

                                                                                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Power-Law (Gamma) Transformations

                                                                                      - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                      Plots of gamma transformation for different values of γ (c=1)

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                      of output values with the opposite being true for higher values of input values The

                                                                                      curves with 1 have the opposite effect of those generated with values of 1

                                                                                      1c - identity transformation

                                                                                      A variety of devices used for image capture printing and display respond according to a

                                                                                      power law The process used to correct these power-law response phenomena is called

                                                                                      gamma correction

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Piecewise-Linear Transformations Functions

                                                                                      Contrast stretching

                                                                                      - a process that expands the range of intensity levels in an image so it spans the full

                                                                                      intensity range of the recording tool or display device

                                                                                      a b c d Fig5

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      11

                                                                                      1

                                                                                      2 1 1 21 2

                                                                                      2 1 2 1

                                                                                      22

                                                                                      2

                                                                                      [0 ]

                                                                                      ( ) ( )( ) [ ]( ) ( )

                                                                                      ( 1 ) [ 1]( 1 )

                                                                                      s r r rrs r r s r rT r r r r

                                                                                      r r r rs L r r r L

                                                                                      L r

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      1 1 2 2r s r s identity transformation (no change)

                                                                                      1 2 1 2 0 1r r s s L thresholding function

                                                                                      Figure 5(b) shows an 8-bit image with low contrast

                                                                                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                      in the image respectively Thus the transformation function stretched the levels linearly

                                                                                      from their original range to the full range [0 L-1]

                                                                                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                      2 2 1r s m L where m is the mean gray level in the image

                                                                                      The original image on which these results are based is a scanning electron microscope

                                                                                      image of pollen magnified approximately 700 times

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Intensity-level slicing

                                                                                      - highlighting a specific range of intensities in an image

                                                                                      There are two approaches for intensity-level slicing

                                                                                      1 display in one value (white for example) all the values in the range of interest and in

                                                                                      another (say black) all other intensities (Figure 311 (a))

                                                                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                      intensities in the image (Figure 311 (b))

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                      the top of the scale of intensities This type of enhancement produces a binary image

                                                                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                      Highlights range [A B] and preserves all other intensities

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                      blockageshellip)

                                                                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                      image around the mean intensity was set to black the other intensities remain unchanged

                                                                                      Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Bit-plane slicing

                                                                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                      This technique highlights the contribution made to the whole image appearances by each

                                                                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      Digital Image Processing

                                                                                      Week 1

                                                                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                      • DIP 1 2017
                                                                                      • DIP 02 (2017)

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Fundamental Steps in DIP

                                                                                        methods whose input and output are images

                                                                                        methods whose inputs are images but whose outputs are attributes extracted from those images

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Outputs are images

                                                                                        bull image acquisition

                                                                                        bull image filtering and enhancement

                                                                                        bull image restoration

                                                                                        bull color image processing

                                                                                        bull wavelets and multiresolution processing

                                                                                        bull compression

                                                                                        bull morphological processing

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Outputs are attributes

                                                                                        bull morphological processing

                                                                                        bull segmentation

                                                                                        bull representation and description

                                                                                        bull object recognition

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Image acquisition - may involve preprocessing such as scaling

                                                                                        Image enhancement

                                                                                        bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                                        bull enhancement is problem oriented

                                                                                        bull there is no general sbquotheoryrsquo of image enhancement

                                                                                        bull enhancement use subjective methods for image emprovement

                                                                                        bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Image restoration

                                                                                        bull improving the appearance of an image

                                                                                        bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                                        Color image processing

                                                                                        bull fundamental concept in color models

                                                                                        bull basic color processing in a digital domain

                                                                                        Wavelets and multiresolution processing

                                                                                        representing images in various degree of resolution

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Compression

                                                                                        reducing the storage required to save an image or the bandwidth required to transmit it

                                                                                        Morphological processing

                                                                                        bull tools for extracting image components that are useful in the representation and description of shape

                                                                                        bull a transition from processes that output images to processes that outputimage attributes

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Segmentation

                                                                                        bull partitioning an image into its constituents parts or objects

                                                                                        bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                                        bull the more accurate the segmentation the more likley recognition is to succeed

                                                                                        Representation and description (almost always follows segmentation)

                                                                                        bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                                        bull converting the data produced by segmentation to a form suitable for computer processing

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                                        bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                                        bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                                        Object recognition

                                                                                        the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                                        Knowledge database

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Simplified diagramof a cross sectionof the human eye

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                        The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                        Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                        The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                        The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                        Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                        Fovea = the place where the image of the object of interest falls on

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                        Blind spot region without receptors

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Image formation in the eye

                                                                                        Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                        Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                        distance between lens and retina along visual axix = 17 mm

                                                                                        range of focal length = 14 mm to 17 mm

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Optical illusions

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                        gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                        quantities that describe the quality of a chromatic light source radiance

                                                                                        the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                        measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                        brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        the physical meaning is determined by the source of the image

                                                                                        ( )f D f x y

                                                                                        Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                        f(xy) ndash characterized by two components

                                                                                        i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                        r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                        ( ) ( ) ( )

                                                                                        0 ( ) 0 ( ) 1

                                                                                        f x y i x y r x y

                                                                                        i x y r x y

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                        i(xy) ndash determined by the illumination source

                                                                                        r(xy) ndash determined by the characteristics of the imaged objects

                                                                                        is called gray (or intensity) scale

                                                                                        In practice

                                                                                        min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                        indoor values without additional illuminationmin max10 1000L L

                                                                                        black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                        min maxL L

                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                        Week 1Week 1

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Image Sampling and Quantization

                                                                                        - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                        scene

                                                                                        converting a continuous image f to digital form

                                                                                        - digitizing (x y) is called sampling

                                                                                        - digitizing f(x y) is called quantization

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                        (00) (01) (0 1)(10) (11) (1 1)

                                                                                        ( )

                                                                                        ( 10) ( 11) ( 1 1)

                                                                                        f f f Nf f f N

                                                                                        f x y

                                                                                        f M f M f M N

                                                                                        image element pixel

                                                                                        00 01 0 1

                                                                                        10 11 1 1

                                                                                        10 11 1 1

                                                                                        ( ) ( )

                                                                                        N

                                                                                        i jN M N

                                                                                        i j

                                                                                        M M M N

                                                                                        a a aa f x i y j f i ja a a

                                                                                        Aa

                                                                                        a a a

                                                                                        f(00) ndash the upper left corner of the image

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        M N ge 0 L=2k

                                                                                        [0 1]i j i ja a L

                                                                                        Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Number of bits required to store a digitized image

                                                                                        for 2 b M N k M N b N k

                                                                                        When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                        Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                        Image resolution = the largest number of discernible line pairs per unit distance

                                                                                        (eg 100 line pairs per mm)

                                                                                        Dots per unit distance are commonly used in printing and publishing

                                                                                        In US the measure is expressed in dots per inch (dpi)

                                                                                        (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                        Intensity resolution ndash the smallest discernible change in intensity level

                                                                                        The number of intensity levels (L) is determined by hardware considerations

                                                                                        L=2k ndash most common k = 8

                                                                                        Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                        150 dpi (lower left) 72 dpi (lower right)

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Reducing the number of gray levels 256 128 64 32

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Reducing the number of gray levels 16 8 4 2

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                        Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                        Interpolation is the process of using known data to estimate values at unknown locations

                                                                                        Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                        750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                        same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                        image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                        Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                        Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                        intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                        This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                        straight edges

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                        where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                        be written using the 4 nearest neighbors of point (x y)

                                                                                        Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                        modest increase in computational effort

                                                                                        Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                        nearest neighbors of the point 3 3

                                                                                        0 0

                                                                                        ( ) i ji j

                                                                                        i jv x y c x y

                                                                                        The coefficients cij are obtained solving a 16x16 linear system

                                                                                        intensity levels of the 16 nearest neighbors of 3 3

                                                                                        0 0

                                                                                        ( )i ji j

                                                                                        i jc x y x y

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                        bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                        programs such as Adobe Photoshop and Corel Photopaint

                                                                                        Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                        the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                        then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                        neighbor interpolation was used (both for shrinking and zooming)

                                                                                        Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                        interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                        from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Neighbors of a Pixel

                                                                                        A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                        This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                        The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                        and are denoted ND(p)

                                                                                        The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                        N8 (p)

                                                                                        If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                        fall outside the image

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Adjacency Connectivity Regions Boundaries

                                                                                        Denote by V the set of intensity levels used to define adjacency

                                                                                        - in a binary image V 01 (V=0 V=1)

                                                                                        - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                        We consider 3 types of adjacency

                                                                                        (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                        (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                        (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                        m-adjacent if

                                                                                        4( )q N p or

                                                                                        ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                        Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                        ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        binary image

                                                                                        0 1 1 0 1 1 0 1 1

                                                                                        1 0 1 0 0 1 0 0 1 0

                                                                                        0 0 1 0 0 1 0 0 1

                                                                                        V

                                                                                        The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                        8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                        m-adjacency

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                        is a sequence of distinct pixels with coordinates

                                                                                        and are adjacent 0 0 1 1

                                                                                        1 1

                                                                                        ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                        n n

                                                                                        i i i i

                                                                                        x y x y x y x y s tx y x y i n

                                                                                        The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                        Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                        Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                        in S if there exists a path between them consisting only of pixels from S

                                                                                        S is a connected set if there is a path in S between any 2 pixels in S

                                                                                        Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                        Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                        that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                        8-adjacency are considered

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                        touches the image border

                                                                                        the complement of 1

                                                                                        ( )K

                                                                                        cu k u u

                                                                                        k

                                                                                        R R R R

                                                                                        We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                        background of the image

                                                                                        The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                        points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                        region that have at least one background neighbor This definition is referred to as the

                                                                                        inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                        border in the background

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Distance measures

                                                                                        For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                        function or metric if

                                                                                        (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                        (b) D(p q) = D(q p)

                                                                                        (c) D(p z) le D(p q) + D(q z)

                                                                                        The Euclidean distance between p and q is defined as 1

                                                                                        2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                        The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                        centered at (x y)

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        The D4 distance (also called city-block distance) between p and q is defined as

                                                                                        4( ) | | | |D p q x s y t

                                                                                        The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                        4

                                                                                        22 1 2

                                                                                        2 2 1 0 1 22 1 2

                                                                                        2

                                                                                        D

                                                                                        The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                        The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                        8( ) max| | | |D p q x s y t

                                                                                        The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        8

                                                                                        2 2 2 2 22 1 1 1 2

                                                                                        2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                        D

                                                                                        The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                        D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                        because these distances involve only the coordinates of the point

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Array versus Matrix Operations

                                                                                        An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                        11 12 11 12

                                                                                        21 22 21 22

                                                                                        a a b ba a b b

                                                                                        Array product

                                                                                        11 12 11 12 11 11 12 12

                                                                                        21 22 21 22 21 21 22 21

                                                                                        a a b b a b a ba a b b a b a b

                                                                                        Matrix product

                                                                                        11 12 11 12 11 11 12 21 11 12 12 21

                                                                                        21 22 21 22 21 11 22 21 21 12 22 22

                                                                                        a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                        We assume array operations unless stated otherwise

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Linear versus Nonlinear Operations

                                                                                        One of the most important classifications of image-processing methods is whether it is

                                                                                        linear or nonlinear

                                                                                        ( ) ( )H f x y g x y

                                                                                        H is said to be a linear operator if

                                                                                        images1 2 1 2

                                                                                        1 2

                                                                                        ( ) ( ) ( ) ( )

                                                                                        H a f x y b f x y a H f x y b H f x y

                                                                                        a b f f

                                                                                        Example of nonlinear operator

                                                                                        the maximum value of the pixels of image max ( )H f f x y f

                                                                                        1 2

                                                                                        0 2 6 5 1 1

                                                                                        2 3 4 7f f a b

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        1 2

                                                                                        0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                        2 3 4 7 2 4a f b f

                                                                                        0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                        2 3 4 7

                                                                                        Arithmetic Operations in Image Processing

                                                                                        Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                        f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                        For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                        value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                        The two random variables are uncorrelated when their covariance is 0

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                        used in image enhancement)

                                                                                        1

                                                                                        1( ) ( )K

                                                                                        ii

                                                                                        g x y g x yK

                                                                                        If the noise satisfies the properties stated above we have

                                                                                        2 2( ) ( )

                                                                                        1( ) ( ) g x y x yE g x y f x yK

                                                                                        ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                        and g respectively The standard deviation (square root of the variance) at any point in

                                                                                        the average image is

                                                                                        ( ) ( )1

                                                                                        g x y x yK

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                        the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                        means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                        averaging process increases

                                                                                        An important application of image averaging is in the field of astronomy where imaging

                                                                                        under very low light levels frequently causes sensor noise to render single images

                                                                                        virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                        was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                        64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                        images respectively

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                        100 noisy images

                                                                                        a b c d e f

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        A frequent application of image subtraction is in the enhancement of differences between

                                                                                        images

                                                                                        (a) (b) (c)

                                                                                        Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                        significant bit of each pixel (c) the difference between the two images

                                                                                        Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                        Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                        images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                        difference between images (a) and (b)

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                        h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                        intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                        source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                        bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                        anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                        images after injection of the contrast medium

                                                                                        In g(x y) we can find the differences between h and f as enhanced detail

                                                                                        Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                        propagates through the various arteries in the area being observed

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        An important application of image multiplication (and division) is shading correction

                                                                                        Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                        f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                        When the shading function is known

                                                                                        ( )( )( )

                                                                                        g x yf x yh x y

                                                                                        h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                        approximation to the shading function by imaging a target of constant intensity When the

                                                                                        sensor is not available often the shading pattern can be estimated from the image

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        (a) (b) (c)

                                                                                        Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                        operations The process consists of multiplying a given image by a mask image that has

                                                                                        1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                        image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                        (a) (b) (c)

                                                                                        Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                        min( )mf f f

                                                                                        0 ( 255)max( )

                                                                                        ms

                                                                                        m

                                                                                        ff K K K

                                                                                        f

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Spatial Operations

                                                                                        - are performed directly on the pixels of a given image

                                                                                        There are three categories of spatial operations

                                                                                        single-pixel operations

                                                                                        neighborhood operations

                                                                                        geometric spatial transformations

                                                                                        Single-pixel operations

                                                                                        - change the values of intensity for the individual pixels ( )s T z

                                                                                        where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                        corresponding pixel in the processed image

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Neighborhood operations

                                                                                        Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                        in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                        based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                        rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                        intensity by computing the average value of the pixels in Sxy

                                                                                        ( )

                                                                                        1( ) ( )xyr c S

                                                                                        g x y f r cm n

                                                                                        The net effect is to perform local blurring in the original image This type of process is

                                                                                        used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                        largest region of an image

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Geometric spatial transformations and image registration

                                                                                        - modify the spatial relationship between pixels in an image

                                                                                        - these transformations are often called rubber-sheet transformations (analogous to

                                                                                        printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                        predefined set of rules

                                                                                        A geometric transformation consists of 2 basic operations

                                                                                        1 a spatial transformation of coordinates

                                                                                        2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                        pixels

                                                                                        The coordinate system transformation ( ) [( )]x y T v w

                                                                                        (v w) ndash pixel coordinates in the original image

                                                                                        (x y) ndash pixel coordinates in the transformed image

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                        Affine transform

                                                                                        11 1211 21 31

                                                                                        21 2212 22 33

                                                                                        31 32

                                                                                        0[ 1] [ 1] [ 1] 0

                                                                                        1

                                                                                        t tx t v t w t

                                                                                        x y v w T v w t ty t v t w t

                                                                                        t t

                                                                                        (AT)

                                                                                        This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                        on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                        result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                        scaling rotation and translation matrices from Table 1

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Affine transformations

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        The preceding transformations relocate pixels on an image to new locations To complete

                                                                                        the process we have to assign intensity values to those locations This task is done by

                                                                                        using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                        In practice we can use equation (AT) in two basic ways

                                                                                        forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                        location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                        Problems

                                                                                        - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                        the same location in the output image

                                                                                        - some output locations have no correspondent in the original image (no intensity

                                                                                        assignment)

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        inverse mapping scans the output pixel locations and at each location (x y)

                                                                                        computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                        It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                        pixel value

                                                                                        Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                        numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Image registration ndash align two or more images of the same scene

                                                                                        In image registration we have available the input and output images but the specific

                                                                                        transformation that produced the output image from the input is generally unknown

                                                                                        The problem is to estimate the transformation function and then use it to register the two

                                                                                        images

                                                                                        - it may be of interest to align (register) two or more image taken at approximately the

                                                                                        same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                        - align images of a given location taken by the same instrument at different moments

                                                                                        of time (satellite images)

                                                                                        Solving the problem using tie points (also called control points) which are

                                                                                        corresponding points whose locations are known precisely in the input and reference

                                                                                        image

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        How to select tie points

                                                                                        - interactively selecting them

                                                                                        - use of algorithms that try to detect these points

                                                                                        - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                        the imaging sensors These objects produce a set of known points (called reseau

                                                                                        marks) directly on all images captured by the system which can be used as guides

                                                                                        for establishing tie points

                                                                                        The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                        of 4 tie points both on the input image and the reference image A simple model based on

                                                                                        a bilinear approximation is given by

                                                                                        1 2 3 4

                                                                                        5 6 7 8

                                                                                        x c v c w c v w cy c v c w c v w c

                                                                                        (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                        frequently is to select a larger number of tie points and using this new set of tie points

                                                                                        subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                        subregions marked by 4 tie points we applied the transformation model described above

                                                                                        The number of tie points and the sophistication of the model required to solve the register

                                                                                        problem depend on the severity of the geometrical distortion

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Probabilistic Methods

                                                                                        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                        p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                        ( ) kk

                                                                                        np zM N

                                                                                        nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                        pixels in the image) 1

                                                                                        0( ) 1

                                                                                        L

                                                                                        kk

                                                                                        p z

                                                                                        The mean (average) intensity of an image is given by 1

                                                                                        0( )

                                                                                        L

                                                                                        k kk

                                                                                        m z p z

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        The variance of the intensities is 1

                                                                                        2 2

                                                                                        0( ) ( )

                                                                                        L

                                                                                        k kk

                                                                                        z m p z

                                                                                        The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                        measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                        ( ) is used

                                                                                        The n-th moment of a random variable z about the mean is defined as 1

                                                                                        0( ) ( ) ( )

                                                                                        Ln

                                                                                        n k kk

                                                                                        z z m p z

                                                                                        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                        3( ) 0z the intensities are biased to values higher than the mean

                                                                                        ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                        mean

                                                                                        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                        Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                        Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                        Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Intensity Transformations and Spatial Filtering

                                                                                        ( ) ( )g x y T f x y

                                                                                        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                        neighborhood of (x y)

                                                                                        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                        and much smaller in size than the image

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                        called spatial filter (spatial mask kernel template or window)

                                                                                        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                        ( )s T r

                                                                                        s and r are denoting respectively the intensity of g and f at (x y)

                                                                                        Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                        darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                        is called contrast stretching

                                                                                        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                        thresholding function

                                                                                        Some Basic Intensity Transformation Functions

                                                                                        Image Negatives

                                                                                        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                        - equivalent of a photographic negative

                                                                                        - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                        image

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Original Negative image

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                        Some basic intensity transformation functions

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                        range An operator of this type is used to expand the values of dark pixels in an image

                                                                                        while compressing the higher-level values The opposite is true for the inverse log

                                                                                        transformation The log functions compress the dynamic range of images with large

                                                                                        variations in pixel values

                                                                                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Power-Law (Gamma) Transformations

                                                                                        - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                        Plots of gamma transformation for different values of γ (c=1)

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                        of output values with the opposite being true for higher values of input values The

                                                                                        curves with 1 have the opposite effect of those generated with values of 1

                                                                                        1c - identity transformation

                                                                                        A variety of devices used for image capture printing and display respond according to a

                                                                                        power law The process used to correct these power-law response phenomena is called

                                                                                        gamma correction

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Piecewise-Linear Transformations Functions

                                                                                        Contrast stretching

                                                                                        - a process that expands the range of intensity levels in an image so it spans the full

                                                                                        intensity range of the recording tool or display device

                                                                                        a b c d Fig5

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        11

                                                                                        1

                                                                                        2 1 1 21 2

                                                                                        2 1 2 1

                                                                                        22

                                                                                        2

                                                                                        [0 ]

                                                                                        ( ) ( )( ) [ ]( ) ( )

                                                                                        ( 1 ) [ 1]( 1 )

                                                                                        s r r rrs r r s r rT r r r r

                                                                                        r r r rs L r r r L

                                                                                        L r

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        1 1 2 2r s r s identity transformation (no change)

                                                                                        1 2 1 2 0 1r r s s L thresholding function

                                                                                        Figure 5(b) shows an 8-bit image with low contrast

                                                                                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                        in the image respectively Thus the transformation function stretched the levels linearly

                                                                                        from their original range to the full range [0 L-1]

                                                                                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                        2 2 1r s m L where m is the mean gray level in the image

                                                                                        The original image on which these results are based is a scanning electron microscope

                                                                                        image of pollen magnified approximately 700 times

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Intensity-level slicing

                                                                                        - highlighting a specific range of intensities in an image

                                                                                        There are two approaches for intensity-level slicing

                                                                                        1 display in one value (white for example) all the values in the range of interest and in

                                                                                        another (say black) all other intensities (Figure 311 (a))

                                                                                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                        intensities in the image (Figure 311 (b))

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                        the top of the scale of intensities This type of enhancement produces a binary image

                                                                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                        Highlights range [A B] and preserves all other intensities

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                        blockageshellip)

                                                                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                        image around the mean intensity was set to black the other intensities remain unchanged

                                                                                        Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                        Digital Image Processing

                                                                                        Week 1

                                                                                        Bit-plane slicing

                                                                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                        This technique highlights the contribution made to the whole image appearances by each

                                                                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                        Digital Image Processing

                                                                                        Week 1

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                                                                                        Week 1

                                                                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                        • DIP 1 2017
                                                                                        • DIP 02 (2017)

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          Outputs are images

                                                                                          bull image acquisition

                                                                                          bull image filtering and enhancement

                                                                                          bull image restoration

                                                                                          bull color image processing

                                                                                          bull wavelets and multiresolution processing

                                                                                          bull compression

                                                                                          bull morphological processing

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                                                                                          Week 1Week 1

                                                                                          Outputs are attributes

                                                                                          bull morphological processing

                                                                                          bull segmentation

                                                                                          bull representation and description

                                                                                          bull object recognition

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                                                                                          Week 1Week 1

                                                                                          Image acquisition - may involve preprocessing such as scaling

                                                                                          Image enhancement

                                                                                          bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                                          bull enhancement is problem oriented

                                                                                          bull there is no general sbquotheoryrsquo of image enhancement

                                                                                          bull enhancement use subjective methods for image emprovement

                                                                                          bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

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                                                                                          Image restoration

                                                                                          bull improving the appearance of an image

                                                                                          bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                                          Color image processing

                                                                                          bull fundamental concept in color models

                                                                                          bull basic color processing in a digital domain

                                                                                          Wavelets and multiresolution processing

                                                                                          representing images in various degree of resolution

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          Compression

                                                                                          reducing the storage required to save an image or the bandwidth required to transmit it

                                                                                          Morphological processing

                                                                                          bull tools for extracting image components that are useful in the representation and description of shape

                                                                                          bull a transition from processes that output images to processes that outputimage attributes

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                                                                                          Week 1Week 1

                                                                                          Segmentation

                                                                                          bull partitioning an image into its constituents parts or objects

                                                                                          bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                                          bull the more accurate the segmentation the more likley recognition is to succeed

                                                                                          Representation and description (almost always follows segmentation)

                                                                                          bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                                          bull converting the data produced by segmentation to a form suitable for computer processing

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                                          bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                                          bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                                          Object recognition

                                                                                          the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                                          Knowledge database

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          Simplified diagramof a cross sectionof the human eye

                                                                                          Digital Image ProcessingDigital Image Processing

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                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                          The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                          Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                          The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                          The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                          Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                          Fovea = the place where the image of the object of interest falls on

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                          Blind spot region without receptors

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          Image formation in the eye

                                                                                          Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                          Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                          distance between lens and retina along visual axix = 17 mm

                                                                                          range of focal length = 14 mm to 17 mm

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          Optical illusions

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                          gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                          quantities that describe the quality of a chromatic light source radiance

                                                                                          the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                          measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                          brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          the physical meaning is determined by the source of the image

                                                                                          ( )f D f x y

                                                                                          Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                          f(xy) ndash characterized by two components

                                                                                          i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                          r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                          ( ) ( ) ( )

                                                                                          0 ( ) 0 ( ) 1

                                                                                          f x y i x y r x y

                                                                                          i x y r x y

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                          i(xy) ndash determined by the illumination source

                                                                                          r(xy) ndash determined by the characteristics of the imaged objects

                                                                                          is called gray (or intensity) scale

                                                                                          In practice

                                                                                          min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                          indoor values without additional illuminationmin max10 1000L L

                                                                                          black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                          min maxL L

                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                          Week 1Week 1

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Image Sampling and Quantization

                                                                                          - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                          scene

                                                                                          converting a continuous image f to digital form

                                                                                          - digitizing (x y) is called sampling

                                                                                          - digitizing f(x y) is called quantization

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                          (00) (01) (0 1)(10) (11) (1 1)

                                                                                          ( )

                                                                                          ( 10) ( 11) ( 1 1)

                                                                                          f f f Nf f f N

                                                                                          f x y

                                                                                          f M f M f M N

                                                                                          image element pixel

                                                                                          00 01 0 1

                                                                                          10 11 1 1

                                                                                          10 11 1 1

                                                                                          ( ) ( )

                                                                                          N

                                                                                          i jN M N

                                                                                          i j

                                                                                          M M M N

                                                                                          a a aa f x i y j f i ja a a

                                                                                          Aa

                                                                                          a a a

                                                                                          f(00) ndash the upper left corner of the image

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          M N ge 0 L=2k

                                                                                          [0 1]i j i ja a L

                                                                                          Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Number of bits required to store a digitized image

                                                                                          for 2 b M N k M N b N k

                                                                                          When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                          Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                          Image resolution = the largest number of discernible line pairs per unit distance

                                                                                          (eg 100 line pairs per mm)

                                                                                          Dots per unit distance are commonly used in printing and publishing

                                                                                          In US the measure is expressed in dots per inch (dpi)

                                                                                          (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                          Intensity resolution ndash the smallest discernible change in intensity level

                                                                                          The number of intensity levels (L) is determined by hardware considerations

                                                                                          L=2k ndash most common k = 8

                                                                                          Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                          150 dpi (lower left) 72 dpi (lower right)

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Reducing the number of gray levels 256 128 64 32

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Reducing the number of gray levels 16 8 4 2

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                          Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                          Interpolation is the process of using known data to estimate values at unknown locations

                                                                                          Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                          750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                          same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                          image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                          Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                          Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                          intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                          This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                          straight edges

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                          where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                          be written using the 4 nearest neighbors of point (x y)

                                                                                          Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                          modest increase in computational effort

                                                                                          Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                          nearest neighbors of the point 3 3

                                                                                          0 0

                                                                                          ( ) i ji j

                                                                                          i jv x y c x y

                                                                                          The coefficients cij are obtained solving a 16x16 linear system

                                                                                          intensity levels of the 16 nearest neighbors of 3 3

                                                                                          0 0

                                                                                          ( )i ji j

                                                                                          i jc x y x y

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                          bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                          programs such as Adobe Photoshop and Corel Photopaint

                                                                                          Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                          the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                          then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                          neighbor interpolation was used (both for shrinking and zooming)

                                                                                          Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                          interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                          from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Neighbors of a Pixel

                                                                                          A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                          This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                          The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                          and are denoted ND(p)

                                                                                          The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                          N8 (p)

                                                                                          If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                          fall outside the image

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Adjacency Connectivity Regions Boundaries

                                                                                          Denote by V the set of intensity levels used to define adjacency

                                                                                          - in a binary image V 01 (V=0 V=1)

                                                                                          - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                          We consider 3 types of adjacency

                                                                                          (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                          (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                          (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                          m-adjacent if

                                                                                          4( )q N p or

                                                                                          ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                          Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                          ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          binary image

                                                                                          0 1 1 0 1 1 0 1 1

                                                                                          1 0 1 0 0 1 0 0 1 0

                                                                                          0 0 1 0 0 1 0 0 1

                                                                                          V

                                                                                          The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                          8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                          m-adjacency

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                          is a sequence of distinct pixels with coordinates

                                                                                          and are adjacent 0 0 1 1

                                                                                          1 1

                                                                                          ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                          n n

                                                                                          i i i i

                                                                                          x y x y x y x y s tx y x y i n

                                                                                          The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                          Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                          Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                          in S if there exists a path between them consisting only of pixels from S

                                                                                          S is a connected set if there is a path in S between any 2 pixels in S

                                                                                          Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                          Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                          that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                          8-adjacency are considered

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                          touches the image border

                                                                                          the complement of 1

                                                                                          ( )K

                                                                                          cu k u u

                                                                                          k

                                                                                          R R R R

                                                                                          We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                          background of the image

                                                                                          The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                          points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                          region that have at least one background neighbor This definition is referred to as the

                                                                                          inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                          border in the background

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Distance measures

                                                                                          For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                          function or metric if

                                                                                          (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                          (b) D(p q) = D(q p)

                                                                                          (c) D(p z) le D(p q) + D(q z)

                                                                                          The Euclidean distance between p and q is defined as 1

                                                                                          2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                          The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                          centered at (x y)

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          The D4 distance (also called city-block distance) between p and q is defined as

                                                                                          4( ) | | | |D p q x s y t

                                                                                          The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                          4

                                                                                          22 1 2

                                                                                          2 2 1 0 1 22 1 2

                                                                                          2

                                                                                          D

                                                                                          The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                          The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                          8( ) max| | | |D p q x s y t

                                                                                          The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          8

                                                                                          2 2 2 2 22 1 1 1 2

                                                                                          2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                          D

                                                                                          The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                          D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                          because these distances involve only the coordinates of the point

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Array versus Matrix Operations

                                                                                          An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                          11 12 11 12

                                                                                          21 22 21 22

                                                                                          a a b ba a b b

                                                                                          Array product

                                                                                          11 12 11 12 11 11 12 12

                                                                                          21 22 21 22 21 21 22 21

                                                                                          a a b b a b a ba a b b a b a b

                                                                                          Matrix product

                                                                                          11 12 11 12 11 11 12 21 11 12 12 21

                                                                                          21 22 21 22 21 11 22 21 21 12 22 22

                                                                                          a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                          We assume array operations unless stated otherwise

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Linear versus Nonlinear Operations

                                                                                          One of the most important classifications of image-processing methods is whether it is

                                                                                          linear or nonlinear

                                                                                          ( ) ( )H f x y g x y

                                                                                          H is said to be a linear operator if

                                                                                          images1 2 1 2

                                                                                          1 2

                                                                                          ( ) ( ) ( ) ( )

                                                                                          H a f x y b f x y a H f x y b H f x y

                                                                                          a b f f

                                                                                          Example of nonlinear operator

                                                                                          the maximum value of the pixels of image max ( )H f f x y f

                                                                                          1 2

                                                                                          0 2 6 5 1 1

                                                                                          2 3 4 7f f a b

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          1 2

                                                                                          0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                          2 3 4 7 2 4a f b f

                                                                                          0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                          2 3 4 7

                                                                                          Arithmetic Operations in Image Processing

                                                                                          Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                          f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                          For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                          value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                          The two random variables are uncorrelated when their covariance is 0

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                          used in image enhancement)

                                                                                          1

                                                                                          1( ) ( )K

                                                                                          ii

                                                                                          g x y g x yK

                                                                                          If the noise satisfies the properties stated above we have

                                                                                          2 2( ) ( )

                                                                                          1( ) ( ) g x y x yE g x y f x yK

                                                                                          ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                          and g respectively The standard deviation (square root of the variance) at any point in

                                                                                          the average image is

                                                                                          ( ) ( )1

                                                                                          g x y x yK

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                          the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                          means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                          averaging process increases

                                                                                          An important application of image averaging is in the field of astronomy where imaging

                                                                                          under very low light levels frequently causes sensor noise to render single images

                                                                                          virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                          was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                          64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                          images respectively

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                          100 noisy images

                                                                                          a b c d e f

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          A frequent application of image subtraction is in the enhancement of differences between

                                                                                          images

                                                                                          (a) (b) (c)

                                                                                          Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                          significant bit of each pixel (c) the difference between the two images

                                                                                          Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                          Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                          images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                          difference between images (a) and (b)

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                          h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                          intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                          source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                          bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                          anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                          images after injection of the contrast medium

                                                                                          In g(x y) we can find the differences between h and f as enhanced detail

                                                                                          Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                          propagates through the various arteries in the area being observed

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          An important application of image multiplication (and division) is shading correction

                                                                                          Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                          f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                          When the shading function is known

                                                                                          ( )( )( )

                                                                                          g x yf x yh x y

                                                                                          h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                          approximation to the shading function by imaging a target of constant intensity When the

                                                                                          sensor is not available often the shading pattern can be estimated from the image

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          (a) (b) (c)

                                                                                          Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                          operations The process consists of multiplying a given image by a mask image that has

                                                                                          1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                          image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                          (a) (b) (c)

                                                                                          Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                          min( )mf f f

                                                                                          0 ( 255)max( )

                                                                                          ms

                                                                                          m

                                                                                          ff K K K

                                                                                          f

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Spatial Operations

                                                                                          - are performed directly on the pixels of a given image

                                                                                          There are three categories of spatial operations

                                                                                          single-pixel operations

                                                                                          neighborhood operations

                                                                                          geometric spatial transformations

                                                                                          Single-pixel operations

                                                                                          - change the values of intensity for the individual pixels ( )s T z

                                                                                          where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                          corresponding pixel in the processed image

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Neighborhood operations

                                                                                          Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                          in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                          based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                          rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                          intensity by computing the average value of the pixels in Sxy

                                                                                          ( )

                                                                                          1( ) ( )xyr c S

                                                                                          g x y f r cm n

                                                                                          The net effect is to perform local blurring in the original image This type of process is

                                                                                          used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                          largest region of an image

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Geometric spatial transformations and image registration

                                                                                          - modify the spatial relationship between pixels in an image

                                                                                          - these transformations are often called rubber-sheet transformations (analogous to

                                                                                          printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                          predefined set of rules

                                                                                          A geometric transformation consists of 2 basic operations

                                                                                          1 a spatial transformation of coordinates

                                                                                          2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                          pixels

                                                                                          The coordinate system transformation ( ) [( )]x y T v w

                                                                                          (v w) ndash pixel coordinates in the original image

                                                                                          (x y) ndash pixel coordinates in the transformed image

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                          Affine transform

                                                                                          11 1211 21 31

                                                                                          21 2212 22 33

                                                                                          31 32

                                                                                          0[ 1] [ 1] [ 1] 0

                                                                                          1

                                                                                          t tx t v t w t

                                                                                          x y v w T v w t ty t v t w t

                                                                                          t t

                                                                                          (AT)

                                                                                          This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                          on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                          result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                          scaling rotation and translation matrices from Table 1

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Affine transformations

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          The preceding transformations relocate pixels on an image to new locations To complete

                                                                                          the process we have to assign intensity values to those locations This task is done by

                                                                                          using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                          In practice we can use equation (AT) in two basic ways

                                                                                          forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                          location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                          Problems

                                                                                          - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                          the same location in the output image

                                                                                          - some output locations have no correspondent in the original image (no intensity

                                                                                          assignment)

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          inverse mapping scans the output pixel locations and at each location (x y)

                                                                                          computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                          It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                          pixel value

                                                                                          Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                          numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Image registration ndash align two or more images of the same scene

                                                                                          In image registration we have available the input and output images but the specific

                                                                                          transformation that produced the output image from the input is generally unknown

                                                                                          The problem is to estimate the transformation function and then use it to register the two

                                                                                          images

                                                                                          - it may be of interest to align (register) two or more image taken at approximately the

                                                                                          same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                          - align images of a given location taken by the same instrument at different moments

                                                                                          of time (satellite images)

                                                                                          Solving the problem using tie points (also called control points) which are

                                                                                          corresponding points whose locations are known precisely in the input and reference

                                                                                          image

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          How to select tie points

                                                                                          - interactively selecting them

                                                                                          - use of algorithms that try to detect these points

                                                                                          - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                          the imaging sensors These objects produce a set of known points (called reseau

                                                                                          marks) directly on all images captured by the system which can be used as guides

                                                                                          for establishing tie points

                                                                                          The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                          of 4 tie points both on the input image and the reference image A simple model based on

                                                                                          a bilinear approximation is given by

                                                                                          1 2 3 4

                                                                                          5 6 7 8

                                                                                          x c v c w c v w cy c v c w c v w c

                                                                                          (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                          frequently is to select a larger number of tie points and using this new set of tie points

                                                                                          subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                          subregions marked by 4 tie points we applied the transformation model described above

                                                                                          The number of tie points and the sophistication of the model required to solve the register

                                                                                          problem depend on the severity of the geometrical distortion

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Probabilistic Methods

                                                                                          zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                          p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                          ( ) kk

                                                                                          np zM N

                                                                                          nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                          pixels in the image) 1

                                                                                          0( ) 1

                                                                                          L

                                                                                          kk

                                                                                          p z

                                                                                          The mean (average) intensity of an image is given by 1

                                                                                          0( )

                                                                                          L

                                                                                          k kk

                                                                                          m z p z

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          The variance of the intensities is 1

                                                                                          2 2

                                                                                          0( ) ( )

                                                                                          L

                                                                                          k kk

                                                                                          z m p z

                                                                                          The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                          measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                          ( ) is used

                                                                                          The n-th moment of a random variable z about the mean is defined as 1

                                                                                          0( ) ( ) ( )

                                                                                          Ln

                                                                                          n k kk

                                                                                          z z m p z

                                                                                          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                          3( ) 0z the intensities are biased to values higher than the mean

                                                                                          ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                          mean

                                                                                          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                          Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                          Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                          Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Intensity Transformations and Spatial Filtering

                                                                                          ( ) ( )g x y T f x y

                                                                                          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                          neighborhood of (x y)

                                                                                          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                          and much smaller in size than the image

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                          called spatial filter (spatial mask kernel template or window)

                                                                                          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                          ( )s T r

                                                                                          s and r are denoting respectively the intensity of g and f at (x y)

                                                                                          Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                          darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                          is called contrast stretching

                                                                                          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                          thresholding function

                                                                                          Some Basic Intensity Transformation Functions

                                                                                          Image Negatives

                                                                                          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                          - equivalent of a photographic negative

                                                                                          - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                          image

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Original Negative image

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                          Some basic intensity transformation functions

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                          range An operator of this type is used to expand the values of dark pixels in an image

                                                                                          while compressing the higher-level values The opposite is true for the inverse log

                                                                                          transformation The log functions compress the dynamic range of images with large

                                                                                          variations in pixel values

                                                                                          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Power-Law (Gamma) Transformations

                                                                                          - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                          Plots of gamma transformation for different values of γ (c=1)

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                          of output values with the opposite being true for higher values of input values The

                                                                                          curves with 1 have the opposite effect of those generated with values of 1

                                                                                          1c - identity transformation

                                                                                          A variety of devices used for image capture printing and display respond according to a

                                                                                          power law The process used to correct these power-law response phenomena is called

                                                                                          gamma correction

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Piecewise-Linear Transformations Functions

                                                                                          Contrast stretching

                                                                                          - a process that expands the range of intensity levels in an image so it spans the full

                                                                                          intensity range of the recording tool or display device

                                                                                          a b c d Fig5

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          11

                                                                                          1

                                                                                          2 1 1 21 2

                                                                                          2 1 2 1

                                                                                          22

                                                                                          2

                                                                                          [0 ]

                                                                                          ( ) ( )( ) [ ]( ) ( )

                                                                                          ( 1 ) [ 1]( 1 )

                                                                                          s r r rrs r r s r rT r r r r

                                                                                          r r r rs L r r r L

                                                                                          L r

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          1 1 2 2r s r s identity transformation (no change)

                                                                                          1 2 1 2 0 1r r s s L thresholding function

                                                                                          Figure 5(b) shows an 8-bit image with low contrast

                                                                                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                          in the image respectively Thus the transformation function stretched the levels linearly

                                                                                          from their original range to the full range [0 L-1]

                                                                                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                          2 2 1r s m L where m is the mean gray level in the image

                                                                                          The original image on which these results are based is a scanning electron microscope

                                                                                          image of pollen magnified approximately 700 times

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Intensity-level slicing

                                                                                          - highlighting a specific range of intensities in an image

                                                                                          There are two approaches for intensity-level slicing

                                                                                          1 display in one value (white for example) all the values in the range of interest and in

                                                                                          another (say black) all other intensities (Figure 311 (a))

                                                                                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                          intensities in the image (Figure 311 (b))

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                          the top of the scale of intensities This type of enhancement produces a binary image

                                                                                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                          Highlights range [A B] and preserves all other intensities

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                          blockageshellip)

                                                                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                          image around the mean intensity was set to black the other intensities remain unchanged

                                                                                          Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Bit-plane slicing

                                                                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                          This technique highlights the contribution made to the whole image appearances by each

                                                                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          Digital Image Processing

                                                                                          Week 1

                                                                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                          • DIP 1 2017
                                                                                          • DIP 02 (2017)

                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                            Week 1Week 1

                                                                                            Outputs are attributes

                                                                                            bull morphological processing

                                                                                            bull segmentation

                                                                                            bull representation and description

                                                                                            bull object recognition

                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                            Week 1Week 1

                                                                                            Image acquisition - may involve preprocessing such as scaling

                                                                                            Image enhancement

                                                                                            bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                                            bull enhancement is problem oriented

                                                                                            bull there is no general sbquotheoryrsquo of image enhancement

                                                                                            bull enhancement use subjective methods for image emprovement

                                                                                            bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                            Week 1Week 1

                                                                                            Image restoration

                                                                                            bull improving the appearance of an image

                                                                                            bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                                            Color image processing

                                                                                            bull fundamental concept in color models

                                                                                            bull basic color processing in a digital domain

                                                                                            Wavelets and multiresolution processing

                                                                                            representing images in various degree of resolution

                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                            Week 1Week 1

                                                                                            Compression

                                                                                            reducing the storage required to save an image or the bandwidth required to transmit it

                                                                                            Morphological processing

                                                                                            bull tools for extracting image components that are useful in the representation and description of shape

                                                                                            bull a transition from processes that output images to processes that outputimage attributes

                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                            Week 1Week 1

                                                                                            Segmentation

                                                                                            bull partitioning an image into its constituents parts or objects

                                                                                            bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                                            bull the more accurate the segmentation the more likley recognition is to succeed

                                                                                            Representation and description (almost always follows segmentation)

                                                                                            bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                                            bull converting the data produced by segmentation to a form suitable for computer processing

                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                            Week 1Week 1

                                                                                            bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                                            bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                                            bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                                            Object recognition

                                                                                            the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                                            Knowledge database

                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                            Week 1Week 1

                                                                                            Simplified diagramof a cross sectionof the human eye

                                                                                            Digital Image ProcessingDigital Image Processing

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                                                                                            Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                            The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                            Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                            The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

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                                                                                            Week 1Week 1

                                                                                            The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                            The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                            Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                            Fovea = the place where the image of the object of interest falls on

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                                                                                            Week 1Week 1

                                                                                            Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                            Blind spot region without receptors

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                                                                                            Week 1Week 1

                                                                                            Image formation in the eye

                                                                                            Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                            Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                            distance between lens and retina along visual axix = 17 mm

                                                                                            range of focal length = 14 mm to 17 mm

                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                            Week 1Week 1

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                                                                                            Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                            Week 1Week 1

                                                                                            All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                            Digital Image ProcessingDigital Image Processing

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                                                                                            Optical illusions

                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                            Week 1Week 1

                                                                                            ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                            Week 1Week 1

                                                                                            Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                            gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                            quantities that describe the quality of a chromatic light source radiance

                                                                                            the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                            measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

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                                                                                            Week 1Week 1

                                                                                            For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                            brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

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                                                                                            Week 1Week 1

                                                                                            the physical meaning is determined by the source of the image

                                                                                            ( )f D f x y

                                                                                            Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                            f(xy) ndash characterized by two components

                                                                                            i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                            r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                            ( ) ( ) ( )

                                                                                            0 ( ) 0 ( ) 1

                                                                                            f x y i x y r x y

                                                                                            i x y r x y

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                                                                                            Week 1Week 1

                                                                                            r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                            i(xy) ndash determined by the illumination source

                                                                                            r(xy) ndash determined by the characteristics of the imaged objects

                                                                                            is called gray (or intensity) scale

                                                                                            In practice

                                                                                            min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                            indoor values without additional illuminationmin max10 1000L L

                                                                                            black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                            min maxL L

                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                            Week 1Week 1

                                                                                            Digital Image Processing

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                                                                                            Image Sampling and Quantization

                                                                                            - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                            scene

                                                                                            converting a continuous image f to digital form

                                                                                            - digitizing (x y) is called sampling

                                                                                            - digitizing f(x y) is called quantization

                                                                                            Digital Image Processing

                                                                                            Week 1

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                                                                                            Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                            (00) (01) (0 1)(10) (11) (1 1)

                                                                                            ( )

                                                                                            ( 10) ( 11) ( 1 1)

                                                                                            f f f Nf f f N

                                                                                            f x y

                                                                                            f M f M f M N

                                                                                            image element pixel

                                                                                            00 01 0 1

                                                                                            10 11 1 1

                                                                                            10 11 1 1

                                                                                            ( ) ( )

                                                                                            N

                                                                                            i jN M N

                                                                                            i j

                                                                                            M M M N

                                                                                            a a aa f x i y j f i ja a a

                                                                                            Aa

                                                                                            a a a

                                                                                            f(00) ndash the upper left corner of the image

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            M N ge 0 L=2k

                                                                                            [0 1]i j i ja a L

                                                                                            Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                            Digital Image Processing

                                                                                            Week 1

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                                                                                            Week 1

                                                                                            Number of bits required to store a digitized image

                                                                                            for 2 b M N k M N b N k

                                                                                            When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                            Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                            Image resolution = the largest number of discernible line pairs per unit distance

                                                                                            (eg 100 line pairs per mm)

                                                                                            Dots per unit distance are commonly used in printing and publishing

                                                                                            In US the measure is expressed in dots per inch (dpi)

                                                                                            (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                            Intensity resolution ndash the smallest discernible change in intensity level

                                                                                            The number of intensity levels (L) is determined by hardware considerations

                                                                                            L=2k ndash most common k = 8

                                                                                            Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                            Week 1

                                                                                            Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                            150 dpi (lower left) 72 dpi (lower right)

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Reducing the number of gray levels 256 128 64 32

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Reducing the number of gray levels 16 8 4 2

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                            Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                            Interpolation is the process of using known data to estimate values at unknown locations

                                                                                            Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                            750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                            same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                            image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                            Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                            Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                            intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                            This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                            straight edges

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                                                                                            Week 1

                                                                                            Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                            where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                            be written using the 4 nearest neighbors of point (x y)

                                                                                            Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                            modest increase in computational effort

                                                                                            Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                            nearest neighbors of the point 3 3

                                                                                            0 0

                                                                                            ( ) i ji j

                                                                                            i jv x y c x y

                                                                                            The coefficients cij are obtained solving a 16x16 linear system

                                                                                            intensity levels of the 16 nearest neighbors of 3 3

                                                                                            0 0

                                                                                            ( )i ji j

                                                                                            i jc x y x y

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                            bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                            programs such as Adobe Photoshop and Corel Photopaint

                                                                                            Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                            the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                            then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                            neighbor interpolation was used (both for shrinking and zooming)

                                                                                            Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                            interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                            from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Neighbors of a Pixel

                                                                                            A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                            This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                            The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                            and are denoted ND(p)

                                                                                            The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                            N8 (p)

                                                                                            If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                            fall outside the image

                                                                                            Digital Image Processing

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                                                                                            Adjacency Connectivity Regions Boundaries

                                                                                            Denote by V the set of intensity levels used to define adjacency

                                                                                            - in a binary image V 01 (V=0 V=1)

                                                                                            - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                            We consider 3 types of adjacency

                                                                                            (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                            (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                            (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                            m-adjacent if

                                                                                            4( )q N p or

                                                                                            ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                            Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                            ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            binary image

                                                                                            0 1 1 0 1 1 0 1 1

                                                                                            1 0 1 0 0 1 0 0 1 0

                                                                                            0 0 1 0 0 1 0 0 1

                                                                                            V

                                                                                            The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                            8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                            m-adjacency

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                            is a sequence of distinct pixels with coordinates

                                                                                            and are adjacent 0 0 1 1

                                                                                            1 1

                                                                                            ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                            n n

                                                                                            i i i i

                                                                                            x y x y x y x y s tx y x y i n

                                                                                            The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                            Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                            Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                            in S if there exists a path between them consisting only of pixels from S

                                                                                            S is a connected set if there is a path in S between any 2 pixels in S

                                                                                            Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                            Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                            that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                            8-adjacency are considered

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                            touches the image border

                                                                                            the complement of 1

                                                                                            ( )K

                                                                                            cu k u u

                                                                                            k

                                                                                            R R R R

                                                                                            We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                            background of the image

                                                                                            The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                            points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                            region that have at least one background neighbor This definition is referred to as the

                                                                                            inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                            border in the background

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Distance measures

                                                                                            For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                            function or metric if

                                                                                            (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                            (b) D(p q) = D(q p)

                                                                                            (c) D(p z) le D(p q) + D(q z)

                                                                                            The Euclidean distance between p and q is defined as 1

                                                                                            2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                            The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                            centered at (x y)

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            The D4 distance (also called city-block distance) between p and q is defined as

                                                                                            4( ) | | | |D p q x s y t

                                                                                            The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                            4

                                                                                            22 1 2

                                                                                            2 2 1 0 1 22 1 2

                                                                                            2

                                                                                            D

                                                                                            The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                            The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                            8( ) max| | | |D p q x s y t

                                                                                            The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            8

                                                                                            2 2 2 2 22 1 1 1 2

                                                                                            2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                            D

                                                                                            The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                            D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                            because these distances involve only the coordinates of the point

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Array versus Matrix Operations

                                                                                            An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                            11 12 11 12

                                                                                            21 22 21 22

                                                                                            a a b ba a b b

                                                                                            Array product

                                                                                            11 12 11 12 11 11 12 12

                                                                                            21 22 21 22 21 21 22 21

                                                                                            a a b b a b a ba a b b a b a b

                                                                                            Matrix product

                                                                                            11 12 11 12 11 11 12 21 11 12 12 21

                                                                                            21 22 21 22 21 11 22 21 21 12 22 22

                                                                                            a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                            We assume array operations unless stated otherwise

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Linear versus Nonlinear Operations

                                                                                            One of the most important classifications of image-processing methods is whether it is

                                                                                            linear or nonlinear

                                                                                            ( ) ( )H f x y g x y

                                                                                            H is said to be a linear operator if

                                                                                            images1 2 1 2

                                                                                            1 2

                                                                                            ( ) ( ) ( ) ( )

                                                                                            H a f x y b f x y a H f x y b H f x y

                                                                                            a b f f

                                                                                            Example of nonlinear operator

                                                                                            the maximum value of the pixels of image max ( )H f f x y f

                                                                                            1 2

                                                                                            0 2 6 5 1 1

                                                                                            2 3 4 7f f a b

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            1 2

                                                                                            0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                            2 3 4 7 2 4a f b f

                                                                                            0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                            2 3 4 7

                                                                                            Arithmetic Operations in Image Processing

                                                                                            Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                            f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                            For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                            value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                            The two random variables are uncorrelated when their covariance is 0

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                            used in image enhancement)

                                                                                            1

                                                                                            1( ) ( )K

                                                                                            ii

                                                                                            g x y g x yK

                                                                                            If the noise satisfies the properties stated above we have

                                                                                            2 2( ) ( )

                                                                                            1( ) ( ) g x y x yE g x y f x yK

                                                                                            ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                            and g respectively The standard deviation (square root of the variance) at any point in

                                                                                            the average image is

                                                                                            ( ) ( )1

                                                                                            g x y x yK

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                            the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                            means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                            averaging process increases

                                                                                            An important application of image averaging is in the field of astronomy where imaging

                                                                                            under very low light levels frequently causes sensor noise to render single images

                                                                                            virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                            was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                            64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                            images respectively

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                            100 noisy images

                                                                                            a b c d e f

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            A frequent application of image subtraction is in the enhancement of differences between

                                                                                            images

                                                                                            (a) (b) (c)

                                                                                            Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                            significant bit of each pixel (c) the difference between the two images

                                                                                            Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                            Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                            images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                            difference between images (a) and (b)

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                            h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                            intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                            source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                            bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                            anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                            images after injection of the contrast medium

                                                                                            In g(x y) we can find the differences between h and f as enhanced detail

                                                                                            Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                            propagates through the various arteries in the area being observed

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            An important application of image multiplication (and division) is shading correction

                                                                                            Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                            f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                            When the shading function is known

                                                                                            ( )( )( )

                                                                                            g x yf x yh x y

                                                                                            h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                            approximation to the shading function by imaging a target of constant intensity When the

                                                                                            sensor is not available often the shading pattern can be estimated from the image

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            (a) (b) (c)

                                                                                            Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                            operations The process consists of multiplying a given image by a mask image that has

                                                                                            1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                            image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                            (a) (b) (c)

                                                                                            Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                            min( )mf f f

                                                                                            0 ( 255)max( )

                                                                                            ms

                                                                                            m

                                                                                            ff K K K

                                                                                            f

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Spatial Operations

                                                                                            - are performed directly on the pixels of a given image

                                                                                            There are three categories of spatial operations

                                                                                            single-pixel operations

                                                                                            neighborhood operations

                                                                                            geometric spatial transformations

                                                                                            Single-pixel operations

                                                                                            - change the values of intensity for the individual pixels ( )s T z

                                                                                            where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                            corresponding pixel in the processed image

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Neighborhood operations

                                                                                            Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                            in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                            based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                            rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                            intensity by computing the average value of the pixels in Sxy

                                                                                            ( )

                                                                                            1( ) ( )xyr c S

                                                                                            g x y f r cm n

                                                                                            The net effect is to perform local blurring in the original image This type of process is

                                                                                            used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                            largest region of an image

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Geometric spatial transformations and image registration

                                                                                            - modify the spatial relationship between pixels in an image

                                                                                            - these transformations are often called rubber-sheet transformations (analogous to

                                                                                            printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                            predefined set of rules

                                                                                            A geometric transformation consists of 2 basic operations

                                                                                            1 a spatial transformation of coordinates

                                                                                            2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                            pixels

                                                                                            The coordinate system transformation ( ) [( )]x y T v w

                                                                                            (v w) ndash pixel coordinates in the original image

                                                                                            (x y) ndash pixel coordinates in the transformed image

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                            Affine transform

                                                                                            11 1211 21 31

                                                                                            21 2212 22 33

                                                                                            31 32

                                                                                            0[ 1] [ 1] [ 1] 0

                                                                                            1

                                                                                            t tx t v t w t

                                                                                            x y v w T v w t ty t v t w t

                                                                                            t t

                                                                                            (AT)

                                                                                            This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                            on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                            result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                            scaling rotation and translation matrices from Table 1

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Affine transformations

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            The preceding transformations relocate pixels on an image to new locations To complete

                                                                                            the process we have to assign intensity values to those locations This task is done by

                                                                                            using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                            In practice we can use equation (AT) in two basic ways

                                                                                            forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                            location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                            Problems

                                                                                            - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                            the same location in the output image

                                                                                            - some output locations have no correspondent in the original image (no intensity

                                                                                            assignment)

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            inverse mapping scans the output pixel locations and at each location (x y)

                                                                                            computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                            It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                            pixel value

                                                                                            Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                            numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Image registration ndash align two or more images of the same scene

                                                                                            In image registration we have available the input and output images but the specific

                                                                                            transformation that produced the output image from the input is generally unknown

                                                                                            The problem is to estimate the transformation function and then use it to register the two

                                                                                            images

                                                                                            - it may be of interest to align (register) two or more image taken at approximately the

                                                                                            same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                            - align images of a given location taken by the same instrument at different moments

                                                                                            of time (satellite images)

                                                                                            Solving the problem using tie points (also called control points) which are

                                                                                            corresponding points whose locations are known precisely in the input and reference

                                                                                            image

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            How to select tie points

                                                                                            - interactively selecting them

                                                                                            - use of algorithms that try to detect these points

                                                                                            - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                            the imaging sensors These objects produce a set of known points (called reseau

                                                                                            marks) directly on all images captured by the system which can be used as guides

                                                                                            for establishing tie points

                                                                                            The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                            of 4 tie points both on the input image and the reference image A simple model based on

                                                                                            a bilinear approximation is given by

                                                                                            1 2 3 4

                                                                                            5 6 7 8

                                                                                            x c v c w c v w cy c v c w c v w c

                                                                                            (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                            frequently is to select a larger number of tie points and using this new set of tie points

                                                                                            subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                            subregions marked by 4 tie points we applied the transformation model described above

                                                                                            The number of tie points and the sophistication of the model required to solve the register

                                                                                            problem depend on the severity of the geometrical distortion

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Probabilistic Methods

                                                                                            zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                            p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                            ( ) kk

                                                                                            np zM N

                                                                                            nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                            pixels in the image) 1

                                                                                            0( ) 1

                                                                                            L

                                                                                            kk

                                                                                            p z

                                                                                            The mean (average) intensity of an image is given by 1

                                                                                            0( )

                                                                                            L

                                                                                            k kk

                                                                                            m z p z

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            The variance of the intensities is 1

                                                                                            2 2

                                                                                            0( ) ( )

                                                                                            L

                                                                                            k kk

                                                                                            z m p z

                                                                                            The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                            measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                            ( ) is used

                                                                                            The n-th moment of a random variable z about the mean is defined as 1

                                                                                            0( ) ( ) ( )

                                                                                            Ln

                                                                                            n k kk

                                                                                            z z m p z

                                                                                            ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                            3( ) 0z the intensities are biased to values higher than the mean

                                                                                            ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                            mean

                                                                                            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                            Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                            Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                            Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Intensity Transformations and Spatial Filtering

                                                                                            ( ) ( )g x y T f x y

                                                                                            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                            neighborhood of (x y)

                                                                                            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                            and much smaller in size than the image

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                            called spatial filter (spatial mask kernel template or window)

                                                                                            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                            ( )s T r

                                                                                            s and r are denoting respectively the intensity of g and f at (x y)

                                                                                            Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                            darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                            is called contrast stretching

                                                                                            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                            thresholding function

                                                                                            Some Basic Intensity Transformation Functions

                                                                                            Image Negatives

                                                                                            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                            - equivalent of a photographic negative

                                                                                            - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                            image

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Original Negative image

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                            Some basic intensity transformation functions

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                            range An operator of this type is used to expand the values of dark pixels in an image

                                                                                            while compressing the higher-level values The opposite is true for the inverse log

                                                                                            transformation The log functions compress the dynamic range of images with large

                                                                                            variations in pixel values

                                                                                            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Power-Law (Gamma) Transformations

                                                                                            - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                            Plots of gamma transformation for different values of γ (c=1)

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                            of output values with the opposite being true for higher values of input values The

                                                                                            curves with 1 have the opposite effect of those generated with values of 1

                                                                                            1c - identity transformation

                                                                                            A variety of devices used for image capture printing and display respond according to a

                                                                                            power law The process used to correct these power-law response phenomena is called

                                                                                            gamma correction

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Piecewise-Linear Transformations Functions

                                                                                            Contrast stretching

                                                                                            - a process that expands the range of intensity levels in an image so it spans the full

                                                                                            intensity range of the recording tool or display device

                                                                                            a b c d Fig5

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            11

                                                                                            1

                                                                                            2 1 1 21 2

                                                                                            2 1 2 1

                                                                                            22

                                                                                            2

                                                                                            [0 ]

                                                                                            ( ) ( )( ) [ ]( ) ( )

                                                                                            ( 1 ) [ 1]( 1 )

                                                                                            s r r rrs r r s r rT r r r r

                                                                                            r r r rs L r r r L

                                                                                            L r

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            1 1 2 2r s r s identity transformation (no change)

                                                                                            1 2 1 2 0 1r r s s L thresholding function

                                                                                            Figure 5(b) shows an 8-bit image with low contrast

                                                                                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                            in the image respectively Thus the transformation function stretched the levels linearly

                                                                                            from their original range to the full range [0 L-1]

                                                                                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                            2 2 1r s m L where m is the mean gray level in the image

                                                                                            The original image on which these results are based is a scanning electron microscope

                                                                                            image of pollen magnified approximately 700 times

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Intensity-level slicing

                                                                                            - highlighting a specific range of intensities in an image

                                                                                            There are two approaches for intensity-level slicing

                                                                                            1 display in one value (white for example) all the values in the range of interest and in

                                                                                            another (say black) all other intensities (Figure 311 (a))

                                                                                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                            intensities in the image (Figure 311 (b))

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                            the top of the scale of intensities This type of enhancement produces a binary image

                                                                                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                            Highlights range [A B] and preserves all other intensities

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                            blockageshellip)

                                                                                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                            image around the mean intensity was set to black the other intensities remain unchanged

                                                                                            Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Bit-plane slicing

                                                                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                            This technique highlights the contribution made to the whole image appearances by each

                                                                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            Digital Image Processing

                                                                                            Week 1

                                                                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                            • DIP 1 2017
                                                                                            • DIP 02 (2017)

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              Image acquisition - may involve preprocessing such as scaling

                                                                                              Image enhancement

                                                                                              bull manipulating an image so that the result is more suitable than the original for a specific operation

                                                                                              bull enhancement is problem oriented

                                                                                              bull there is no general sbquotheoryrsquo of image enhancement

                                                                                              bull enhancement use subjective methods for image emprovement

                                                                                              bull enhancement is based on human subjective preferences regarding what is a bdquogoodrdquo enhancement result

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              Image restoration

                                                                                              bull improving the appearance of an image

                                                                                              bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                                              Color image processing

                                                                                              bull fundamental concept in color models

                                                                                              bull basic color processing in a digital domain

                                                                                              Wavelets and multiresolution processing

                                                                                              representing images in various degree of resolution

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              Compression

                                                                                              reducing the storage required to save an image or the bandwidth required to transmit it

                                                                                              Morphological processing

                                                                                              bull tools for extracting image components that are useful in the representation and description of shape

                                                                                              bull a transition from processes that output images to processes that outputimage attributes

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              Segmentation

                                                                                              bull partitioning an image into its constituents parts or objects

                                                                                              bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                                              bull the more accurate the segmentation the more likley recognition is to succeed

                                                                                              Representation and description (almost always follows segmentation)

                                                                                              bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                                              bull converting the data produced by segmentation to a form suitable for computer processing

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                                              bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                                              bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                                              Object recognition

                                                                                              the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                                              Knowledge database

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              Simplified diagramof a cross sectionof the human eye

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                              The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                              Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                              The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                              The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                              Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                              Fovea = the place where the image of the object of interest falls on

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                              Blind spot region without receptors

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              Image formation in the eye

                                                                                              Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                              Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                              distance between lens and retina along visual axix = 17 mm

                                                                                              range of focal length = 14 mm to 17 mm

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              Optical illusions

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                              gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                              quantities that describe the quality of a chromatic light source radiance

                                                                                              the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                              measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                              brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              the physical meaning is determined by the source of the image

                                                                                              ( )f D f x y

                                                                                              Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                              f(xy) ndash characterized by two components

                                                                                              i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                              r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                              ( ) ( ) ( )

                                                                                              0 ( ) 0 ( ) 1

                                                                                              f x y i x y r x y

                                                                                              i x y r x y

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                              i(xy) ndash determined by the illumination source

                                                                                              r(xy) ndash determined by the characteristics of the imaged objects

                                                                                              is called gray (or intensity) scale

                                                                                              In practice

                                                                                              min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                              indoor values without additional illuminationmin max10 1000L L

                                                                                              black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                              min maxL L

                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                              Week 1Week 1

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Image Sampling and Quantization

                                                                                              - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                              scene

                                                                                              converting a continuous image f to digital form

                                                                                              - digitizing (x y) is called sampling

                                                                                              - digitizing f(x y) is called quantization

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                              (00) (01) (0 1)(10) (11) (1 1)

                                                                                              ( )

                                                                                              ( 10) ( 11) ( 1 1)

                                                                                              f f f Nf f f N

                                                                                              f x y

                                                                                              f M f M f M N

                                                                                              image element pixel

                                                                                              00 01 0 1

                                                                                              10 11 1 1

                                                                                              10 11 1 1

                                                                                              ( ) ( )

                                                                                              N

                                                                                              i jN M N

                                                                                              i j

                                                                                              M M M N

                                                                                              a a aa f x i y j f i ja a a

                                                                                              Aa

                                                                                              a a a

                                                                                              f(00) ndash the upper left corner of the image

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              M N ge 0 L=2k

                                                                                              [0 1]i j i ja a L

                                                                                              Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Number of bits required to store a digitized image

                                                                                              for 2 b M N k M N b N k

                                                                                              When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                              Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                              Image resolution = the largest number of discernible line pairs per unit distance

                                                                                              (eg 100 line pairs per mm)

                                                                                              Dots per unit distance are commonly used in printing and publishing

                                                                                              In US the measure is expressed in dots per inch (dpi)

                                                                                              (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                              Intensity resolution ndash the smallest discernible change in intensity level

                                                                                              The number of intensity levels (L) is determined by hardware considerations

                                                                                              L=2k ndash most common k = 8

                                                                                              Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                              150 dpi (lower left) 72 dpi (lower right)

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Reducing the number of gray levels 256 128 64 32

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Reducing the number of gray levels 16 8 4 2

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                              Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                              Interpolation is the process of using known data to estimate values at unknown locations

                                                                                              Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                              750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                              same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                              image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                              Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                              Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                              intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                              This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                              straight edges

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                              where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                              be written using the 4 nearest neighbors of point (x y)

                                                                                              Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                              modest increase in computational effort

                                                                                              Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                              nearest neighbors of the point 3 3

                                                                                              0 0

                                                                                              ( ) i ji j

                                                                                              i jv x y c x y

                                                                                              The coefficients cij are obtained solving a 16x16 linear system

                                                                                              intensity levels of the 16 nearest neighbors of 3 3

                                                                                              0 0

                                                                                              ( )i ji j

                                                                                              i jc x y x y

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                              bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                              programs such as Adobe Photoshop and Corel Photopaint

                                                                                              Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                              the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                              then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                              neighbor interpolation was used (both for shrinking and zooming)

                                                                                              Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                              interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                              from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Neighbors of a Pixel

                                                                                              A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                              This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                              The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                              and are denoted ND(p)

                                                                                              The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                              N8 (p)

                                                                                              If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                              fall outside the image

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Adjacency Connectivity Regions Boundaries

                                                                                              Denote by V the set of intensity levels used to define adjacency

                                                                                              - in a binary image V 01 (V=0 V=1)

                                                                                              - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                              We consider 3 types of adjacency

                                                                                              (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                              (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                              (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                              m-adjacent if

                                                                                              4( )q N p or

                                                                                              ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                              Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                              ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              binary image

                                                                                              0 1 1 0 1 1 0 1 1

                                                                                              1 0 1 0 0 1 0 0 1 0

                                                                                              0 0 1 0 0 1 0 0 1

                                                                                              V

                                                                                              The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                              8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                              m-adjacency

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                              is a sequence of distinct pixels with coordinates

                                                                                              and are adjacent 0 0 1 1

                                                                                              1 1

                                                                                              ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                              n n

                                                                                              i i i i

                                                                                              x y x y x y x y s tx y x y i n

                                                                                              The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                              Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                              Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                              in S if there exists a path between them consisting only of pixels from S

                                                                                              S is a connected set if there is a path in S between any 2 pixels in S

                                                                                              Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                              Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                              that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                              8-adjacency are considered

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                              touches the image border

                                                                                              the complement of 1

                                                                                              ( )K

                                                                                              cu k u u

                                                                                              k

                                                                                              R R R R

                                                                                              We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                              background of the image

                                                                                              The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                              points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                              region that have at least one background neighbor This definition is referred to as the

                                                                                              inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                              border in the background

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Distance measures

                                                                                              For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                              function or metric if

                                                                                              (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                              (b) D(p q) = D(q p)

                                                                                              (c) D(p z) le D(p q) + D(q z)

                                                                                              The Euclidean distance between p and q is defined as 1

                                                                                              2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                              The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                              centered at (x y)

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              The D4 distance (also called city-block distance) between p and q is defined as

                                                                                              4( ) | | | |D p q x s y t

                                                                                              The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                              4

                                                                                              22 1 2

                                                                                              2 2 1 0 1 22 1 2

                                                                                              2

                                                                                              D

                                                                                              The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                              The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                              8( ) max| | | |D p q x s y t

                                                                                              The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              8

                                                                                              2 2 2 2 22 1 1 1 2

                                                                                              2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                              D

                                                                                              The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                              D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                              because these distances involve only the coordinates of the point

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Array versus Matrix Operations

                                                                                              An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                              11 12 11 12

                                                                                              21 22 21 22

                                                                                              a a b ba a b b

                                                                                              Array product

                                                                                              11 12 11 12 11 11 12 12

                                                                                              21 22 21 22 21 21 22 21

                                                                                              a a b b a b a ba a b b a b a b

                                                                                              Matrix product

                                                                                              11 12 11 12 11 11 12 21 11 12 12 21

                                                                                              21 22 21 22 21 11 22 21 21 12 22 22

                                                                                              a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                              We assume array operations unless stated otherwise

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Linear versus Nonlinear Operations

                                                                                              One of the most important classifications of image-processing methods is whether it is

                                                                                              linear or nonlinear

                                                                                              ( ) ( )H f x y g x y

                                                                                              H is said to be a linear operator if

                                                                                              images1 2 1 2

                                                                                              1 2

                                                                                              ( ) ( ) ( ) ( )

                                                                                              H a f x y b f x y a H f x y b H f x y

                                                                                              a b f f

                                                                                              Example of nonlinear operator

                                                                                              the maximum value of the pixels of image max ( )H f f x y f

                                                                                              1 2

                                                                                              0 2 6 5 1 1

                                                                                              2 3 4 7f f a b

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              1 2

                                                                                              0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                              2 3 4 7 2 4a f b f

                                                                                              0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                              2 3 4 7

                                                                                              Arithmetic Operations in Image Processing

                                                                                              Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                              f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                              For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                              value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                              The two random variables are uncorrelated when their covariance is 0

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                              used in image enhancement)

                                                                                              1

                                                                                              1( ) ( )K

                                                                                              ii

                                                                                              g x y g x yK

                                                                                              If the noise satisfies the properties stated above we have

                                                                                              2 2( ) ( )

                                                                                              1( ) ( ) g x y x yE g x y f x yK

                                                                                              ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                              and g respectively The standard deviation (square root of the variance) at any point in

                                                                                              the average image is

                                                                                              ( ) ( )1

                                                                                              g x y x yK

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                              the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                              means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                              averaging process increases

                                                                                              An important application of image averaging is in the field of astronomy where imaging

                                                                                              under very low light levels frequently causes sensor noise to render single images

                                                                                              virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                              was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                              64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                              images respectively

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                              100 noisy images

                                                                                              a b c d e f

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              A frequent application of image subtraction is in the enhancement of differences between

                                                                                              images

                                                                                              (a) (b) (c)

                                                                                              Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                              significant bit of each pixel (c) the difference between the two images

                                                                                              Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                              Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                              images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                              difference between images (a) and (b)

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                              h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                              intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                              source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                              bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                              anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                              images after injection of the contrast medium

                                                                                              In g(x y) we can find the differences between h and f as enhanced detail

                                                                                              Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                              propagates through the various arteries in the area being observed

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              An important application of image multiplication (and division) is shading correction

                                                                                              Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                              f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                              When the shading function is known

                                                                                              ( )( )( )

                                                                                              g x yf x yh x y

                                                                                              h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                              approximation to the shading function by imaging a target of constant intensity When the

                                                                                              sensor is not available often the shading pattern can be estimated from the image

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              (a) (b) (c)

                                                                                              Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                              operations The process consists of multiplying a given image by a mask image that has

                                                                                              1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                              image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                              (a) (b) (c)

                                                                                              Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                              min( )mf f f

                                                                                              0 ( 255)max( )

                                                                                              ms

                                                                                              m

                                                                                              ff K K K

                                                                                              f

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Spatial Operations

                                                                                              - are performed directly on the pixels of a given image

                                                                                              There are three categories of spatial operations

                                                                                              single-pixel operations

                                                                                              neighborhood operations

                                                                                              geometric spatial transformations

                                                                                              Single-pixel operations

                                                                                              - change the values of intensity for the individual pixels ( )s T z

                                                                                              where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                              corresponding pixel in the processed image

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Neighborhood operations

                                                                                              Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                              in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                              based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                              rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                              intensity by computing the average value of the pixels in Sxy

                                                                                              ( )

                                                                                              1( ) ( )xyr c S

                                                                                              g x y f r cm n

                                                                                              The net effect is to perform local blurring in the original image This type of process is

                                                                                              used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                              largest region of an image

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Geometric spatial transformations and image registration

                                                                                              - modify the spatial relationship between pixels in an image

                                                                                              - these transformations are often called rubber-sheet transformations (analogous to

                                                                                              printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                              predefined set of rules

                                                                                              A geometric transformation consists of 2 basic operations

                                                                                              1 a spatial transformation of coordinates

                                                                                              2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                              pixels

                                                                                              The coordinate system transformation ( ) [( )]x y T v w

                                                                                              (v w) ndash pixel coordinates in the original image

                                                                                              (x y) ndash pixel coordinates in the transformed image

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                              Affine transform

                                                                                              11 1211 21 31

                                                                                              21 2212 22 33

                                                                                              31 32

                                                                                              0[ 1] [ 1] [ 1] 0

                                                                                              1

                                                                                              t tx t v t w t

                                                                                              x y v w T v w t ty t v t w t

                                                                                              t t

                                                                                              (AT)

                                                                                              This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                              on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                              result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                              scaling rotation and translation matrices from Table 1

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Affine transformations

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              The preceding transformations relocate pixels on an image to new locations To complete

                                                                                              the process we have to assign intensity values to those locations This task is done by

                                                                                              using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                              In practice we can use equation (AT) in two basic ways

                                                                                              forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                              location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                              Problems

                                                                                              - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                              the same location in the output image

                                                                                              - some output locations have no correspondent in the original image (no intensity

                                                                                              assignment)

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              inverse mapping scans the output pixel locations and at each location (x y)

                                                                                              computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                              It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                              pixel value

                                                                                              Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                              numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Image registration ndash align two or more images of the same scene

                                                                                              In image registration we have available the input and output images but the specific

                                                                                              transformation that produced the output image from the input is generally unknown

                                                                                              The problem is to estimate the transformation function and then use it to register the two

                                                                                              images

                                                                                              - it may be of interest to align (register) two or more image taken at approximately the

                                                                                              same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                              - align images of a given location taken by the same instrument at different moments

                                                                                              of time (satellite images)

                                                                                              Solving the problem using tie points (also called control points) which are

                                                                                              corresponding points whose locations are known precisely in the input and reference

                                                                                              image

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              How to select tie points

                                                                                              - interactively selecting them

                                                                                              - use of algorithms that try to detect these points

                                                                                              - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                              the imaging sensors These objects produce a set of known points (called reseau

                                                                                              marks) directly on all images captured by the system which can be used as guides

                                                                                              for establishing tie points

                                                                                              The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                              of 4 tie points both on the input image and the reference image A simple model based on

                                                                                              a bilinear approximation is given by

                                                                                              1 2 3 4

                                                                                              5 6 7 8

                                                                                              x c v c w c v w cy c v c w c v w c

                                                                                              (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                              frequently is to select a larger number of tie points and using this new set of tie points

                                                                                              subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                              subregions marked by 4 tie points we applied the transformation model described above

                                                                                              The number of tie points and the sophistication of the model required to solve the register

                                                                                              problem depend on the severity of the geometrical distortion

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Probabilistic Methods

                                                                                              zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                              p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                              ( ) kk

                                                                                              np zM N

                                                                                              nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                              pixels in the image) 1

                                                                                              0( ) 1

                                                                                              L

                                                                                              kk

                                                                                              p z

                                                                                              The mean (average) intensity of an image is given by 1

                                                                                              0( )

                                                                                              L

                                                                                              k kk

                                                                                              m z p z

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              The variance of the intensities is 1

                                                                                              2 2

                                                                                              0( ) ( )

                                                                                              L

                                                                                              k kk

                                                                                              z m p z

                                                                                              The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                              measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                              ( ) is used

                                                                                              The n-th moment of a random variable z about the mean is defined as 1

                                                                                              0( ) ( ) ( )

                                                                                              Ln

                                                                                              n k kk

                                                                                              z z m p z

                                                                                              ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                              3( ) 0z the intensities are biased to values higher than the mean

                                                                                              ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                              mean

                                                                                              Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                              Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                              Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                              Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Intensity Transformations and Spatial Filtering

                                                                                              ( ) ( )g x y T f x y

                                                                                              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                              neighborhood of (x y)

                                                                                              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                              and much smaller in size than the image

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                              called spatial filter (spatial mask kernel template or window)

                                                                                              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                              ( )s T r

                                                                                              s and r are denoting respectively the intensity of g and f at (x y)

                                                                                              Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                              darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                              is called contrast stretching

                                                                                              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                              thresholding function

                                                                                              Some Basic Intensity Transformation Functions

                                                                                              Image Negatives

                                                                                              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                              - equivalent of a photographic negative

                                                                                              - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                              image

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Original Negative image

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                              Some basic intensity transformation functions

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                              range An operator of this type is used to expand the values of dark pixels in an image

                                                                                              while compressing the higher-level values The opposite is true for the inverse log

                                                                                              transformation The log functions compress the dynamic range of images with large

                                                                                              variations in pixel values

                                                                                              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Power-Law (Gamma) Transformations

                                                                                              - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                              Plots of gamma transformation for different values of γ (c=1)

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                              of output values with the opposite being true for higher values of input values The

                                                                                              curves with 1 have the opposite effect of those generated with values of 1

                                                                                              1c - identity transformation

                                                                                              A variety of devices used for image capture printing and display respond according to a

                                                                                              power law The process used to correct these power-law response phenomena is called

                                                                                              gamma correction

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Piecewise-Linear Transformations Functions

                                                                                              Contrast stretching

                                                                                              - a process that expands the range of intensity levels in an image so it spans the full

                                                                                              intensity range of the recording tool or display device

                                                                                              a b c d Fig5

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              11

                                                                                              1

                                                                                              2 1 1 21 2

                                                                                              2 1 2 1

                                                                                              22

                                                                                              2

                                                                                              [0 ]

                                                                                              ( ) ( )( ) [ ]( ) ( )

                                                                                              ( 1 ) [ 1]( 1 )

                                                                                              s r r rrs r r s r rT r r r r

                                                                                              r r r rs L r r r L

                                                                                              L r

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              1 1 2 2r s r s identity transformation (no change)

                                                                                              1 2 1 2 0 1r r s s L thresholding function

                                                                                              Figure 5(b) shows an 8-bit image with low contrast

                                                                                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                              in the image respectively Thus the transformation function stretched the levels linearly

                                                                                              from their original range to the full range [0 L-1]

                                                                                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                              2 2 1r s m L where m is the mean gray level in the image

                                                                                              The original image on which these results are based is a scanning electron microscope

                                                                                              image of pollen magnified approximately 700 times

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              Intensity-level slicing

                                                                                              - highlighting a specific range of intensities in an image

                                                                                              There are two approaches for intensity-level slicing

                                                                                              1 display in one value (white for example) all the values in the range of interest and in

                                                                                              another (say black) all other intensities (Figure 311 (a))

                                                                                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                              intensities in the image (Figure 311 (b))

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                                                                                              Week 1

                                                                                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                              the top of the scale of intensities This type of enhancement produces a binary image

                                                                                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                              Highlights range [A B] and preserves all other intensities

                                                                                              Digital Image Processing

                                                                                              Week 1

                                                                                              which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                              blockageshellip)

                                                                                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                              image around the mean intensity was set to black the other intensities remain unchanged

                                                                                              Fig 6 - Aortic angiogram and intensity sliced versions

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                                                                                              Week 1

                                                                                              Bit-plane slicing

                                                                                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                              This technique highlights the contribution made to the whole image appearances by each

                                                                                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

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                                                                                              Week 1

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                                                                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                              • DIP 1 2017
                                                                                              • DIP 02 (2017)

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                                                                                                Week 1Week 1

                                                                                                Image restoration

                                                                                                bull improving the appearance of an image

                                                                                                bull restoration is objective - the techniques for restoration are based on mathematical or probabilistic models of image degradation

                                                                                                Color image processing

                                                                                                bull fundamental concept in color models

                                                                                                bull basic color processing in a digital domain

                                                                                                Wavelets and multiresolution processing

                                                                                                representing images in various degree of resolution

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                                                                                                Compression

                                                                                                reducing the storage required to save an image or the bandwidth required to transmit it

                                                                                                Morphological processing

                                                                                                bull tools for extracting image components that are useful in the representation and description of shape

                                                                                                bull a transition from processes that output images to processes that outputimage attributes

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                                                                                                Segmentation

                                                                                                bull partitioning an image into its constituents parts or objects

                                                                                                bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                                                bull the more accurate the segmentation the more likley recognition is to succeed

                                                                                                Representation and description (almost always follows segmentation)

                                                                                                bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                                                bull converting the data produced by segmentation to a form suitable for computer processing

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                                                                                                Week 1Week 1

                                                                                                bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                                                bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                                                bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                                                Object recognition

                                                                                                the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                                                Knowledge database

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                                                                                                Simplified diagramof a cross sectionof the human eye

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                                                                                                Week 1Week 1

                                                                                                Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                                The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                                Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                                The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                                                Digital Image ProcessingDigital Image Processing

                                                                                                Week 1Week 1

                                                                                                The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                                The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                                Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                                Fovea = the place where the image of the object of interest falls on

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                                                                                                Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                                Blind spot region without receptors

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                                                                                                Week 1Week 1

                                                                                                Image formation in the eye

                                                                                                Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                                Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                                distance between lens and retina along visual axix = 17 mm

                                                                                                range of focal length = 14 mm to 17 mm

                                                                                                Digital Image ProcessingDigital Image Processing

                                                                                                Week 1Week 1

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                                                                                                Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                                Digital Image ProcessingDigital Image Processing

                                                                                                Week 1Week 1

                                                                                                All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                                Digital Image ProcessingDigital Image Processing

                                                                                                Week 1Week 1

                                                                                                Optical illusions

                                                                                                Digital Image ProcessingDigital Image Processing

                                                                                                Week 1Week 1

                                                                                                ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                                Digital Image ProcessingDigital Image Processing

                                                                                                Week 1Week 1

                                                                                                Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                quantities that describe the quality of a chromatic light source radiance

                                                                                                the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

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                                                                                                Week 1Week 1

                                                                                                For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

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                                                                                                Week 1Week 1

                                                                                                the physical meaning is determined by the source of the image

                                                                                                ( )f D f x y

                                                                                                Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                f(xy) ndash characterized by two components

                                                                                                i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                ( ) ( ) ( )

                                                                                                0 ( ) 0 ( ) 1

                                                                                                f x y i x y r x y

                                                                                                i x y r x y

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                                                                                                Week 1Week 1

                                                                                                r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                i(xy) ndash determined by the illumination source

                                                                                                r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                is called gray (or intensity) scale

                                                                                                In practice

                                                                                                min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                indoor values without additional illuminationmin max10 1000L L

                                                                                                black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                min maxL L

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                                                                                                Week 1Week 1

                                                                                                Digital Image Processing

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                                                                                                Image Sampling and Quantization

                                                                                                - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                scene

                                                                                                converting a continuous image f to digital form

                                                                                                - digitizing (x y) is called sampling

                                                                                                - digitizing f(x y) is called quantization

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Digital Image Processing

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                                                                                                Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                (00) (01) (0 1)(10) (11) (1 1)

                                                                                                ( )

                                                                                                ( 10) ( 11) ( 1 1)

                                                                                                f f f Nf f f N

                                                                                                f x y

                                                                                                f M f M f M N

                                                                                                image element pixel

                                                                                                00 01 0 1

                                                                                                10 11 1 1

                                                                                                10 11 1 1

                                                                                                ( ) ( )

                                                                                                N

                                                                                                i jN M N

                                                                                                i j

                                                                                                M M M N

                                                                                                a a aa f x i y j f i ja a a

                                                                                                Aa

                                                                                                a a a

                                                                                                f(00) ndash the upper left corner of the image

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                M N ge 0 L=2k

                                                                                                [0 1]i j i ja a L

                                                                                                Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                Digital Image Processing

                                                                                                Week 1

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                                                                                                Number of bits required to store a digitized image

                                                                                                for 2 b M N k M N b N k

                                                                                                When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                (eg 100 line pairs per mm)

                                                                                                Dots per unit distance are commonly used in printing and publishing

                                                                                                In US the measure is expressed in dots per inch (dpi)

                                                                                                (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                The number of intensity levels (L) is determined by hardware considerations

                                                                                                L=2k ndash most common k = 8

                                                                                                Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                150 dpi (lower left) 72 dpi (lower right)

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Reducing the number of gray levels 256 128 64 32

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                                                                                                Week 1

                                                                                                Reducing the number of gray levels 16 8 4 2

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                straight edges

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                be written using the 4 nearest neighbors of point (x y)

                                                                                                Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                modest increase in computational effort

                                                                                                Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                nearest neighbors of the point 3 3

                                                                                                0 0

                                                                                                ( ) i ji j

                                                                                                i jv x y c x y

                                                                                                The coefficients cij are obtained solving a 16x16 linear system

                                                                                                intensity levels of the 16 nearest neighbors of 3 3

                                                                                                0 0

                                                                                                ( )i ji j

                                                                                                i jc x y x y

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                programs such as Adobe Photoshop and Corel Photopaint

                                                                                                Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                neighbor interpolation was used (both for shrinking and zooming)

                                                                                                Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Neighbors of a Pixel

                                                                                                A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                and are denoted ND(p)

                                                                                                The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                N8 (p)

                                                                                                If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                fall outside the image

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Adjacency Connectivity Regions Boundaries

                                                                                                Denote by V the set of intensity levels used to define adjacency

                                                                                                - in a binary image V 01 (V=0 V=1)

                                                                                                - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                We consider 3 types of adjacency

                                                                                                (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                m-adjacent if

                                                                                                4( )q N p or

                                                                                                ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                binary image

                                                                                                0 1 1 0 1 1 0 1 1

                                                                                                1 0 1 0 0 1 0 0 1 0

                                                                                                0 0 1 0 0 1 0 0 1

                                                                                                V

                                                                                                The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                m-adjacency

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                is a sequence of distinct pixels with coordinates

                                                                                                and are adjacent 0 0 1 1

                                                                                                1 1

                                                                                                ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                n n

                                                                                                i i i i

                                                                                                x y x y x y x y s tx y x y i n

                                                                                                The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                in S if there exists a path between them consisting only of pixels from S

                                                                                                S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                8-adjacency are considered

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                touches the image border

                                                                                                the complement of 1

                                                                                                ( )K

                                                                                                cu k u u

                                                                                                k

                                                                                                R R R R

                                                                                                We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                background of the image

                                                                                                The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                region that have at least one background neighbor This definition is referred to as the

                                                                                                inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                border in the background

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Distance measures

                                                                                                For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                function or metric if

                                                                                                (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                (b) D(p q) = D(q p)

                                                                                                (c) D(p z) le D(p q) + D(q z)

                                                                                                The Euclidean distance between p and q is defined as 1

                                                                                                2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                centered at (x y)

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                4( ) | | | |D p q x s y t

                                                                                                The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                4

                                                                                                22 1 2

                                                                                                2 2 1 0 1 22 1 2

                                                                                                2

                                                                                                D

                                                                                                The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                8( ) max| | | |D p q x s y t

                                                                                                The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                8

                                                                                                2 2 2 2 22 1 1 1 2

                                                                                                2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                D

                                                                                                The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                because these distances involve only the coordinates of the point

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Array versus Matrix Operations

                                                                                                An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                11 12 11 12

                                                                                                21 22 21 22

                                                                                                a a b ba a b b

                                                                                                Array product

                                                                                                11 12 11 12 11 11 12 12

                                                                                                21 22 21 22 21 21 22 21

                                                                                                a a b b a b a ba a b b a b a b

                                                                                                Matrix product

                                                                                                11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                We assume array operations unless stated otherwise

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Linear versus Nonlinear Operations

                                                                                                One of the most important classifications of image-processing methods is whether it is

                                                                                                linear or nonlinear

                                                                                                ( ) ( )H f x y g x y

                                                                                                H is said to be a linear operator if

                                                                                                images1 2 1 2

                                                                                                1 2

                                                                                                ( ) ( ) ( ) ( )

                                                                                                H a f x y b f x y a H f x y b H f x y

                                                                                                a b f f

                                                                                                Example of nonlinear operator

                                                                                                the maximum value of the pixels of image max ( )H f f x y f

                                                                                                1 2

                                                                                                0 2 6 5 1 1

                                                                                                2 3 4 7f f a b

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                1 2

                                                                                                0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                2 3 4 7 2 4a f b f

                                                                                                0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                2 3 4 7

                                                                                                Arithmetic Operations in Image Processing

                                                                                                Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                The two random variables are uncorrelated when their covariance is 0

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                used in image enhancement)

                                                                                                1

                                                                                                1( ) ( )K

                                                                                                ii

                                                                                                g x y g x yK

                                                                                                If the noise satisfies the properties stated above we have

                                                                                                2 2( ) ( )

                                                                                                1( ) ( ) g x y x yE g x y f x yK

                                                                                                ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                the average image is

                                                                                                ( ) ( )1

                                                                                                g x y x yK

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                averaging process increases

                                                                                                An important application of image averaging is in the field of astronomy where imaging

                                                                                                under very low light levels frequently causes sensor noise to render single images

                                                                                                virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                images respectively

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                100 noisy images

                                                                                                a b c d e f

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                A frequent application of image subtraction is in the enhancement of differences between

                                                                                                images

                                                                                                (a) (b) (c)

                                                                                                Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                significant bit of each pixel (c) the difference between the two images

                                                                                                Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                difference between images (a) and (b)

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                images after injection of the contrast medium

                                                                                                In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                propagates through the various arteries in the area being observed

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                An important application of image multiplication (and division) is shading correction

                                                                                                Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                When the shading function is known

                                                                                                ( )( )( )

                                                                                                g x yf x yh x y

                                                                                                h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                approximation to the shading function by imaging a target of constant intensity When the

                                                                                                sensor is not available often the shading pattern can be estimated from the image

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                (a) (b) (c)

                                                                                                Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                operations The process consists of multiplying a given image by a mask image that has

                                                                                                1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                (a) (b) (c)

                                                                                                Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                min( )mf f f

                                                                                                0 ( 255)max( )

                                                                                                ms

                                                                                                m

                                                                                                ff K K K

                                                                                                f

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Spatial Operations

                                                                                                - are performed directly on the pixels of a given image

                                                                                                There are three categories of spatial operations

                                                                                                single-pixel operations

                                                                                                neighborhood operations

                                                                                                geometric spatial transformations

                                                                                                Single-pixel operations

                                                                                                - change the values of intensity for the individual pixels ( )s T z

                                                                                                where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                corresponding pixel in the processed image

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Neighborhood operations

                                                                                                Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                intensity by computing the average value of the pixels in Sxy

                                                                                                ( )

                                                                                                1( ) ( )xyr c S

                                                                                                g x y f r cm n

                                                                                                The net effect is to perform local blurring in the original image This type of process is

                                                                                                used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                largest region of an image

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Geometric spatial transformations and image registration

                                                                                                - modify the spatial relationship between pixels in an image

                                                                                                - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                predefined set of rules

                                                                                                A geometric transformation consists of 2 basic operations

                                                                                                1 a spatial transformation of coordinates

                                                                                                2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                pixels

                                                                                                The coordinate system transformation ( ) [( )]x y T v w

                                                                                                (v w) ndash pixel coordinates in the original image

                                                                                                (x y) ndash pixel coordinates in the transformed image

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                Affine transform

                                                                                                11 1211 21 31

                                                                                                21 2212 22 33

                                                                                                31 32

                                                                                                0[ 1] [ 1] [ 1] 0

                                                                                                1

                                                                                                t tx t v t w t

                                                                                                x y v w T v w t ty t v t w t

                                                                                                t t

                                                                                                (AT)

                                                                                                This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                scaling rotation and translation matrices from Table 1

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Affine transformations

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                the process we have to assign intensity values to those locations This task is done by

                                                                                                using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                In practice we can use equation (AT) in two basic ways

                                                                                                forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                Problems

                                                                                                - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                the same location in the output image

                                                                                                - some output locations have no correspondent in the original image (no intensity

                                                                                                assignment)

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                pixel value

                                                                                                Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Image registration ndash align two or more images of the same scene

                                                                                                In image registration we have available the input and output images but the specific

                                                                                                transformation that produced the output image from the input is generally unknown

                                                                                                The problem is to estimate the transformation function and then use it to register the two

                                                                                                images

                                                                                                - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                - align images of a given location taken by the same instrument at different moments

                                                                                                of time (satellite images)

                                                                                                Solving the problem using tie points (also called control points) which are

                                                                                                corresponding points whose locations are known precisely in the input and reference

                                                                                                image

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                How to select tie points

                                                                                                - interactively selecting them

                                                                                                - use of algorithms that try to detect these points

                                                                                                - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                the imaging sensors These objects produce a set of known points (called reseau

                                                                                                marks) directly on all images captured by the system which can be used as guides

                                                                                                for establishing tie points

                                                                                                The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                a bilinear approximation is given by

                                                                                                1 2 3 4

                                                                                                5 6 7 8

                                                                                                x c v c w c v w cy c v c w c v w c

                                                                                                (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                subregions marked by 4 tie points we applied the transformation model described above

                                                                                                The number of tie points and the sophistication of the model required to solve the register

                                                                                                problem depend on the severity of the geometrical distortion

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Probabilistic Methods

                                                                                                zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                ( ) kk

                                                                                                np zM N

                                                                                                nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                pixels in the image) 1

                                                                                                0( ) 1

                                                                                                L

                                                                                                kk

                                                                                                p z

                                                                                                The mean (average) intensity of an image is given by 1

                                                                                                0( )

                                                                                                L

                                                                                                k kk

                                                                                                m z p z

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                The variance of the intensities is 1

                                                                                                2 2

                                                                                                0( ) ( )

                                                                                                L

                                                                                                k kk

                                                                                                z m p z

                                                                                                The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                ( ) is used

                                                                                                The n-th moment of a random variable z about the mean is defined as 1

                                                                                                0( ) ( ) ( )

                                                                                                Ln

                                                                                                n k kk

                                                                                                z z m p z

                                                                                                ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                3( ) 0z the intensities are biased to values higher than the mean

                                                                                                ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                mean

                                                                                                Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Intensity Transformations and Spatial Filtering

                                                                                                ( ) ( )g x y T f x y

                                                                                                f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                neighborhood of (x y)

                                                                                                - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                and much smaller in size than the image

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                called spatial filter (spatial mask kernel template or window)

                                                                                                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                ( )s T r

                                                                                                s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                is called contrast stretching

                                                                                                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                thresholding function

                                                                                                Some Basic Intensity Transformation Functions

                                                                                                Image Negatives

                                                                                                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                - equivalent of a photographic negative

                                                                                                - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                image

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Original Negative image

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                Some basic intensity transformation functions

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                while compressing the higher-level values The opposite is true for the inverse log

                                                                                                transformation The log functions compress the dynamic range of images with large

                                                                                                variations in pixel values

                                                                                                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Power-Law (Gamma) Transformations

                                                                                                - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                Plots of gamma transformation for different values of γ (c=1)

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                of output values with the opposite being true for higher values of input values The

                                                                                                curves with 1 have the opposite effect of those generated with values of 1

                                                                                                1c - identity transformation

                                                                                                A variety of devices used for image capture printing and display respond according to a

                                                                                                power law The process used to correct these power-law response phenomena is called

                                                                                                gamma correction

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Piecewise-Linear Transformations Functions

                                                                                                Contrast stretching

                                                                                                - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                intensity range of the recording tool or display device

                                                                                                a b c d Fig5

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                11

                                                                                                1

                                                                                                2 1 1 21 2

                                                                                                2 1 2 1

                                                                                                22

                                                                                                2

                                                                                                [0 ]

                                                                                                ( ) ( )( ) [ ]( ) ( )

                                                                                                ( 1 ) [ 1]( 1 )

                                                                                                s r r rrs r r s r rT r r r r

                                                                                                r r r rs L r r r L

                                                                                                L r

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                1 1 2 2r s r s identity transformation (no change)

                                                                                                1 2 1 2 0 1r r s s L thresholding function

                                                                                                Figure 5(b) shows an 8-bit image with low contrast

                                                                                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                from their original range to the full range [0 L-1]

                                                                                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                2 2 1r s m L where m is the mean gray level in the image

                                                                                                The original image on which these results are based is a scanning electron microscope

                                                                                                image of pollen magnified approximately 700 times

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Intensity-level slicing

                                                                                                - highlighting a specific range of intensities in an image

                                                                                                There are two approaches for intensity-level slicing

                                                                                                1 display in one value (white for example) all the values in the range of interest and in

                                                                                                another (say black) all other intensities (Figure 311 (a))

                                                                                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                intensities in the image (Figure 311 (b))

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                Highlights range [A B] and preserves all other intensities

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                blockageshellip)

                                                                                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Bit-plane slicing

                                                                                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                This technique highlights the contribution made to the whole image appearances by each

                                                                                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                Digital Image Processing

                                                                                                Week 1

                                                                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                • DIP 1 2017
                                                                                                • DIP 02 (2017)

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  Compression

                                                                                                  reducing the storage required to save an image or the bandwidth required to transmit it

                                                                                                  Morphological processing

                                                                                                  bull tools for extracting image components that are useful in the representation and description of shape

                                                                                                  bull a transition from processes that output images to processes that outputimage attributes

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  Segmentation

                                                                                                  bull partitioning an image into its constituents parts or objects

                                                                                                  bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                                                  bull the more accurate the segmentation the more likley recognition is to succeed

                                                                                                  Representation and description (almost always follows segmentation)

                                                                                                  bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                                                  bull converting the data produced by segmentation to a form suitable for computer processing

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                                                  bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                                                  bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                                                  Object recognition

                                                                                                  the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                                                  Knowledge database

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  Simplified diagramof a cross sectionof the human eye

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                                  The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                                  Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                                  The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                                  The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                                  Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                                  Fovea = the place where the image of the object of interest falls on

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                                  Blind spot region without receptors

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  Image formation in the eye

                                                                                                  Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                                  Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                                  distance between lens and retina along visual axix = 17 mm

                                                                                                  range of focal length = 14 mm to 17 mm

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  Optical illusions

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                  gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                  quantities that describe the quality of a chromatic light source radiance

                                                                                                  the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                  measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                  brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  the physical meaning is determined by the source of the image

                                                                                                  ( )f D f x y

                                                                                                  Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                  f(xy) ndash characterized by two components

                                                                                                  i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                  r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                  ( ) ( ) ( )

                                                                                                  0 ( ) 0 ( ) 1

                                                                                                  f x y i x y r x y

                                                                                                  i x y r x y

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                  i(xy) ndash determined by the illumination source

                                                                                                  r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                  is called gray (or intensity) scale

                                                                                                  In practice

                                                                                                  min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                  indoor values without additional illuminationmin max10 1000L L

                                                                                                  black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                  min maxL L

                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                  Week 1Week 1

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Image Sampling and Quantization

                                                                                                  - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                  scene

                                                                                                  converting a continuous image f to digital form

                                                                                                  - digitizing (x y) is called sampling

                                                                                                  - digitizing f(x y) is called quantization

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                  (00) (01) (0 1)(10) (11) (1 1)

                                                                                                  ( )

                                                                                                  ( 10) ( 11) ( 1 1)

                                                                                                  f f f Nf f f N

                                                                                                  f x y

                                                                                                  f M f M f M N

                                                                                                  image element pixel

                                                                                                  00 01 0 1

                                                                                                  10 11 1 1

                                                                                                  10 11 1 1

                                                                                                  ( ) ( )

                                                                                                  N

                                                                                                  i jN M N

                                                                                                  i j

                                                                                                  M M M N

                                                                                                  a a aa f x i y j f i ja a a

                                                                                                  Aa

                                                                                                  a a a

                                                                                                  f(00) ndash the upper left corner of the image

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  M N ge 0 L=2k

                                                                                                  [0 1]i j i ja a L

                                                                                                  Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Number of bits required to store a digitized image

                                                                                                  for 2 b M N k M N b N k

                                                                                                  When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                  Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                  Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                  (eg 100 line pairs per mm)

                                                                                                  Dots per unit distance are commonly used in printing and publishing

                                                                                                  In US the measure is expressed in dots per inch (dpi)

                                                                                                  (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                  Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                  The number of intensity levels (L) is determined by hardware considerations

                                                                                                  L=2k ndash most common k = 8

                                                                                                  Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                  150 dpi (lower left) 72 dpi (lower right)

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Reducing the number of gray levels 256 128 64 32

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Reducing the number of gray levels 16 8 4 2

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                  Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                  Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                  Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                  750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                  same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                  image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                  Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                  Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                  intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                  This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                  straight edges

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                  where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                  be written using the 4 nearest neighbors of point (x y)

                                                                                                  Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                  modest increase in computational effort

                                                                                                  Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                  nearest neighbors of the point 3 3

                                                                                                  0 0

                                                                                                  ( ) i ji j

                                                                                                  i jv x y c x y

                                                                                                  The coefficients cij are obtained solving a 16x16 linear system

                                                                                                  intensity levels of the 16 nearest neighbors of 3 3

                                                                                                  0 0

                                                                                                  ( )i ji j

                                                                                                  i jc x y x y

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                  bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                  programs such as Adobe Photoshop and Corel Photopaint

                                                                                                  Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                  the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                  then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                  neighbor interpolation was used (both for shrinking and zooming)

                                                                                                  Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                  interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                  from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Neighbors of a Pixel

                                                                                                  A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                  This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                  The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                  and are denoted ND(p)

                                                                                                  The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                  N8 (p)

                                                                                                  If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                  fall outside the image

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Adjacency Connectivity Regions Boundaries

                                                                                                  Denote by V the set of intensity levels used to define adjacency

                                                                                                  - in a binary image V 01 (V=0 V=1)

                                                                                                  - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                  We consider 3 types of adjacency

                                                                                                  (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                  (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                  (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                  m-adjacent if

                                                                                                  4( )q N p or

                                                                                                  ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                  Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                  ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  binary image

                                                                                                  0 1 1 0 1 1 0 1 1

                                                                                                  1 0 1 0 0 1 0 0 1 0

                                                                                                  0 0 1 0 0 1 0 0 1

                                                                                                  V

                                                                                                  The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                  8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                  m-adjacency

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                  is a sequence of distinct pixels with coordinates

                                                                                                  and are adjacent 0 0 1 1

                                                                                                  1 1

                                                                                                  ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                  n n

                                                                                                  i i i i

                                                                                                  x y x y x y x y s tx y x y i n

                                                                                                  The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                  Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                  Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                  in S if there exists a path between them consisting only of pixels from S

                                                                                                  S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                  Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                  Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                  that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                  8-adjacency are considered

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                  touches the image border

                                                                                                  the complement of 1

                                                                                                  ( )K

                                                                                                  cu k u u

                                                                                                  k

                                                                                                  R R R R

                                                                                                  We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                  background of the image

                                                                                                  The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                  points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                  region that have at least one background neighbor This definition is referred to as the

                                                                                                  inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                  border in the background

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Distance measures

                                                                                                  For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                  function or metric if

                                                                                                  (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                  (b) D(p q) = D(q p)

                                                                                                  (c) D(p z) le D(p q) + D(q z)

                                                                                                  The Euclidean distance between p and q is defined as 1

                                                                                                  2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                  The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                  centered at (x y)

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                  4( ) | | | |D p q x s y t

                                                                                                  The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                  4

                                                                                                  22 1 2

                                                                                                  2 2 1 0 1 22 1 2

                                                                                                  2

                                                                                                  D

                                                                                                  The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                  The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                  8( ) max| | | |D p q x s y t

                                                                                                  The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  8

                                                                                                  2 2 2 2 22 1 1 1 2

                                                                                                  2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                  D

                                                                                                  The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                  D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                  because these distances involve only the coordinates of the point

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Array versus Matrix Operations

                                                                                                  An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                  11 12 11 12

                                                                                                  21 22 21 22

                                                                                                  a a b ba a b b

                                                                                                  Array product

                                                                                                  11 12 11 12 11 11 12 12

                                                                                                  21 22 21 22 21 21 22 21

                                                                                                  a a b b a b a ba a b b a b a b

                                                                                                  Matrix product

                                                                                                  11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                  21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                  a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                  We assume array operations unless stated otherwise

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Linear versus Nonlinear Operations

                                                                                                  One of the most important classifications of image-processing methods is whether it is

                                                                                                  linear or nonlinear

                                                                                                  ( ) ( )H f x y g x y

                                                                                                  H is said to be a linear operator if

                                                                                                  images1 2 1 2

                                                                                                  1 2

                                                                                                  ( ) ( ) ( ) ( )

                                                                                                  H a f x y b f x y a H f x y b H f x y

                                                                                                  a b f f

                                                                                                  Example of nonlinear operator

                                                                                                  the maximum value of the pixels of image max ( )H f f x y f

                                                                                                  1 2

                                                                                                  0 2 6 5 1 1

                                                                                                  2 3 4 7f f a b

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  1 2

                                                                                                  0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                  2 3 4 7 2 4a f b f

                                                                                                  0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                  2 3 4 7

                                                                                                  Arithmetic Operations in Image Processing

                                                                                                  Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                  f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                  For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                  value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                  The two random variables are uncorrelated when their covariance is 0

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                  used in image enhancement)

                                                                                                  1

                                                                                                  1( ) ( )K

                                                                                                  ii

                                                                                                  g x y g x yK

                                                                                                  If the noise satisfies the properties stated above we have

                                                                                                  2 2( ) ( )

                                                                                                  1( ) ( ) g x y x yE g x y f x yK

                                                                                                  ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                  and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                  the average image is

                                                                                                  ( ) ( )1

                                                                                                  g x y x yK

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                  the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                  means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                  averaging process increases

                                                                                                  An important application of image averaging is in the field of astronomy where imaging

                                                                                                  under very low light levels frequently causes sensor noise to render single images

                                                                                                  virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                  was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                  64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                  images respectively

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                  100 noisy images

                                                                                                  a b c d e f

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  A frequent application of image subtraction is in the enhancement of differences between

                                                                                                  images

                                                                                                  (a) (b) (c)

                                                                                                  Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                  significant bit of each pixel (c) the difference between the two images

                                                                                                  Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                  Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                  images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                  difference between images (a) and (b)

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                  h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                  intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                  source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                  bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                  anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                  images after injection of the contrast medium

                                                                                                  In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                  Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                  propagates through the various arteries in the area being observed

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  An important application of image multiplication (and division) is shading correction

                                                                                                  Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                  f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                  When the shading function is known

                                                                                                  ( )( )( )

                                                                                                  g x yf x yh x y

                                                                                                  h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                  approximation to the shading function by imaging a target of constant intensity When the

                                                                                                  sensor is not available often the shading pattern can be estimated from the image

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  (a) (b) (c)

                                                                                                  Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                  operations The process consists of multiplying a given image by a mask image that has

                                                                                                  1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                  image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                  (a) (b) (c)

                                                                                                  Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                  min( )mf f f

                                                                                                  0 ( 255)max( )

                                                                                                  ms

                                                                                                  m

                                                                                                  ff K K K

                                                                                                  f

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Spatial Operations

                                                                                                  - are performed directly on the pixels of a given image

                                                                                                  There are three categories of spatial operations

                                                                                                  single-pixel operations

                                                                                                  neighborhood operations

                                                                                                  geometric spatial transformations

                                                                                                  Single-pixel operations

                                                                                                  - change the values of intensity for the individual pixels ( )s T z

                                                                                                  where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                  corresponding pixel in the processed image

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Neighborhood operations

                                                                                                  Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                  in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                  based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                  rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                  intensity by computing the average value of the pixels in Sxy

                                                                                                  ( )

                                                                                                  1( ) ( )xyr c S

                                                                                                  g x y f r cm n

                                                                                                  The net effect is to perform local blurring in the original image This type of process is

                                                                                                  used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                  largest region of an image

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Geometric spatial transformations and image registration

                                                                                                  - modify the spatial relationship between pixels in an image

                                                                                                  - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                  printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                  predefined set of rules

                                                                                                  A geometric transformation consists of 2 basic operations

                                                                                                  1 a spatial transformation of coordinates

                                                                                                  2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                  pixels

                                                                                                  The coordinate system transformation ( ) [( )]x y T v w

                                                                                                  (v w) ndash pixel coordinates in the original image

                                                                                                  (x y) ndash pixel coordinates in the transformed image

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                  Affine transform

                                                                                                  11 1211 21 31

                                                                                                  21 2212 22 33

                                                                                                  31 32

                                                                                                  0[ 1] [ 1] [ 1] 0

                                                                                                  1

                                                                                                  t tx t v t w t

                                                                                                  x y v w T v w t ty t v t w t

                                                                                                  t t

                                                                                                  (AT)

                                                                                                  This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                  on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                  result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                  scaling rotation and translation matrices from Table 1

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Affine transformations

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                  the process we have to assign intensity values to those locations This task is done by

                                                                                                  using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                  In practice we can use equation (AT) in two basic ways

                                                                                                  forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                  location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                  Problems

                                                                                                  - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                  the same location in the output image

                                                                                                  - some output locations have no correspondent in the original image (no intensity

                                                                                                  assignment)

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                  computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                  It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                  pixel value

                                                                                                  Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                  numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Image registration ndash align two or more images of the same scene

                                                                                                  In image registration we have available the input and output images but the specific

                                                                                                  transformation that produced the output image from the input is generally unknown

                                                                                                  The problem is to estimate the transformation function and then use it to register the two

                                                                                                  images

                                                                                                  - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                  same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                  - align images of a given location taken by the same instrument at different moments

                                                                                                  of time (satellite images)

                                                                                                  Solving the problem using tie points (also called control points) which are

                                                                                                  corresponding points whose locations are known precisely in the input and reference

                                                                                                  image

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  How to select tie points

                                                                                                  - interactively selecting them

                                                                                                  - use of algorithms that try to detect these points

                                                                                                  - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                  the imaging sensors These objects produce a set of known points (called reseau

                                                                                                  marks) directly on all images captured by the system which can be used as guides

                                                                                                  for establishing tie points

                                                                                                  The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                  of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                  a bilinear approximation is given by

                                                                                                  1 2 3 4

                                                                                                  5 6 7 8

                                                                                                  x c v c w c v w cy c v c w c v w c

                                                                                                  (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                  frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                  subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                  subregions marked by 4 tie points we applied the transformation model described above

                                                                                                  The number of tie points and the sophistication of the model required to solve the register

                                                                                                  problem depend on the severity of the geometrical distortion

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Probabilistic Methods

                                                                                                  zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                  p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                  ( ) kk

                                                                                                  np zM N

                                                                                                  nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                  pixels in the image) 1

                                                                                                  0( ) 1

                                                                                                  L

                                                                                                  kk

                                                                                                  p z

                                                                                                  The mean (average) intensity of an image is given by 1

                                                                                                  0( )

                                                                                                  L

                                                                                                  k kk

                                                                                                  m z p z

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  The variance of the intensities is 1

                                                                                                  2 2

                                                                                                  0( ) ( )

                                                                                                  L

                                                                                                  k kk

                                                                                                  z m p z

                                                                                                  The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                  measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                  ( ) is used

                                                                                                  The n-th moment of a random variable z about the mean is defined as 1

                                                                                                  0( ) ( ) ( )

                                                                                                  Ln

                                                                                                  n k kk

                                                                                                  z z m p z

                                                                                                  ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                  3( ) 0z the intensities are biased to values higher than the mean

                                                                                                  ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                  mean

                                                                                                  Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                  Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                  Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                  Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Intensity Transformations and Spatial Filtering

                                                                                                  ( ) ( )g x y T f x y

                                                                                                  f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                  neighborhood of (x y)

                                                                                                  - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                  and much smaller in size than the image

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                  called spatial filter (spatial mask kernel template or window)

                                                                                                  ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                  ( )s T r

                                                                                                  s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                  Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                  darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                  is called contrast stretching

                                                                                                  Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                  thresholding function

                                                                                                  Some Basic Intensity Transformation Functions

                                                                                                  Image Negatives

                                                                                                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                  - equivalent of a photographic negative

                                                                                                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                  image

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Original Negative image

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                  Some basic intensity transformation functions

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                  range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                  while compressing the higher-level values The opposite is true for the inverse log

                                                                                                  transformation The log functions compress the dynamic range of images with large

                                                                                                  variations in pixel values

                                                                                                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Power-Law (Gamma) Transformations

                                                                                                  - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                  Plots of gamma transformation for different values of γ (c=1)

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                  of output values with the opposite being true for higher values of input values The

                                                                                                  curves with 1 have the opposite effect of those generated with values of 1

                                                                                                  1c - identity transformation

                                                                                                  A variety of devices used for image capture printing and display respond according to a

                                                                                                  power law The process used to correct these power-law response phenomena is called

                                                                                                  gamma correction

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Piecewise-Linear Transformations Functions

                                                                                                  Contrast stretching

                                                                                                  - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                  intensity range of the recording tool or display device

                                                                                                  a b c d Fig5

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  11

                                                                                                  1

                                                                                                  2 1 1 21 2

                                                                                                  2 1 2 1

                                                                                                  22

                                                                                                  2

                                                                                                  [0 ]

                                                                                                  ( ) ( )( ) [ ]( ) ( )

                                                                                                  ( 1 ) [ 1]( 1 )

                                                                                                  s r r rrs r r s r rT r r r r

                                                                                                  r r r rs L r r r L

                                                                                                  L r

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  1 1 2 2r s r s identity transformation (no change)

                                                                                                  1 2 1 2 0 1r r s s L thresholding function

                                                                                                  Figure 5(b) shows an 8-bit image with low contrast

                                                                                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                  in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                  from their original range to the full range [0 L-1]

                                                                                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                  2 2 1r s m L where m is the mean gray level in the image

                                                                                                  The original image on which these results are based is a scanning electron microscope

                                                                                                  image of pollen magnified approximately 700 times

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Intensity-level slicing

                                                                                                  - highlighting a specific range of intensities in an image

                                                                                                  There are two approaches for intensity-level slicing

                                                                                                  1 display in one value (white for example) all the values in the range of interest and in

                                                                                                  another (say black) all other intensities (Figure 311 (a))

                                                                                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                  intensities in the image (Figure 311 (b))

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                  the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                  Highlights range [A B] and preserves all other intensities

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                  blockageshellip)

                                                                                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                  image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                  Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Bit-plane slicing

                                                                                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                  This technique highlights the contribution made to the whole image appearances by each

                                                                                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  Digital Image Processing

                                                                                                  Week 1

                                                                                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                  • DIP 1 2017
                                                                                                  • DIP 02 (2017)

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    Segmentation

                                                                                                    bull partitioning an image into its constituents parts or objects

                                                                                                    bull autonomous segmentation is one of the most difficult tasks of DIP

                                                                                                    bull the more accurate the segmentation the more likley recognition is to succeed

                                                                                                    Representation and description (almost always follows segmentation)

                                                                                                    bull segmentation produces either the boundary of a region or all the poits in the region itself

                                                                                                    bull converting the data produced by segmentation to a form suitable for computer processing

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                                                                                                    Week 1Week 1

                                                                                                    bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                                                    bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                                                    bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                                                    Object recognition

                                                                                                    the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                                                    Knowledge database

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    Simplified diagramof a cross sectionof the human eye

                                                                                                    Digital Image ProcessingDigital Image Processing

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                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                                    The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                                    Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                                    The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                                    The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                                    Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                                    Fovea = the place where the image of the object of interest falls on

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                                    Blind spot region without receptors

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    Image formation in the eye

                                                                                                    Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                                    Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                                    distance between lens and retina along visual axix = 17 mm

                                                                                                    range of focal length = 14 mm to 17 mm

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    Optical illusions

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                    gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                    quantities that describe the quality of a chromatic light source radiance

                                                                                                    the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                    measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                    brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    the physical meaning is determined by the source of the image

                                                                                                    ( )f D f x y

                                                                                                    Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                    f(xy) ndash characterized by two components

                                                                                                    i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                    r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                    ( ) ( ) ( )

                                                                                                    0 ( ) 0 ( ) 1

                                                                                                    f x y i x y r x y

                                                                                                    i x y r x y

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                    i(xy) ndash determined by the illumination source

                                                                                                    r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                    is called gray (or intensity) scale

                                                                                                    In practice

                                                                                                    min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                    indoor values without additional illuminationmin max10 1000L L

                                                                                                    black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                    min maxL L

                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                    Week 1Week 1

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Image Sampling and Quantization

                                                                                                    - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                    scene

                                                                                                    converting a continuous image f to digital form

                                                                                                    - digitizing (x y) is called sampling

                                                                                                    - digitizing f(x y) is called quantization

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                    (00) (01) (0 1)(10) (11) (1 1)

                                                                                                    ( )

                                                                                                    ( 10) ( 11) ( 1 1)

                                                                                                    f f f Nf f f N

                                                                                                    f x y

                                                                                                    f M f M f M N

                                                                                                    image element pixel

                                                                                                    00 01 0 1

                                                                                                    10 11 1 1

                                                                                                    10 11 1 1

                                                                                                    ( ) ( )

                                                                                                    N

                                                                                                    i jN M N

                                                                                                    i j

                                                                                                    M M M N

                                                                                                    a a aa f x i y j f i ja a a

                                                                                                    Aa

                                                                                                    a a a

                                                                                                    f(00) ndash the upper left corner of the image

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    M N ge 0 L=2k

                                                                                                    [0 1]i j i ja a L

                                                                                                    Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Number of bits required to store a digitized image

                                                                                                    for 2 b M N k M N b N k

                                                                                                    When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                    Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                    Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                    (eg 100 line pairs per mm)

                                                                                                    Dots per unit distance are commonly used in printing and publishing

                                                                                                    In US the measure is expressed in dots per inch (dpi)

                                                                                                    (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                    Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                    The number of intensity levels (L) is determined by hardware considerations

                                                                                                    L=2k ndash most common k = 8

                                                                                                    Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                    150 dpi (lower left) 72 dpi (lower right)

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Reducing the number of gray levels 256 128 64 32

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Reducing the number of gray levels 16 8 4 2

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                    Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                    Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                    Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                    750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                    same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                    image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                    Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                    Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                    intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                    This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                    straight edges

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                    where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                    be written using the 4 nearest neighbors of point (x y)

                                                                                                    Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                    modest increase in computational effort

                                                                                                    Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                    nearest neighbors of the point 3 3

                                                                                                    0 0

                                                                                                    ( ) i ji j

                                                                                                    i jv x y c x y

                                                                                                    The coefficients cij are obtained solving a 16x16 linear system

                                                                                                    intensity levels of the 16 nearest neighbors of 3 3

                                                                                                    0 0

                                                                                                    ( )i ji j

                                                                                                    i jc x y x y

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                    bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                    programs such as Adobe Photoshop and Corel Photopaint

                                                                                                    Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                    the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                    then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                    neighbor interpolation was used (both for shrinking and zooming)

                                                                                                    Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                    interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                    from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Neighbors of a Pixel

                                                                                                    A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                    This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                    The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                    and are denoted ND(p)

                                                                                                    The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                    N8 (p)

                                                                                                    If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                    fall outside the image

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Adjacency Connectivity Regions Boundaries

                                                                                                    Denote by V the set of intensity levels used to define adjacency

                                                                                                    - in a binary image V 01 (V=0 V=1)

                                                                                                    - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                    We consider 3 types of adjacency

                                                                                                    (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                    (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                    (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                    m-adjacent if

                                                                                                    4( )q N p or

                                                                                                    ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                    Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                    ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    binary image

                                                                                                    0 1 1 0 1 1 0 1 1

                                                                                                    1 0 1 0 0 1 0 0 1 0

                                                                                                    0 0 1 0 0 1 0 0 1

                                                                                                    V

                                                                                                    The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                    8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                    m-adjacency

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                    is a sequence of distinct pixels with coordinates

                                                                                                    and are adjacent 0 0 1 1

                                                                                                    1 1

                                                                                                    ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                    n n

                                                                                                    i i i i

                                                                                                    x y x y x y x y s tx y x y i n

                                                                                                    The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                    Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                    Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                    in S if there exists a path between them consisting only of pixels from S

                                                                                                    S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                    Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                    Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                    that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                    8-adjacency are considered

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                    touches the image border

                                                                                                    the complement of 1

                                                                                                    ( )K

                                                                                                    cu k u u

                                                                                                    k

                                                                                                    R R R R

                                                                                                    We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                    background of the image

                                                                                                    The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                    points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                    region that have at least one background neighbor This definition is referred to as the

                                                                                                    inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                    border in the background

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Distance measures

                                                                                                    For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                    function or metric if

                                                                                                    (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                    (b) D(p q) = D(q p)

                                                                                                    (c) D(p z) le D(p q) + D(q z)

                                                                                                    The Euclidean distance between p and q is defined as 1

                                                                                                    2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                    The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                    centered at (x y)

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                    4( ) | | | |D p q x s y t

                                                                                                    The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                    4

                                                                                                    22 1 2

                                                                                                    2 2 1 0 1 22 1 2

                                                                                                    2

                                                                                                    D

                                                                                                    The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                    The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                    8( ) max| | | |D p q x s y t

                                                                                                    The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    8

                                                                                                    2 2 2 2 22 1 1 1 2

                                                                                                    2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                    D

                                                                                                    The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                    D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                    because these distances involve only the coordinates of the point

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Array versus Matrix Operations

                                                                                                    An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                    11 12 11 12

                                                                                                    21 22 21 22

                                                                                                    a a b ba a b b

                                                                                                    Array product

                                                                                                    11 12 11 12 11 11 12 12

                                                                                                    21 22 21 22 21 21 22 21

                                                                                                    a a b b a b a ba a b b a b a b

                                                                                                    Matrix product

                                                                                                    11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                    21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                    a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                    We assume array operations unless stated otherwise

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Linear versus Nonlinear Operations

                                                                                                    One of the most important classifications of image-processing methods is whether it is

                                                                                                    linear or nonlinear

                                                                                                    ( ) ( )H f x y g x y

                                                                                                    H is said to be a linear operator if

                                                                                                    images1 2 1 2

                                                                                                    1 2

                                                                                                    ( ) ( ) ( ) ( )

                                                                                                    H a f x y b f x y a H f x y b H f x y

                                                                                                    a b f f

                                                                                                    Example of nonlinear operator

                                                                                                    the maximum value of the pixels of image max ( )H f f x y f

                                                                                                    1 2

                                                                                                    0 2 6 5 1 1

                                                                                                    2 3 4 7f f a b

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    1 2

                                                                                                    0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                    2 3 4 7 2 4a f b f

                                                                                                    0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                    2 3 4 7

                                                                                                    Arithmetic Operations in Image Processing

                                                                                                    Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                    f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                    For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                    value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                    The two random variables are uncorrelated when their covariance is 0

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                    used in image enhancement)

                                                                                                    1

                                                                                                    1( ) ( )K

                                                                                                    ii

                                                                                                    g x y g x yK

                                                                                                    If the noise satisfies the properties stated above we have

                                                                                                    2 2( ) ( )

                                                                                                    1( ) ( ) g x y x yE g x y f x yK

                                                                                                    ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                    and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                    the average image is

                                                                                                    ( ) ( )1

                                                                                                    g x y x yK

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                    the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                    means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                    averaging process increases

                                                                                                    An important application of image averaging is in the field of astronomy where imaging

                                                                                                    under very low light levels frequently causes sensor noise to render single images

                                                                                                    virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                    was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                    64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                    images respectively

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                    100 noisy images

                                                                                                    a b c d e f

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    A frequent application of image subtraction is in the enhancement of differences between

                                                                                                    images

                                                                                                    (a) (b) (c)

                                                                                                    Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                    significant bit of each pixel (c) the difference between the two images

                                                                                                    Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                    Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                    images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                    difference between images (a) and (b)

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                    h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                    intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                    source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                    bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                    anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                    images after injection of the contrast medium

                                                                                                    In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                    Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                    propagates through the various arteries in the area being observed

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    An important application of image multiplication (and division) is shading correction

                                                                                                    Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                    f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                    When the shading function is known

                                                                                                    ( )( )( )

                                                                                                    g x yf x yh x y

                                                                                                    h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                    approximation to the shading function by imaging a target of constant intensity When the

                                                                                                    sensor is not available often the shading pattern can be estimated from the image

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    (a) (b) (c)

                                                                                                    Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                    operations The process consists of multiplying a given image by a mask image that has

                                                                                                    1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                    image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                    (a) (b) (c)

                                                                                                    Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                    min( )mf f f

                                                                                                    0 ( 255)max( )

                                                                                                    ms

                                                                                                    m

                                                                                                    ff K K K

                                                                                                    f

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Spatial Operations

                                                                                                    - are performed directly on the pixels of a given image

                                                                                                    There are three categories of spatial operations

                                                                                                    single-pixel operations

                                                                                                    neighborhood operations

                                                                                                    geometric spatial transformations

                                                                                                    Single-pixel operations

                                                                                                    - change the values of intensity for the individual pixels ( )s T z

                                                                                                    where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                    corresponding pixel in the processed image

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Neighborhood operations

                                                                                                    Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                    in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                    based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                    rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                    intensity by computing the average value of the pixels in Sxy

                                                                                                    ( )

                                                                                                    1( ) ( )xyr c S

                                                                                                    g x y f r cm n

                                                                                                    The net effect is to perform local blurring in the original image This type of process is

                                                                                                    used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                    largest region of an image

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Geometric spatial transformations and image registration

                                                                                                    - modify the spatial relationship between pixels in an image

                                                                                                    - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                    printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                    predefined set of rules

                                                                                                    A geometric transformation consists of 2 basic operations

                                                                                                    1 a spatial transformation of coordinates

                                                                                                    2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                    pixels

                                                                                                    The coordinate system transformation ( ) [( )]x y T v w

                                                                                                    (v w) ndash pixel coordinates in the original image

                                                                                                    (x y) ndash pixel coordinates in the transformed image

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                    Affine transform

                                                                                                    11 1211 21 31

                                                                                                    21 2212 22 33

                                                                                                    31 32

                                                                                                    0[ 1] [ 1] [ 1] 0

                                                                                                    1

                                                                                                    t tx t v t w t

                                                                                                    x y v w T v w t ty t v t w t

                                                                                                    t t

                                                                                                    (AT)

                                                                                                    This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                    on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                    result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                    scaling rotation and translation matrices from Table 1

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Affine transformations

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                    the process we have to assign intensity values to those locations This task is done by

                                                                                                    using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                    In practice we can use equation (AT) in two basic ways

                                                                                                    forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                    location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                    Problems

                                                                                                    - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                    the same location in the output image

                                                                                                    - some output locations have no correspondent in the original image (no intensity

                                                                                                    assignment)

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                    computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                    It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                    pixel value

                                                                                                    Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                    numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Image registration ndash align two or more images of the same scene

                                                                                                    In image registration we have available the input and output images but the specific

                                                                                                    transformation that produced the output image from the input is generally unknown

                                                                                                    The problem is to estimate the transformation function and then use it to register the two

                                                                                                    images

                                                                                                    - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                    same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                    - align images of a given location taken by the same instrument at different moments

                                                                                                    of time (satellite images)

                                                                                                    Solving the problem using tie points (also called control points) which are

                                                                                                    corresponding points whose locations are known precisely in the input and reference

                                                                                                    image

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    How to select tie points

                                                                                                    - interactively selecting them

                                                                                                    - use of algorithms that try to detect these points

                                                                                                    - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                    the imaging sensors These objects produce a set of known points (called reseau

                                                                                                    marks) directly on all images captured by the system which can be used as guides

                                                                                                    for establishing tie points

                                                                                                    The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                    of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                    a bilinear approximation is given by

                                                                                                    1 2 3 4

                                                                                                    5 6 7 8

                                                                                                    x c v c w c v w cy c v c w c v w c

                                                                                                    (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                    frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                    subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                    subregions marked by 4 tie points we applied the transformation model described above

                                                                                                    The number of tie points and the sophistication of the model required to solve the register

                                                                                                    problem depend on the severity of the geometrical distortion

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Probabilistic Methods

                                                                                                    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                    p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                    ( ) kk

                                                                                                    np zM N

                                                                                                    nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                    pixels in the image) 1

                                                                                                    0( ) 1

                                                                                                    L

                                                                                                    kk

                                                                                                    p z

                                                                                                    The mean (average) intensity of an image is given by 1

                                                                                                    0( )

                                                                                                    L

                                                                                                    k kk

                                                                                                    m z p z

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    The variance of the intensities is 1

                                                                                                    2 2

                                                                                                    0( ) ( )

                                                                                                    L

                                                                                                    k kk

                                                                                                    z m p z

                                                                                                    The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                    measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                    ( ) is used

                                                                                                    The n-th moment of a random variable z about the mean is defined as 1

                                                                                                    0( ) ( ) ( )

                                                                                                    Ln

                                                                                                    n k kk

                                                                                                    z z m p z

                                                                                                    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                    3( ) 0z the intensities are biased to values higher than the mean

                                                                                                    ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                    mean

                                                                                                    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                    Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                    Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                    Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Intensity Transformations and Spatial Filtering

                                                                                                    ( ) ( )g x y T f x y

                                                                                                    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                    neighborhood of (x y)

                                                                                                    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                    and much smaller in size than the image

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                    called spatial filter (spatial mask kernel template or window)

                                                                                                    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                    ( )s T r

                                                                                                    s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                    Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                    darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                    is called contrast stretching

                                                                                                    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                    thresholding function

                                                                                                    Some Basic Intensity Transformation Functions

                                                                                                    Image Negatives

                                                                                                    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                    - equivalent of a photographic negative

                                                                                                    - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                    image

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Original Negative image

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                    Some basic intensity transformation functions

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                    range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                    while compressing the higher-level values The opposite is true for the inverse log

                                                                                                    transformation The log functions compress the dynamic range of images with large

                                                                                                    variations in pixel values

                                                                                                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Power-Law (Gamma) Transformations

                                                                                                    - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                    Plots of gamma transformation for different values of γ (c=1)

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                    of output values with the opposite being true for higher values of input values The

                                                                                                    curves with 1 have the opposite effect of those generated with values of 1

                                                                                                    1c - identity transformation

                                                                                                    A variety of devices used for image capture printing and display respond according to a

                                                                                                    power law The process used to correct these power-law response phenomena is called

                                                                                                    gamma correction

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    Piecewise-Linear Transformations Functions

                                                                                                    Contrast stretching

                                                                                                    - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                    intensity range of the recording tool or display device

                                                                                                    a b c d Fig5

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    11

                                                                                                    1

                                                                                                    2 1 1 21 2

                                                                                                    2 1 2 1

                                                                                                    22

                                                                                                    2

                                                                                                    [0 ]

                                                                                                    ( ) ( )( ) [ ]( ) ( )

                                                                                                    ( 1 ) [ 1]( 1 )

                                                                                                    s r r rrs r r s r rT r r r r

                                                                                                    r r r rs L r r r L

                                                                                                    L r

                                                                                                    Digital Image Processing

                                                                                                    Week 1

                                                                                                    1 1 2 2r s r s identity transformation (no change)

                                                                                                    1 2 1 2 0 1r r s s L thresholding function

                                                                                                    Figure 5(b) shows an 8-bit image with low contrast

                                                                                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                    in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                    from their original range to the full range [0 L-1]

                                                                                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                    2 2 1r s m L where m is the mean gray level in the image

                                                                                                    The original image on which these results are based is a scanning electron microscope

                                                                                                    image of pollen magnified approximately 700 times

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                                                                                                    Intensity-level slicing

                                                                                                    - highlighting a specific range of intensities in an image

                                                                                                    There are two approaches for intensity-level slicing

                                                                                                    1 display in one value (white for example) all the values in the range of interest and in

                                                                                                    another (say black) all other intensities (Figure 311 (a))

                                                                                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                    intensities in the image (Figure 311 (b))

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                                                                                                    Week 1

                                                                                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                    the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                    Highlights range [A B] and preserves all other intensities

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                                                                                                    Week 1

                                                                                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                    blockageshellip)

                                                                                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                    image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                    Fig 6 - Aortic angiogram and intensity sliced versions

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                                                                                                    Week 1

                                                                                                    Bit-plane slicing

                                                                                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                    This technique highlights the contribution made to the whole image appearances by each

                                                                                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

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                                                                                                    Week 1

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                                                                                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                    • DIP 1 2017
                                                                                                    • DIP 02 (2017)

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                                                                                                      bull boundary representation the focus is on external shape characteristics such as corners or inflections

                                                                                                      bull complete region the focus is on internal properties such as texture or skeletal shape

                                                                                                      bull description is also called feature extraction ndash extracting attributes that result in some quantitative information of interest or are basic for differentiating one class of objects from another

                                                                                                      Object recognition

                                                                                                      the process of assigning a label (eg bdquovehiclerdquo) to an object based on itsdescriptors

                                                                                                      Knowledge database

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                                                                                                      Simplified diagramof a cross sectionof the human eye

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                                                                                                      Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                                      The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                                      Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                                      The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

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                                                                                                      The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                                      The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                                      Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                                      Fovea = the place where the image of the object of interest falls on

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                                                                                                      Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                                      Blind spot region without receptors

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                                                                                                      Image formation in the eye

                                                                                                      Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                                      Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                                      distance between lens and retina along visual axix = 17 mm

                                                                                                      range of focal length = 14 mm to 17 mm

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                                                                                                      Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

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                                                                                                      All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

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                                                                                                      Optical illusions

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                                                                                                      ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

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                                                                                                      Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                      gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                      quantities that describe the quality of a chromatic light source radiance

                                                                                                      the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                      measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

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                                                                                                      For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                      brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

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                                                                                                      the physical meaning is determined by the source of the image

                                                                                                      ( )f D f x y

                                                                                                      Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                      f(xy) ndash characterized by two components

                                                                                                      i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                      r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                      ( ) ( ) ( )

                                                                                                      0 ( ) 0 ( ) 1

                                                                                                      f x y i x y r x y

                                                                                                      i x y r x y

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                                                                                                      r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                      i(xy) ndash determined by the illumination source

                                                                                                      r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                      is called gray (or intensity) scale

                                                                                                      In practice

                                                                                                      min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                      indoor values without additional illuminationmin max10 1000L L

                                                                                                      black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                      min maxL L

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                                                                                                      Image Sampling and Quantization

                                                                                                      - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                      scene

                                                                                                      converting a continuous image f to digital form

                                                                                                      - digitizing (x y) is called sampling

                                                                                                      - digitizing f(x y) is called quantization

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                                                                                                      Continuous image projected onto a sensor array Result of image sampling and quantization

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                                                                                                      Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                      (00) (01) (0 1)(10) (11) (1 1)

                                                                                                      ( )

                                                                                                      ( 10) ( 11) ( 1 1)

                                                                                                      f f f Nf f f N

                                                                                                      f x y

                                                                                                      f M f M f M N

                                                                                                      image element pixel

                                                                                                      00 01 0 1

                                                                                                      10 11 1 1

                                                                                                      10 11 1 1

                                                                                                      ( ) ( )

                                                                                                      N

                                                                                                      i jN M N

                                                                                                      i j

                                                                                                      M M M N

                                                                                                      a a aa f x i y j f i ja a a

                                                                                                      Aa

                                                                                                      a a a

                                                                                                      f(00) ndash the upper left corner of the image

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                                                                                                      M N ge 0 L=2k

                                                                                                      [0 1]i j i ja a L

                                                                                                      Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

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                                                                                                      Number of bits required to store a digitized image

                                                                                                      for 2 b M N k M N b N k

                                                                                                      When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

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                                                                                                      Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                      Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                      Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                      (eg 100 line pairs per mm)

                                                                                                      Dots per unit distance are commonly used in printing and publishing

                                                                                                      In US the measure is expressed in dots per inch (dpi)

                                                                                                      (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                      Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                      The number of intensity levels (L) is determined by hardware considerations

                                                                                                      L=2k ndash most common k = 8

                                                                                                      Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                      Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                      150 dpi (lower left) 72 dpi (lower right)

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                                                                                                      Reducing the number of gray levels 256 128 64 32

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                                                                                                      Reducing the number of gray levels 16 8 4 2

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                                                                                                      Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                      Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                      Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                      Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                      750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                      same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                      image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                      Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                      Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                      intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                      This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                      straight edges

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                                                                                                      Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                      where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                      be written using the 4 nearest neighbors of point (x y)

                                                                                                      Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                      modest increase in computational effort

                                                                                                      Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                      nearest neighbors of the point 3 3

                                                                                                      0 0

                                                                                                      ( ) i ji j

                                                                                                      i jv x y c x y

                                                                                                      The coefficients cij are obtained solving a 16x16 linear system

                                                                                                      intensity levels of the 16 nearest neighbors of 3 3

                                                                                                      0 0

                                                                                                      ( )i ji j

                                                                                                      i jc x y x y

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                                                                                                      Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                      bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                      programs such as Adobe Photoshop and Corel Photopaint

                                                                                                      Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                      the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                      then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                      neighbor interpolation was used (both for shrinking and zooming)

                                                                                                      Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                      interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                      from 1250 dpi to 150 dpi (instead of 72 dpi)

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                                                                                                      Week 1

                                                                                                      Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

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                                                                                                      Neighbors of a Pixel

                                                                                                      A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                      This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                      The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                      and are denoted ND(p)

                                                                                                      The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                      N8 (p)

                                                                                                      If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                      fall outside the image

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                                                                                                      Adjacency Connectivity Regions Boundaries

                                                                                                      Denote by V the set of intensity levels used to define adjacency

                                                                                                      - in a binary image V 01 (V=0 V=1)

                                                                                                      - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                      We consider 3 types of adjacency

                                                                                                      (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                      (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                      (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                      m-adjacent if

                                                                                                      4( )q N p or

                                                                                                      ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                      Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                      ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      binary image

                                                                                                      0 1 1 0 1 1 0 1 1

                                                                                                      1 0 1 0 0 1 0 0 1 0

                                                                                                      0 0 1 0 0 1 0 0 1

                                                                                                      V

                                                                                                      The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                      8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                      m-adjacency

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                      is a sequence of distinct pixels with coordinates

                                                                                                      and are adjacent 0 0 1 1

                                                                                                      1 1

                                                                                                      ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                      n n

                                                                                                      i i i i

                                                                                                      x y x y x y x y s tx y x y i n

                                                                                                      The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                      Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                      Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                      in S if there exists a path between them consisting only of pixels from S

                                                                                                      S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                      Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                      Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                      that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                      8-adjacency are considered

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                      touches the image border

                                                                                                      the complement of 1

                                                                                                      ( )K

                                                                                                      cu k u u

                                                                                                      k

                                                                                                      R R R R

                                                                                                      We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                      background of the image

                                                                                                      The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                      points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                      region that have at least one background neighbor This definition is referred to as the

                                                                                                      inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                      border in the background

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Distance measures

                                                                                                      For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                      function or metric if

                                                                                                      (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                      (b) D(p q) = D(q p)

                                                                                                      (c) D(p z) le D(p q) + D(q z)

                                                                                                      The Euclidean distance between p and q is defined as 1

                                                                                                      2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                      The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                      centered at (x y)

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                      4( ) | | | |D p q x s y t

                                                                                                      The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                      4

                                                                                                      22 1 2

                                                                                                      2 2 1 0 1 22 1 2

                                                                                                      2

                                                                                                      D

                                                                                                      The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                      The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                      8( ) max| | | |D p q x s y t

                                                                                                      The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      8

                                                                                                      2 2 2 2 22 1 1 1 2

                                                                                                      2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                      D

                                                                                                      The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                      D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                      because these distances involve only the coordinates of the point

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Array versus Matrix Operations

                                                                                                      An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                      11 12 11 12

                                                                                                      21 22 21 22

                                                                                                      a a b ba a b b

                                                                                                      Array product

                                                                                                      11 12 11 12 11 11 12 12

                                                                                                      21 22 21 22 21 21 22 21

                                                                                                      a a b b a b a ba a b b a b a b

                                                                                                      Matrix product

                                                                                                      11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                      21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                      a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                      We assume array operations unless stated otherwise

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Linear versus Nonlinear Operations

                                                                                                      One of the most important classifications of image-processing methods is whether it is

                                                                                                      linear or nonlinear

                                                                                                      ( ) ( )H f x y g x y

                                                                                                      H is said to be a linear operator if

                                                                                                      images1 2 1 2

                                                                                                      1 2

                                                                                                      ( ) ( ) ( ) ( )

                                                                                                      H a f x y b f x y a H f x y b H f x y

                                                                                                      a b f f

                                                                                                      Example of nonlinear operator

                                                                                                      the maximum value of the pixels of image max ( )H f f x y f

                                                                                                      1 2

                                                                                                      0 2 6 5 1 1

                                                                                                      2 3 4 7f f a b

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      1 2

                                                                                                      0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                      2 3 4 7 2 4a f b f

                                                                                                      0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                      2 3 4 7

                                                                                                      Arithmetic Operations in Image Processing

                                                                                                      Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                      f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                      For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                      value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                      The two random variables are uncorrelated when their covariance is 0

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                      used in image enhancement)

                                                                                                      1

                                                                                                      1( ) ( )K

                                                                                                      ii

                                                                                                      g x y g x yK

                                                                                                      If the noise satisfies the properties stated above we have

                                                                                                      2 2( ) ( )

                                                                                                      1( ) ( ) g x y x yE g x y f x yK

                                                                                                      ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                      and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                      the average image is

                                                                                                      ( ) ( )1

                                                                                                      g x y x yK

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                      the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                      means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                      averaging process increases

                                                                                                      An important application of image averaging is in the field of astronomy where imaging

                                                                                                      under very low light levels frequently causes sensor noise to render single images

                                                                                                      virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                      was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                      64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                      images respectively

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                      100 noisy images

                                                                                                      a b c d e f

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      A frequent application of image subtraction is in the enhancement of differences between

                                                                                                      images

                                                                                                      (a) (b) (c)

                                                                                                      Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                      significant bit of each pixel (c) the difference between the two images

                                                                                                      Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                      Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                      images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                      difference between images (a) and (b)

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                      h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                      intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                      source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                      bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                      anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                      images after injection of the contrast medium

                                                                                                      In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                      Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                      propagates through the various arteries in the area being observed

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      An important application of image multiplication (and division) is shading correction

                                                                                                      Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                      f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                      When the shading function is known

                                                                                                      ( )( )( )

                                                                                                      g x yf x yh x y

                                                                                                      h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                      approximation to the shading function by imaging a target of constant intensity When the

                                                                                                      sensor is not available often the shading pattern can be estimated from the image

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      (a) (b) (c)

                                                                                                      Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                      operations The process consists of multiplying a given image by a mask image that has

                                                                                                      1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                      image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                      (a) (b) (c)

                                                                                                      Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                      min( )mf f f

                                                                                                      0 ( 255)max( )

                                                                                                      ms

                                                                                                      m

                                                                                                      ff K K K

                                                                                                      f

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Spatial Operations

                                                                                                      - are performed directly on the pixels of a given image

                                                                                                      There are three categories of spatial operations

                                                                                                      single-pixel operations

                                                                                                      neighborhood operations

                                                                                                      geometric spatial transformations

                                                                                                      Single-pixel operations

                                                                                                      - change the values of intensity for the individual pixels ( )s T z

                                                                                                      where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                      corresponding pixel in the processed image

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Neighborhood operations

                                                                                                      Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                      in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                      based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                      rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                      intensity by computing the average value of the pixels in Sxy

                                                                                                      ( )

                                                                                                      1( ) ( )xyr c S

                                                                                                      g x y f r cm n

                                                                                                      The net effect is to perform local blurring in the original image This type of process is

                                                                                                      used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                      largest region of an image

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Geometric spatial transformations and image registration

                                                                                                      - modify the spatial relationship between pixels in an image

                                                                                                      - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                      printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                      predefined set of rules

                                                                                                      A geometric transformation consists of 2 basic operations

                                                                                                      1 a spatial transformation of coordinates

                                                                                                      2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                      pixels

                                                                                                      The coordinate system transformation ( ) [( )]x y T v w

                                                                                                      (v w) ndash pixel coordinates in the original image

                                                                                                      (x y) ndash pixel coordinates in the transformed image

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                      Affine transform

                                                                                                      11 1211 21 31

                                                                                                      21 2212 22 33

                                                                                                      31 32

                                                                                                      0[ 1] [ 1] [ 1] 0

                                                                                                      1

                                                                                                      t tx t v t w t

                                                                                                      x y v w T v w t ty t v t w t

                                                                                                      t t

                                                                                                      (AT)

                                                                                                      This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                      on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                      result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                      scaling rotation and translation matrices from Table 1

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Affine transformations

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                      the process we have to assign intensity values to those locations This task is done by

                                                                                                      using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                      In practice we can use equation (AT) in two basic ways

                                                                                                      forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                      location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                      Problems

                                                                                                      - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                      the same location in the output image

                                                                                                      - some output locations have no correspondent in the original image (no intensity

                                                                                                      assignment)

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                      computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                      It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                      pixel value

                                                                                                      Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                      numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Image registration ndash align two or more images of the same scene

                                                                                                      In image registration we have available the input and output images but the specific

                                                                                                      transformation that produced the output image from the input is generally unknown

                                                                                                      The problem is to estimate the transformation function and then use it to register the two

                                                                                                      images

                                                                                                      - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                      same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                      - align images of a given location taken by the same instrument at different moments

                                                                                                      of time (satellite images)

                                                                                                      Solving the problem using tie points (also called control points) which are

                                                                                                      corresponding points whose locations are known precisely in the input and reference

                                                                                                      image

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      How to select tie points

                                                                                                      - interactively selecting them

                                                                                                      - use of algorithms that try to detect these points

                                                                                                      - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                      the imaging sensors These objects produce a set of known points (called reseau

                                                                                                      marks) directly on all images captured by the system which can be used as guides

                                                                                                      for establishing tie points

                                                                                                      The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                      of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                      a bilinear approximation is given by

                                                                                                      1 2 3 4

                                                                                                      5 6 7 8

                                                                                                      x c v c w c v w cy c v c w c v w c

                                                                                                      (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                      frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                      subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                      subregions marked by 4 tie points we applied the transformation model described above

                                                                                                      The number of tie points and the sophistication of the model required to solve the register

                                                                                                      problem depend on the severity of the geometrical distortion

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Probabilistic Methods

                                                                                                      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                      p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                      ( ) kk

                                                                                                      np zM N

                                                                                                      nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                      pixels in the image) 1

                                                                                                      0( ) 1

                                                                                                      L

                                                                                                      kk

                                                                                                      p z

                                                                                                      The mean (average) intensity of an image is given by 1

                                                                                                      0( )

                                                                                                      L

                                                                                                      k kk

                                                                                                      m z p z

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      The variance of the intensities is 1

                                                                                                      2 2

                                                                                                      0( ) ( )

                                                                                                      L

                                                                                                      k kk

                                                                                                      z m p z

                                                                                                      The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                      measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                      ( ) is used

                                                                                                      The n-th moment of a random variable z about the mean is defined as 1

                                                                                                      0( ) ( ) ( )

                                                                                                      Ln

                                                                                                      n k kk

                                                                                                      z z m p z

                                                                                                      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                      3( ) 0z the intensities are biased to values higher than the mean

                                                                                                      ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                      mean

                                                                                                      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                      Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                      Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                      Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Intensity Transformations and Spatial Filtering

                                                                                                      ( ) ( )g x y T f x y

                                                                                                      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                      neighborhood of (x y)

                                                                                                      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                      and much smaller in size than the image

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                      called spatial filter (spatial mask kernel template or window)

                                                                                                      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                      ( )s T r

                                                                                                      s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                      Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                      darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                      is called contrast stretching

                                                                                                      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                      thresholding function

                                                                                                      Some Basic Intensity Transformation Functions

                                                                                                      Image Negatives

                                                                                                      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                      - equivalent of a photographic negative

                                                                                                      - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                      image

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Original Negative image

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                      Some basic intensity transformation functions

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                      range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                      while compressing the higher-level values The opposite is true for the inverse log

                                                                                                      transformation The log functions compress the dynamic range of images with large

                                                                                                      variations in pixel values

                                                                                                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Power-Law (Gamma) Transformations

                                                                                                      - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                      Plots of gamma transformation for different values of γ (c=1)

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                      of output values with the opposite being true for higher values of input values The

                                                                                                      curves with 1 have the opposite effect of those generated with values of 1

                                                                                                      1c - identity transformation

                                                                                                      A variety of devices used for image capture printing and display respond according to a

                                                                                                      power law The process used to correct these power-law response phenomena is called

                                                                                                      gamma correction

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Piecewise-Linear Transformations Functions

                                                                                                      Contrast stretching

                                                                                                      - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                      intensity range of the recording tool or display device

                                                                                                      a b c d Fig5

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      11

                                                                                                      1

                                                                                                      2 1 1 21 2

                                                                                                      2 1 2 1

                                                                                                      22

                                                                                                      2

                                                                                                      [0 ]

                                                                                                      ( ) ( )( ) [ ]( ) ( )

                                                                                                      ( 1 ) [ 1]( 1 )

                                                                                                      s r r rrs r r s r rT r r r r

                                                                                                      r r r rs L r r r L

                                                                                                      L r

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      1 1 2 2r s r s identity transformation (no change)

                                                                                                      1 2 1 2 0 1r r s s L thresholding function

                                                                                                      Figure 5(b) shows an 8-bit image with low contrast

                                                                                                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                      in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                      from their original range to the full range [0 L-1]

                                                                                                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                      2 2 1r s m L where m is the mean gray level in the image

                                                                                                      The original image on which these results are based is a scanning electron microscope

                                                                                                      image of pollen magnified approximately 700 times

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Intensity-level slicing

                                                                                                      - highlighting a specific range of intensities in an image

                                                                                                      There are two approaches for intensity-level slicing

                                                                                                      1 display in one value (white for example) all the values in the range of interest and in

                                                                                                      another (say black) all other intensities (Figure 311 (a))

                                                                                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                      intensities in the image (Figure 311 (b))

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                      the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                      Highlights range [A B] and preserves all other intensities

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                      blockageshellip)

                                                                                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                      image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                      Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Bit-plane slicing

                                                                                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                      This technique highlights the contribution made to the whole image appearances by each

                                                                                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      Digital Image Processing

                                                                                                      Week 1

                                                                                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                      • DIP 1 2017
                                                                                                      • DIP 02 (2017)

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        Simplified diagramof a cross sectionof the human eye

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                                        The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                                        Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                                        The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                                        The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                                        Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                                        Fovea = the place where the image of the object of interest falls on

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                                        Blind spot region without receptors

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        Image formation in the eye

                                                                                                        Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                                        Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                                        distance between lens and retina along visual axix = 17 mm

                                                                                                        range of focal length = 14 mm to 17 mm

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        Optical illusions

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                        gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                        quantities that describe the quality of a chromatic light source radiance

                                                                                                        the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                        measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                        brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        the physical meaning is determined by the source of the image

                                                                                                        ( )f D f x y

                                                                                                        Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                        f(xy) ndash characterized by two components

                                                                                                        i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                        r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                        ( ) ( ) ( )

                                                                                                        0 ( ) 0 ( ) 1

                                                                                                        f x y i x y r x y

                                                                                                        i x y r x y

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                        i(xy) ndash determined by the illumination source

                                                                                                        r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                        is called gray (or intensity) scale

                                                                                                        In practice

                                                                                                        min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                        indoor values without additional illuminationmin max10 1000L L

                                                                                                        black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                        min maxL L

                                                                                                        Digital Image ProcessingDigital Image Processing

                                                                                                        Week 1Week 1

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Image Sampling and Quantization

                                                                                                        - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                        scene

                                                                                                        converting a continuous image f to digital form

                                                                                                        - digitizing (x y) is called sampling

                                                                                                        - digitizing f(x y) is called quantization

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                        (00) (01) (0 1)(10) (11) (1 1)

                                                                                                        ( )

                                                                                                        ( 10) ( 11) ( 1 1)

                                                                                                        f f f Nf f f N

                                                                                                        f x y

                                                                                                        f M f M f M N

                                                                                                        image element pixel

                                                                                                        00 01 0 1

                                                                                                        10 11 1 1

                                                                                                        10 11 1 1

                                                                                                        ( ) ( )

                                                                                                        N

                                                                                                        i jN M N

                                                                                                        i j

                                                                                                        M M M N

                                                                                                        a a aa f x i y j f i ja a a

                                                                                                        Aa

                                                                                                        a a a

                                                                                                        f(00) ndash the upper left corner of the image

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        M N ge 0 L=2k

                                                                                                        [0 1]i j i ja a L

                                                                                                        Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Number of bits required to store a digitized image

                                                                                                        for 2 b M N k M N b N k

                                                                                                        When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                        Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                        Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                        (eg 100 line pairs per mm)

                                                                                                        Dots per unit distance are commonly used in printing and publishing

                                                                                                        In US the measure is expressed in dots per inch (dpi)

                                                                                                        (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                        Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                        The number of intensity levels (L) is determined by hardware considerations

                                                                                                        L=2k ndash most common k = 8

                                                                                                        Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                        150 dpi (lower left) 72 dpi (lower right)

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Reducing the number of gray levels 256 128 64 32

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Reducing the number of gray levels 16 8 4 2

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                        Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                        Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                        Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                        750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                        same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                        image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                        Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                        Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                        intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                        This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                        straight edges

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                        where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                        be written using the 4 nearest neighbors of point (x y)

                                                                                                        Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                        modest increase in computational effort

                                                                                                        Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                        nearest neighbors of the point 3 3

                                                                                                        0 0

                                                                                                        ( ) i ji j

                                                                                                        i jv x y c x y

                                                                                                        The coefficients cij are obtained solving a 16x16 linear system

                                                                                                        intensity levels of the 16 nearest neighbors of 3 3

                                                                                                        0 0

                                                                                                        ( )i ji j

                                                                                                        i jc x y x y

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                        bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                        programs such as Adobe Photoshop and Corel Photopaint

                                                                                                        Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                        the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                        then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                        neighbor interpolation was used (both for shrinking and zooming)

                                                                                                        Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                        interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                        from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Neighbors of a Pixel

                                                                                                        A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                        This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                        The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                        and are denoted ND(p)

                                                                                                        The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                        N8 (p)

                                                                                                        If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                        fall outside the image

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Adjacency Connectivity Regions Boundaries

                                                                                                        Denote by V the set of intensity levels used to define adjacency

                                                                                                        - in a binary image V 01 (V=0 V=1)

                                                                                                        - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                        We consider 3 types of adjacency

                                                                                                        (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                        (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                        (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                        m-adjacent if

                                                                                                        4( )q N p or

                                                                                                        ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                        Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                        ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        binary image

                                                                                                        0 1 1 0 1 1 0 1 1

                                                                                                        1 0 1 0 0 1 0 0 1 0

                                                                                                        0 0 1 0 0 1 0 0 1

                                                                                                        V

                                                                                                        The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                        8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                        m-adjacency

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                        is a sequence of distinct pixels with coordinates

                                                                                                        and are adjacent 0 0 1 1

                                                                                                        1 1

                                                                                                        ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                        n n

                                                                                                        i i i i

                                                                                                        x y x y x y x y s tx y x y i n

                                                                                                        The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                        Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                        Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                        in S if there exists a path between them consisting only of pixels from S

                                                                                                        S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                        Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                        Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                        that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                        8-adjacency are considered

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                        touches the image border

                                                                                                        the complement of 1

                                                                                                        ( )K

                                                                                                        cu k u u

                                                                                                        k

                                                                                                        R R R R

                                                                                                        We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                        background of the image

                                                                                                        The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                        points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                        region that have at least one background neighbor This definition is referred to as the

                                                                                                        inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                        border in the background

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Distance measures

                                                                                                        For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                        function or metric if

                                                                                                        (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                        (b) D(p q) = D(q p)

                                                                                                        (c) D(p z) le D(p q) + D(q z)

                                                                                                        The Euclidean distance between p and q is defined as 1

                                                                                                        2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                        The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                        centered at (x y)

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                        4( ) | | | |D p q x s y t

                                                                                                        The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                        4

                                                                                                        22 1 2

                                                                                                        2 2 1 0 1 22 1 2

                                                                                                        2

                                                                                                        D

                                                                                                        The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                        The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                        8( ) max| | | |D p q x s y t

                                                                                                        The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        8

                                                                                                        2 2 2 2 22 1 1 1 2

                                                                                                        2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                        D

                                                                                                        The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                        D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                        because these distances involve only the coordinates of the point

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Array versus Matrix Operations

                                                                                                        An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                        11 12 11 12

                                                                                                        21 22 21 22

                                                                                                        a a b ba a b b

                                                                                                        Array product

                                                                                                        11 12 11 12 11 11 12 12

                                                                                                        21 22 21 22 21 21 22 21

                                                                                                        a a b b a b a ba a b b a b a b

                                                                                                        Matrix product

                                                                                                        11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                        21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                        a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                        We assume array operations unless stated otherwise

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Linear versus Nonlinear Operations

                                                                                                        One of the most important classifications of image-processing methods is whether it is

                                                                                                        linear or nonlinear

                                                                                                        ( ) ( )H f x y g x y

                                                                                                        H is said to be a linear operator if

                                                                                                        images1 2 1 2

                                                                                                        1 2

                                                                                                        ( ) ( ) ( ) ( )

                                                                                                        H a f x y b f x y a H f x y b H f x y

                                                                                                        a b f f

                                                                                                        Example of nonlinear operator

                                                                                                        the maximum value of the pixels of image max ( )H f f x y f

                                                                                                        1 2

                                                                                                        0 2 6 5 1 1

                                                                                                        2 3 4 7f f a b

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        1 2

                                                                                                        0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                        2 3 4 7 2 4a f b f

                                                                                                        0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                        2 3 4 7

                                                                                                        Arithmetic Operations in Image Processing

                                                                                                        Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                        f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                        For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                        value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                        The two random variables are uncorrelated when their covariance is 0

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                        used in image enhancement)

                                                                                                        1

                                                                                                        1( ) ( )K

                                                                                                        ii

                                                                                                        g x y g x yK

                                                                                                        If the noise satisfies the properties stated above we have

                                                                                                        2 2( ) ( )

                                                                                                        1( ) ( ) g x y x yE g x y f x yK

                                                                                                        ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                        and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                        the average image is

                                                                                                        ( ) ( )1

                                                                                                        g x y x yK

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                        the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                        means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                        averaging process increases

                                                                                                        An important application of image averaging is in the field of astronomy where imaging

                                                                                                        under very low light levels frequently causes sensor noise to render single images

                                                                                                        virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                        was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                        64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                        images respectively

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                        100 noisy images

                                                                                                        a b c d e f

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        A frequent application of image subtraction is in the enhancement of differences between

                                                                                                        images

                                                                                                        (a) (b) (c)

                                                                                                        Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                        significant bit of each pixel (c) the difference between the two images

                                                                                                        Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                        Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                        images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                        difference between images (a) and (b)

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                        h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                        intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                        source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                        bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                        anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                        images after injection of the contrast medium

                                                                                                        In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                        Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                        propagates through the various arteries in the area being observed

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        An important application of image multiplication (and division) is shading correction

                                                                                                        Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                        f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                        When the shading function is known

                                                                                                        ( )( )( )

                                                                                                        g x yf x yh x y

                                                                                                        h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                        approximation to the shading function by imaging a target of constant intensity When the

                                                                                                        sensor is not available often the shading pattern can be estimated from the image

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        (a) (b) (c)

                                                                                                        Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                        operations The process consists of multiplying a given image by a mask image that has

                                                                                                        1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                        image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                        (a) (b) (c)

                                                                                                        Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                        min( )mf f f

                                                                                                        0 ( 255)max( )

                                                                                                        ms

                                                                                                        m

                                                                                                        ff K K K

                                                                                                        f

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Spatial Operations

                                                                                                        - are performed directly on the pixels of a given image

                                                                                                        There are three categories of spatial operations

                                                                                                        single-pixel operations

                                                                                                        neighborhood operations

                                                                                                        geometric spatial transformations

                                                                                                        Single-pixel operations

                                                                                                        - change the values of intensity for the individual pixels ( )s T z

                                                                                                        where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                        corresponding pixel in the processed image

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Neighborhood operations

                                                                                                        Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                        in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                        based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                        rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                        intensity by computing the average value of the pixels in Sxy

                                                                                                        ( )

                                                                                                        1( ) ( )xyr c S

                                                                                                        g x y f r cm n

                                                                                                        The net effect is to perform local blurring in the original image This type of process is

                                                                                                        used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                        largest region of an image

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Geometric spatial transformations and image registration

                                                                                                        - modify the spatial relationship between pixels in an image

                                                                                                        - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                        printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                        predefined set of rules

                                                                                                        A geometric transformation consists of 2 basic operations

                                                                                                        1 a spatial transformation of coordinates

                                                                                                        2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                        pixels

                                                                                                        The coordinate system transformation ( ) [( )]x y T v w

                                                                                                        (v w) ndash pixel coordinates in the original image

                                                                                                        (x y) ndash pixel coordinates in the transformed image

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                        Affine transform

                                                                                                        11 1211 21 31

                                                                                                        21 2212 22 33

                                                                                                        31 32

                                                                                                        0[ 1] [ 1] [ 1] 0

                                                                                                        1

                                                                                                        t tx t v t w t

                                                                                                        x y v w T v w t ty t v t w t

                                                                                                        t t

                                                                                                        (AT)

                                                                                                        This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                        on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                        result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                        scaling rotation and translation matrices from Table 1

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Affine transformations

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                        the process we have to assign intensity values to those locations This task is done by

                                                                                                        using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                        In practice we can use equation (AT) in two basic ways

                                                                                                        forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                        location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                        Problems

                                                                                                        - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                        the same location in the output image

                                                                                                        - some output locations have no correspondent in the original image (no intensity

                                                                                                        assignment)

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                        computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                        It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                        pixel value

                                                                                                        Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                        numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Image registration ndash align two or more images of the same scene

                                                                                                        In image registration we have available the input and output images but the specific

                                                                                                        transformation that produced the output image from the input is generally unknown

                                                                                                        The problem is to estimate the transformation function and then use it to register the two

                                                                                                        images

                                                                                                        - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                        same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                        - align images of a given location taken by the same instrument at different moments

                                                                                                        of time (satellite images)

                                                                                                        Solving the problem using tie points (also called control points) which are

                                                                                                        corresponding points whose locations are known precisely in the input and reference

                                                                                                        image

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        How to select tie points

                                                                                                        - interactively selecting them

                                                                                                        - use of algorithms that try to detect these points

                                                                                                        - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                        the imaging sensors These objects produce a set of known points (called reseau

                                                                                                        marks) directly on all images captured by the system which can be used as guides

                                                                                                        for establishing tie points

                                                                                                        The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                        of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                        a bilinear approximation is given by

                                                                                                        1 2 3 4

                                                                                                        5 6 7 8

                                                                                                        x c v c w c v w cy c v c w c v w c

                                                                                                        (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                        frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                        subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                        subregions marked by 4 tie points we applied the transformation model described above

                                                                                                        The number of tie points and the sophistication of the model required to solve the register

                                                                                                        problem depend on the severity of the geometrical distortion

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Probabilistic Methods

                                                                                                        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                        p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                        ( ) kk

                                                                                                        np zM N

                                                                                                        nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                        pixels in the image) 1

                                                                                                        0( ) 1

                                                                                                        L

                                                                                                        kk

                                                                                                        p z

                                                                                                        The mean (average) intensity of an image is given by 1

                                                                                                        0( )

                                                                                                        L

                                                                                                        k kk

                                                                                                        m z p z

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        The variance of the intensities is 1

                                                                                                        2 2

                                                                                                        0( ) ( )

                                                                                                        L

                                                                                                        k kk

                                                                                                        z m p z

                                                                                                        The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                        measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                        ( ) is used

                                                                                                        The n-th moment of a random variable z about the mean is defined as 1

                                                                                                        0( ) ( ) ( )

                                                                                                        Ln

                                                                                                        n k kk

                                                                                                        z z m p z

                                                                                                        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                        3( ) 0z the intensities are biased to values higher than the mean

                                                                                                        ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                        mean

                                                                                                        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                        Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                        Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                        Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Intensity Transformations and Spatial Filtering

                                                                                                        ( ) ( )g x y T f x y

                                                                                                        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                        neighborhood of (x y)

                                                                                                        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                        and much smaller in size than the image

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                                                                                                        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                        called spatial filter (spatial mask kernel template or window)

                                                                                                        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                        ( )s T r

                                                                                                        s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                        Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                        darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                        is called contrast stretching

                                                                                                        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                        thresholding function

                                                                                                        Some Basic Intensity Transformation Functions

                                                                                                        Image Negatives

                                                                                                        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                        - equivalent of a photographic negative

                                                                                                        - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                        image

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Original Negative image

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                        Some basic intensity transformation functions

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                                                                                                        Week 1

                                                                                                        This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                        range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                        while compressing the higher-level values The opposite is true for the inverse log

                                                                                                        transformation The log functions compress the dynamic range of images with large

                                                                                                        variations in pixel values

                                                                                                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Power-Law (Gamma) Transformations

                                                                                                        - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                        Plots of gamma transformation for different values of γ (c=1)

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                        of output values with the opposite being true for higher values of input values The

                                                                                                        curves with 1 have the opposite effect of those generated with values of 1

                                                                                                        1c - identity transformation

                                                                                                        A variety of devices used for image capture printing and display respond according to a

                                                                                                        power law The process used to correct these power-law response phenomena is called

                                                                                                        gamma correction

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Piecewise-Linear Transformations Functions

                                                                                                        Contrast stretching

                                                                                                        - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                        intensity range of the recording tool or display device

                                                                                                        a b c d Fig5

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                                                                                                        Week 1

                                                                                                        11

                                                                                                        1

                                                                                                        2 1 1 21 2

                                                                                                        2 1 2 1

                                                                                                        22

                                                                                                        2

                                                                                                        [0 ]

                                                                                                        ( ) ( )( ) [ ]( ) ( )

                                                                                                        ( 1 ) [ 1]( 1 )

                                                                                                        s r r rrs r r s r rT r r r r

                                                                                                        r r r rs L r r r L

                                                                                                        L r

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        1 1 2 2r s r s identity transformation (no change)

                                                                                                        1 2 1 2 0 1r r s s L thresholding function

                                                                                                        Figure 5(b) shows an 8-bit image with low contrast

                                                                                                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                        in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                        from their original range to the full range [0 L-1]

                                                                                                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                        2 2 1r s m L where m is the mean gray level in the image

                                                                                                        The original image on which these results are based is a scanning electron microscope

                                                                                                        image of pollen magnified approximately 700 times

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        Intensity-level slicing

                                                                                                        - highlighting a specific range of intensities in an image

                                                                                                        There are two approaches for intensity-level slicing

                                                                                                        1 display in one value (white for example) all the values in the range of interest and in

                                                                                                        another (say black) all other intensities (Figure 311 (a))

                                                                                                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                        intensities in the image (Figure 311 (b))

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                                                                                                        Week 1

                                                                                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                        the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                        Highlights range [A B] and preserves all other intensities

                                                                                                        Digital Image Processing

                                                                                                        Week 1

                                                                                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                        blockageshellip)

                                                                                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                        image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                        Fig 6 - Aortic angiogram and intensity sliced versions

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                                                                                                        Bit-plane slicing

                                                                                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                        This technique highlights the contribution made to the whole image appearances by each

                                                                                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                        Digital Image Processing

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                                                                                                        Digital Image Processing

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                                                                                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                        • DIP 1 2017
                                                                                                        • DIP 02 (2017)

                                                                                                          Digital Image ProcessingDigital Image Processing

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                                                                                                          Digital Image ProcessingDigital Image Processing

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                                                                                                          Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                                          The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                                          Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                                          The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

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                                                                                                          The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                                          The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                                          Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                                          Fovea = the place where the image of the object of interest falls on

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                                                                                                          Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                                          Blind spot region without receptors

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                                                                                                          Image formation in the eye

                                                                                                          Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                                          Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                                          distance between lens and retina along visual axix = 17 mm

                                                                                                          range of focal length = 14 mm to 17 mm

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                                                                                                          Digital Image ProcessingDigital Image Processing

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                                                                                                          Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                                          Digital Image ProcessingDigital Image Processing

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                                                                                                          All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                                          Digital Image ProcessingDigital Image Processing

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                                                                                                          Optical illusions

                                                                                                          Digital Image ProcessingDigital Image Processing

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                                                                                                          ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                                          Digital Image ProcessingDigital Image Processing

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                                                                                                          Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                          gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                          quantities that describe the quality of a chromatic light source radiance

                                                                                                          the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                          measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

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                                                                                                          For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                          brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

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                                                                                                          the physical meaning is determined by the source of the image

                                                                                                          ( )f D f x y

                                                                                                          Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                          f(xy) ndash characterized by two components

                                                                                                          i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                          r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                          ( ) ( ) ( )

                                                                                                          0 ( ) 0 ( ) 1

                                                                                                          f x y i x y r x y

                                                                                                          i x y r x y

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                                                                                                          r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                          i(xy) ndash determined by the illumination source

                                                                                                          r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                          is called gray (or intensity) scale

                                                                                                          In practice

                                                                                                          min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                          indoor values without additional illuminationmin max10 1000L L

                                                                                                          black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                          min maxL L

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                                                                                                          Digital Image Processing

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                                                                                                          Image Sampling and Quantization

                                                                                                          - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                          scene

                                                                                                          converting a continuous image f to digital form

                                                                                                          - digitizing (x y) is called sampling

                                                                                                          - digitizing f(x y) is called quantization

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                                                                                                          Digital Image Processing

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                                                                                                          Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                          (00) (01) (0 1)(10) (11) (1 1)

                                                                                                          ( )

                                                                                                          ( 10) ( 11) ( 1 1)

                                                                                                          f f f Nf f f N

                                                                                                          f x y

                                                                                                          f M f M f M N

                                                                                                          image element pixel

                                                                                                          00 01 0 1

                                                                                                          10 11 1 1

                                                                                                          10 11 1 1

                                                                                                          ( ) ( )

                                                                                                          N

                                                                                                          i jN M N

                                                                                                          i j

                                                                                                          M M M N

                                                                                                          a a aa f x i y j f i ja a a

                                                                                                          Aa

                                                                                                          a a a

                                                                                                          f(00) ndash the upper left corner of the image

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                                                                                                          Week 1

                                                                                                          M N ge 0 L=2k

                                                                                                          [0 1]i j i ja a L

                                                                                                          Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

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                                                                                                          Number of bits required to store a digitized image

                                                                                                          for 2 b M N k M N b N k

                                                                                                          When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                          Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                          Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                          (eg 100 line pairs per mm)

                                                                                                          Dots per unit distance are commonly used in printing and publishing

                                                                                                          In US the measure is expressed in dots per inch (dpi)

                                                                                                          (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                          Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                          The number of intensity levels (L) is determined by hardware considerations

                                                                                                          L=2k ndash most common k = 8

                                                                                                          Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                          Week 1

                                                                                                          Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                          150 dpi (lower left) 72 dpi (lower right)

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                                                                                                          Week 1

                                                                                                          Reducing the number of gray levels 256 128 64 32

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                                                                                                          Reducing the number of gray levels 16 8 4 2

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                                                                                                          Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                          Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                          Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                          Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                          750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                          same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                          image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                          Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                          Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                          intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                          This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                          straight edges

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                                                                                                          Week 1

                                                                                                          Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                          where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                          be written using the 4 nearest neighbors of point (x y)

                                                                                                          Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                          modest increase in computational effort

                                                                                                          Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                          nearest neighbors of the point 3 3

                                                                                                          0 0

                                                                                                          ( ) i ji j

                                                                                                          i jv x y c x y

                                                                                                          The coefficients cij are obtained solving a 16x16 linear system

                                                                                                          intensity levels of the 16 nearest neighbors of 3 3

                                                                                                          0 0

                                                                                                          ( )i ji j

                                                                                                          i jc x y x y

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                                                                                                          Week 1

                                                                                                          Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                          bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                          programs such as Adobe Photoshop and Corel Photopaint

                                                                                                          Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                          the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                          then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                          neighbor interpolation was used (both for shrinking and zooming)

                                                                                                          Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                          interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                          from 1250 dpi to 150 dpi (instead of 72 dpi)

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                                                                                                          Week 1

                                                                                                          Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

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                                                                                                          Neighbors of a Pixel

                                                                                                          A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                          This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                          The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                          and are denoted ND(p)

                                                                                                          The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                          N8 (p)

                                                                                                          If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                          fall outside the image

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                                                                                                          Adjacency Connectivity Regions Boundaries

                                                                                                          Denote by V the set of intensity levels used to define adjacency

                                                                                                          - in a binary image V 01 (V=0 V=1)

                                                                                                          - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                          We consider 3 types of adjacency

                                                                                                          (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                          (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                          (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                          m-adjacent if

                                                                                                          4( )q N p or

                                                                                                          ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                          Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                          ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          binary image

                                                                                                          0 1 1 0 1 1 0 1 1

                                                                                                          1 0 1 0 0 1 0 0 1 0

                                                                                                          0 0 1 0 0 1 0 0 1

                                                                                                          V

                                                                                                          The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                          8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                          m-adjacency

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                                                                                                          A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                          is a sequence of distinct pixels with coordinates

                                                                                                          and are adjacent 0 0 1 1

                                                                                                          1 1

                                                                                                          ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                          n n

                                                                                                          i i i i

                                                                                                          x y x y x y x y s tx y x y i n

                                                                                                          The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                          Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                          Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                          in S if there exists a path between them consisting only of pixels from S

                                                                                                          S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                          Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                          Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                          that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                          8-adjacency are considered

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                          touches the image border

                                                                                                          the complement of 1

                                                                                                          ( )K

                                                                                                          cu k u u

                                                                                                          k

                                                                                                          R R R R

                                                                                                          We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                          background of the image

                                                                                                          The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                          points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                          region that have at least one background neighbor This definition is referred to as the

                                                                                                          inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                          border in the background

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Distance measures

                                                                                                          For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                          function or metric if

                                                                                                          (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                          (b) D(p q) = D(q p)

                                                                                                          (c) D(p z) le D(p q) + D(q z)

                                                                                                          The Euclidean distance between p and q is defined as 1

                                                                                                          2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                          The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                          centered at (x y)

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                          4( ) | | | |D p q x s y t

                                                                                                          The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                          4

                                                                                                          22 1 2

                                                                                                          2 2 1 0 1 22 1 2

                                                                                                          2

                                                                                                          D

                                                                                                          The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                          The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                          8( ) max| | | |D p q x s y t

                                                                                                          The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          8

                                                                                                          2 2 2 2 22 1 1 1 2

                                                                                                          2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                          D

                                                                                                          The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                          D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                          because these distances involve only the coordinates of the point

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Array versus Matrix Operations

                                                                                                          An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                          11 12 11 12

                                                                                                          21 22 21 22

                                                                                                          a a b ba a b b

                                                                                                          Array product

                                                                                                          11 12 11 12 11 11 12 12

                                                                                                          21 22 21 22 21 21 22 21

                                                                                                          a a b b a b a ba a b b a b a b

                                                                                                          Matrix product

                                                                                                          11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                          21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                          a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                          We assume array operations unless stated otherwise

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Linear versus Nonlinear Operations

                                                                                                          One of the most important classifications of image-processing methods is whether it is

                                                                                                          linear or nonlinear

                                                                                                          ( ) ( )H f x y g x y

                                                                                                          H is said to be a linear operator if

                                                                                                          images1 2 1 2

                                                                                                          1 2

                                                                                                          ( ) ( ) ( ) ( )

                                                                                                          H a f x y b f x y a H f x y b H f x y

                                                                                                          a b f f

                                                                                                          Example of nonlinear operator

                                                                                                          the maximum value of the pixels of image max ( )H f f x y f

                                                                                                          1 2

                                                                                                          0 2 6 5 1 1

                                                                                                          2 3 4 7f f a b

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          1 2

                                                                                                          0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                          2 3 4 7 2 4a f b f

                                                                                                          0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                          2 3 4 7

                                                                                                          Arithmetic Operations in Image Processing

                                                                                                          Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                          f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                          For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                          value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                          The two random variables are uncorrelated when their covariance is 0

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                          used in image enhancement)

                                                                                                          1

                                                                                                          1( ) ( )K

                                                                                                          ii

                                                                                                          g x y g x yK

                                                                                                          If the noise satisfies the properties stated above we have

                                                                                                          2 2( ) ( )

                                                                                                          1( ) ( ) g x y x yE g x y f x yK

                                                                                                          ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                          and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                          the average image is

                                                                                                          ( ) ( )1

                                                                                                          g x y x yK

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                          the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                          means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                          averaging process increases

                                                                                                          An important application of image averaging is in the field of astronomy where imaging

                                                                                                          under very low light levels frequently causes sensor noise to render single images

                                                                                                          virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                          was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                          64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                          images respectively

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                          100 noisy images

                                                                                                          a b c d e f

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          A frequent application of image subtraction is in the enhancement of differences between

                                                                                                          images

                                                                                                          (a) (b) (c)

                                                                                                          Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                          significant bit of each pixel (c) the difference between the two images

                                                                                                          Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                          Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                          images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                          difference between images (a) and (b)

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                          h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                          intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                          source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                          bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                          anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                          images after injection of the contrast medium

                                                                                                          In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                          Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                          propagates through the various arteries in the area being observed

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          An important application of image multiplication (and division) is shading correction

                                                                                                          Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                          f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                          When the shading function is known

                                                                                                          ( )( )( )

                                                                                                          g x yf x yh x y

                                                                                                          h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                          approximation to the shading function by imaging a target of constant intensity When the

                                                                                                          sensor is not available often the shading pattern can be estimated from the image

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          (a) (b) (c)

                                                                                                          Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                          operations The process consists of multiplying a given image by a mask image that has

                                                                                                          1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                          image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                          (a) (b) (c)

                                                                                                          Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                          min( )mf f f

                                                                                                          0 ( 255)max( )

                                                                                                          ms

                                                                                                          m

                                                                                                          ff K K K

                                                                                                          f

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Spatial Operations

                                                                                                          - are performed directly on the pixels of a given image

                                                                                                          There are three categories of spatial operations

                                                                                                          single-pixel operations

                                                                                                          neighborhood operations

                                                                                                          geometric spatial transformations

                                                                                                          Single-pixel operations

                                                                                                          - change the values of intensity for the individual pixels ( )s T z

                                                                                                          where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                          corresponding pixel in the processed image

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Neighborhood operations

                                                                                                          Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                          in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                          based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                          rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                          intensity by computing the average value of the pixels in Sxy

                                                                                                          ( )

                                                                                                          1( ) ( )xyr c S

                                                                                                          g x y f r cm n

                                                                                                          The net effect is to perform local blurring in the original image This type of process is

                                                                                                          used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                          largest region of an image

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Geometric spatial transformations and image registration

                                                                                                          - modify the spatial relationship between pixels in an image

                                                                                                          - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                          printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                          predefined set of rules

                                                                                                          A geometric transformation consists of 2 basic operations

                                                                                                          1 a spatial transformation of coordinates

                                                                                                          2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                          pixels

                                                                                                          The coordinate system transformation ( ) [( )]x y T v w

                                                                                                          (v w) ndash pixel coordinates in the original image

                                                                                                          (x y) ndash pixel coordinates in the transformed image

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                          Affine transform

                                                                                                          11 1211 21 31

                                                                                                          21 2212 22 33

                                                                                                          31 32

                                                                                                          0[ 1] [ 1] [ 1] 0

                                                                                                          1

                                                                                                          t tx t v t w t

                                                                                                          x y v w T v w t ty t v t w t

                                                                                                          t t

                                                                                                          (AT)

                                                                                                          This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                          on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                          result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                          scaling rotation and translation matrices from Table 1

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Affine transformations

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                          the process we have to assign intensity values to those locations This task is done by

                                                                                                          using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                          In practice we can use equation (AT) in two basic ways

                                                                                                          forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                          location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                          Problems

                                                                                                          - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                          the same location in the output image

                                                                                                          - some output locations have no correspondent in the original image (no intensity

                                                                                                          assignment)

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                          computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                          It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                          pixel value

                                                                                                          Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                          numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Image registration ndash align two or more images of the same scene

                                                                                                          In image registration we have available the input and output images but the specific

                                                                                                          transformation that produced the output image from the input is generally unknown

                                                                                                          The problem is to estimate the transformation function and then use it to register the two

                                                                                                          images

                                                                                                          - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                          same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                          - align images of a given location taken by the same instrument at different moments

                                                                                                          of time (satellite images)

                                                                                                          Solving the problem using tie points (also called control points) which are

                                                                                                          corresponding points whose locations are known precisely in the input and reference

                                                                                                          image

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          How to select tie points

                                                                                                          - interactively selecting them

                                                                                                          - use of algorithms that try to detect these points

                                                                                                          - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                          the imaging sensors These objects produce a set of known points (called reseau

                                                                                                          marks) directly on all images captured by the system which can be used as guides

                                                                                                          for establishing tie points

                                                                                                          The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                          of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                          a bilinear approximation is given by

                                                                                                          1 2 3 4

                                                                                                          5 6 7 8

                                                                                                          x c v c w c v w cy c v c w c v w c

                                                                                                          (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                          frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                          subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                          subregions marked by 4 tie points we applied the transformation model described above

                                                                                                          The number of tie points and the sophistication of the model required to solve the register

                                                                                                          problem depend on the severity of the geometrical distortion

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Probabilistic Methods

                                                                                                          zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                          p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                          ( ) kk

                                                                                                          np zM N

                                                                                                          nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                          pixels in the image) 1

                                                                                                          0( ) 1

                                                                                                          L

                                                                                                          kk

                                                                                                          p z

                                                                                                          The mean (average) intensity of an image is given by 1

                                                                                                          0( )

                                                                                                          L

                                                                                                          k kk

                                                                                                          m z p z

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          The variance of the intensities is 1

                                                                                                          2 2

                                                                                                          0( ) ( )

                                                                                                          L

                                                                                                          k kk

                                                                                                          z m p z

                                                                                                          The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                          measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                          ( ) is used

                                                                                                          The n-th moment of a random variable z about the mean is defined as 1

                                                                                                          0( ) ( ) ( )

                                                                                                          Ln

                                                                                                          n k kk

                                                                                                          z z m p z

                                                                                                          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                          3( ) 0z the intensities are biased to values higher than the mean

                                                                                                          ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                          mean

                                                                                                          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                          Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                          Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                          Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Intensity Transformations and Spatial Filtering

                                                                                                          ( ) ( )g x y T f x y

                                                                                                          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                          neighborhood of (x y)

                                                                                                          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                          and much smaller in size than the image

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                          called spatial filter (spatial mask kernel template or window)

                                                                                                          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                          ( )s T r

                                                                                                          s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                          Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                          darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                          is called contrast stretching

                                                                                                          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                          thresholding function

                                                                                                          Some Basic Intensity Transformation Functions

                                                                                                          Image Negatives

                                                                                                          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                          - equivalent of a photographic negative

                                                                                                          - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                          image

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Original Negative image

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                          Some basic intensity transformation functions

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                          range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                          while compressing the higher-level values The opposite is true for the inverse log

                                                                                                          transformation The log functions compress the dynamic range of images with large

                                                                                                          variations in pixel values

                                                                                                          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Power-Law (Gamma) Transformations

                                                                                                          - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                          Plots of gamma transformation for different values of γ (c=1)

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                          of output values with the opposite being true for higher values of input values The

                                                                                                          curves with 1 have the opposite effect of those generated with values of 1

                                                                                                          1c - identity transformation

                                                                                                          A variety of devices used for image capture printing and display respond according to a

                                                                                                          power law The process used to correct these power-law response phenomena is called

                                                                                                          gamma correction

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Piecewise-Linear Transformations Functions

                                                                                                          Contrast stretching

                                                                                                          - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                          intensity range of the recording tool or display device

                                                                                                          a b c d Fig5

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          11

                                                                                                          1

                                                                                                          2 1 1 21 2

                                                                                                          2 1 2 1

                                                                                                          22

                                                                                                          2

                                                                                                          [0 ]

                                                                                                          ( ) ( )( ) [ ]( ) ( )

                                                                                                          ( 1 ) [ 1]( 1 )

                                                                                                          s r r rrs r r s r rT r r r r

                                                                                                          r r r rs L r r r L

                                                                                                          L r

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          1 1 2 2r s r s identity transformation (no change)

                                                                                                          1 2 1 2 0 1r r s s L thresholding function

                                                                                                          Figure 5(b) shows an 8-bit image with low contrast

                                                                                                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                          in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                          from their original range to the full range [0 L-1]

                                                                                                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                          2 2 1r s m L where m is the mean gray level in the image

                                                                                                          The original image on which these results are based is a scanning electron microscope

                                                                                                          image of pollen magnified approximately 700 times

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Intensity-level slicing

                                                                                                          - highlighting a specific range of intensities in an image

                                                                                                          There are two approaches for intensity-level slicing

                                                                                                          1 display in one value (white for example) all the values in the range of interest and in

                                                                                                          another (say black) all other intensities (Figure 311 (a))

                                                                                                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                          intensities in the image (Figure 311 (b))

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                          the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                          Highlights range [A B] and preserves all other intensities

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                          blockageshellip)

                                                                                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                          image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                          Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                          Digital Image Processing

                                                                                                          Week 1

                                                                                                          Bit-plane slicing

                                                                                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                          This technique highlights the contribution made to the whole image appearances by each

                                                                                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                          Digital Image Processing

                                                                                                          Week 1

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                                                                                                          Week 1

                                                                                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                          • DIP 1 2017
                                                                                                          • DIP 02 (2017)

                                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                                            Week 1Week 1

                                                                                                            Three membranes enclose the eye the cornea and sclera outer cover the choroid the retina

                                                                                                            The cornea is a tough transparent tissue that covers the anterior surface of the eye

                                                                                                            Continuous with the cornea the sclera is an opaque membrane that enclosesthe remainder of the optic globe

                                                                                                            The choroid lies directly below the sclera This membrane contains a network of blood vessels (major nutrition of the eye) The choroid is pigmented and help reduce the amount of light entering the eyeThe choroid is divided (at its anterior extreme) into the ciliary body and the irisThe iris contracts and expands to control the amount of light

                                                                                                            Digital Image ProcessingDigital Image Processing

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                                                                                                            The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                                            The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                                            Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                                            Fovea = the place where the image of the object of interest falls on

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                                                                                                            Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                                            Blind spot region without receptors

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                                                                                                            Week 1Week 1

                                                                                                            Image formation in the eye

                                                                                                            Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                                            Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                                            distance between lens and retina along visual axix = 17 mm

                                                                                                            range of focal length = 14 mm to 17 mm

                                                                                                            Digital Image ProcessingDigital Image Processing

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                                                                                                            Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                                            Digital Image ProcessingDigital Image Processing

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                                                                                                            All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                                            Week 1Week 1

                                                                                                            Optical illusions

                                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                                            Week 1Week 1

                                                                                                            ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                                            Week 1Week 1

                                                                                                            Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                            gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                            quantities that describe the quality of a chromatic light source radiance

                                                                                                            the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                            measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

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                                                                                                            Week 1Week 1

                                                                                                            For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                            brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

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                                                                                                            Week 1Week 1

                                                                                                            the physical meaning is determined by the source of the image

                                                                                                            ( )f D f x y

                                                                                                            Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                            f(xy) ndash characterized by two components

                                                                                                            i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                            r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                            ( ) ( ) ( )

                                                                                                            0 ( ) 0 ( ) 1

                                                                                                            f x y i x y r x y

                                                                                                            i x y r x y

                                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                                            Week 1Week 1

                                                                                                            r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                            i(xy) ndash determined by the illumination source

                                                                                                            r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                            is called gray (or intensity) scale

                                                                                                            In practice

                                                                                                            min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                            indoor values without additional illuminationmin max10 1000L L

                                                                                                            black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                            min maxL L

                                                                                                            Digital Image ProcessingDigital Image Processing

                                                                                                            Week 1Week 1

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Image Sampling and Quantization

                                                                                                            - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                            scene

                                                                                                            converting a continuous image f to digital form

                                                                                                            - digitizing (x y) is called sampling

                                                                                                            - digitizing f(x y) is called quantization

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                            (00) (01) (0 1)(10) (11) (1 1)

                                                                                                            ( )

                                                                                                            ( 10) ( 11) ( 1 1)

                                                                                                            f f f Nf f f N

                                                                                                            f x y

                                                                                                            f M f M f M N

                                                                                                            image element pixel

                                                                                                            00 01 0 1

                                                                                                            10 11 1 1

                                                                                                            10 11 1 1

                                                                                                            ( ) ( )

                                                                                                            N

                                                                                                            i jN M N

                                                                                                            i j

                                                                                                            M M M N

                                                                                                            a a aa f x i y j f i ja a a

                                                                                                            Aa

                                                                                                            a a a

                                                                                                            f(00) ndash the upper left corner of the image

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            M N ge 0 L=2k

                                                                                                            [0 1]i j i ja a L

                                                                                                            Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Number of bits required to store a digitized image

                                                                                                            for 2 b M N k M N b N k

                                                                                                            When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                            Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                            Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                            (eg 100 line pairs per mm)

                                                                                                            Dots per unit distance are commonly used in printing and publishing

                                                                                                            In US the measure is expressed in dots per inch (dpi)

                                                                                                            (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                            Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                            The number of intensity levels (L) is determined by hardware considerations

                                                                                                            L=2k ndash most common k = 8

                                                                                                            Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                            150 dpi (lower left) 72 dpi (lower right)

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Reducing the number of gray levels 256 128 64 32

                                                                                                            Digital Image Processing

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                                                                                                            Reducing the number of gray levels 16 8 4 2

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                            Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                            Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                            Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                            750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                            same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                            image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                            Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                            Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                            intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                            This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                            straight edges

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                            where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                            be written using the 4 nearest neighbors of point (x y)

                                                                                                            Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                            modest increase in computational effort

                                                                                                            Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                            nearest neighbors of the point 3 3

                                                                                                            0 0

                                                                                                            ( ) i ji j

                                                                                                            i jv x y c x y

                                                                                                            The coefficients cij are obtained solving a 16x16 linear system

                                                                                                            intensity levels of the 16 nearest neighbors of 3 3

                                                                                                            0 0

                                                                                                            ( )i ji j

                                                                                                            i jc x y x y

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                            bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                            programs such as Adobe Photoshop and Corel Photopaint

                                                                                                            Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                            the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                            then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                            neighbor interpolation was used (both for shrinking and zooming)

                                                                                                            Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                            interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                            from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                            Digital Image Processing

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                                                                                                            Neighbors of a Pixel

                                                                                                            A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                            This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                            The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                            and are denoted ND(p)

                                                                                                            The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                            N8 (p)

                                                                                                            If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                            fall outside the image

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Adjacency Connectivity Regions Boundaries

                                                                                                            Denote by V the set of intensity levels used to define adjacency

                                                                                                            - in a binary image V 01 (V=0 V=1)

                                                                                                            - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                            We consider 3 types of adjacency

                                                                                                            (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                            (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                            (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                            m-adjacent if

                                                                                                            4( )q N p or

                                                                                                            ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                            Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                            ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            binary image

                                                                                                            0 1 1 0 1 1 0 1 1

                                                                                                            1 0 1 0 0 1 0 0 1 0

                                                                                                            0 0 1 0 0 1 0 0 1

                                                                                                            V

                                                                                                            The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                            8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                            m-adjacency

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                            is a sequence of distinct pixels with coordinates

                                                                                                            and are adjacent 0 0 1 1

                                                                                                            1 1

                                                                                                            ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                            n n

                                                                                                            i i i i

                                                                                                            x y x y x y x y s tx y x y i n

                                                                                                            The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                            Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                            Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                            in S if there exists a path between them consisting only of pixels from S

                                                                                                            S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                            Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                            Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                            that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                            8-adjacency are considered

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                            touches the image border

                                                                                                            the complement of 1

                                                                                                            ( )K

                                                                                                            cu k u u

                                                                                                            k

                                                                                                            R R R R

                                                                                                            We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                            background of the image

                                                                                                            The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                            points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                            region that have at least one background neighbor This definition is referred to as the

                                                                                                            inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                            border in the background

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Distance measures

                                                                                                            For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                            function or metric if

                                                                                                            (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                            (b) D(p q) = D(q p)

                                                                                                            (c) D(p z) le D(p q) + D(q z)

                                                                                                            The Euclidean distance between p and q is defined as 1

                                                                                                            2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                            The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                            centered at (x y)

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                            4( ) | | | |D p q x s y t

                                                                                                            The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                            4

                                                                                                            22 1 2

                                                                                                            2 2 1 0 1 22 1 2

                                                                                                            2

                                                                                                            D

                                                                                                            The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                            The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                            8( ) max| | | |D p q x s y t

                                                                                                            The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            8

                                                                                                            2 2 2 2 22 1 1 1 2

                                                                                                            2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                            D

                                                                                                            The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                            D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                            because these distances involve only the coordinates of the point

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Array versus Matrix Operations

                                                                                                            An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                            11 12 11 12

                                                                                                            21 22 21 22

                                                                                                            a a b ba a b b

                                                                                                            Array product

                                                                                                            11 12 11 12 11 11 12 12

                                                                                                            21 22 21 22 21 21 22 21

                                                                                                            a a b b a b a ba a b b a b a b

                                                                                                            Matrix product

                                                                                                            11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                            21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                            a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                            We assume array operations unless stated otherwise

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Linear versus Nonlinear Operations

                                                                                                            One of the most important classifications of image-processing methods is whether it is

                                                                                                            linear or nonlinear

                                                                                                            ( ) ( )H f x y g x y

                                                                                                            H is said to be a linear operator if

                                                                                                            images1 2 1 2

                                                                                                            1 2

                                                                                                            ( ) ( ) ( ) ( )

                                                                                                            H a f x y b f x y a H f x y b H f x y

                                                                                                            a b f f

                                                                                                            Example of nonlinear operator

                                                                                                            the maximum value of the pixels of image max ( )H f f x y f

                                                                                                            1 2

                                                                                                            0 2 6 5 1 1

                                                                                                            2 3 4 7f f a b

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            1 2

                                                                                                            0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                            2 3 4 7 2 4a f b f

                                                                                                            0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                            2 3 4 7

                                                                                                            Arithmetic Operations in Image Processing

                                                                                                            Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                            f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                            For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                            value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                            The two random variables are uncorrelated when their covariance is 0

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                            used in image enhancement)

                                                                                                            1

                                                                                                            1( ) ( )K

                                                                                                            ii

                                                                                                            g x y g x yK

                                                                                                            If the noise satisfies the properties stated above we have

                                                                                                            2 2( ) ( )

                                                                                                            1( ) ( ) g x y x yE g x y f x yK

                                                                                                            ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                            and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                            the average image is

                                                                                                            ( ) ( )1

                                                                                                            g x y x yK

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                            the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                            means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                            averaging process increases

                                                                                                            An important application of image averaging is in the field of astronomy where imaging

                                                                                                            under very low light levels frequently causes sensor noise to render single images

                                                                                                            virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                            was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                            64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                            images respectively

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                            100 noisy images

                                                                                                            a b c d e f

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            A frequent application of image subtraction is in the enhancement of differences between

                                                                                                            images

                                                                                                            (a) (b) (c)

                                                                                                            Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                            significant bit of each pixel (c) the difference between the two images

                                                                                                            Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                            Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                            images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                            difference between images (a) and (b)

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                            h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                            intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                            source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                            bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                            anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                            images after injection of the contrast medium

                                                                                                            In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                            Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                            propagates through the various arteries in the area being observed

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            An important application of image multiplication (and division) is shading correction

                                                                                                            Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                            f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                            When the shading function is known

                                                                                                            ( )( )( )

                                                                                                            g x yf x yh x y

                                                                                                            h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                            approximation to the shading function by imaging a target of constant intensity When the

                                                                                                            sensor is not available often the shading pattern can be estimated from the image

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            (a) (b) (c)

                                                                                                            Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                            operations The process consists of multiplying a given image by a mask image that has

                                                                                                            1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                            image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                            (a) (b) (c)

                                                                                                            Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                            min( )mf f f

                                                                                                            0 ( 255)max( )

                                                                                                            ms

                                                                                                            m

                                                                                                            ff K K K

                                                                                                            f

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Spatial Operations

                                                                                                            - are performed directly on the pixels of a given image

                                                                                                            There are three categories of spatial operations

                                                                                                            single-pixel operations

                                                                                                            neighborhood operations

                                                                                                            geometric spatial transformations

                                                                                                            Single-pixel operations

                                                                                                            - change the values of intensity for the individual pixels ( )s T z

                                                                                                            where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                            corresponding pixel in the processed image

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Neighborhood operations

                                                                                                            Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                            in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                            based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                            rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                            intensity by computing the average value of the pixels in Sxy

                                                                                                            ( )

                                                                                                            1( ) ( )xyr c S

                                                                                                            g x y f r cm n

                                                                                                            The net effect is to perform local blurring in the original image This type of process is

                                                                                                            used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                            largest region of an image

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Geometric spatial transformations and image registration

                                                                                                            - modify the spatial relationship between pixels in an image

                                                                                                            - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                            printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                            predefined set of rules

                                                                                                            A geometric transformation consists of 2 basic operations

                                                                                                            1 a spatial transformation of coordinates

                                                                                                            2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                            pixels

                                                                                                            The coordinate system transformation ( ) [( )]x y T v w

                                                                                                            (v w) ndash pixel coordinates in the original image

                                                                                                            (x y) ndash pixel coordinates in the transformed image

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                            Affine transform

                                                                                                            11 1211 21 31

                                                                                                            21 2212 22 33

                                                                                                            31 32

                                                                                                            0[ 1] [ 1] [ 1] 0

                                                                                                            1

                                                                                                            t tx t v t w t

                                                                                                            x y v w T v w t ty t v t w t

                                                                                                            t t

                                                                                                            (AT)

                                                                                                            This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                            on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                            result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                            scaling rotation and translation matrices from Table 1

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Affine transformations

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                            the process we have to assign intensity values to those locations This task is done by

                                                                                                            using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                            In practice we can use equation (AT) in two basic ways

                                                                                                            forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                            location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                            Problems

                                                                                                            - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                            the same location in the output image

                                                                                                            - some output locations have no correspondent in the original image (no intensity

                                                                                                            assignment)

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                            computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                            It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                            pixel value

                                                                                                            Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                            numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Image registration ndash align two or more images of the same scene

                                                                                                            In image registration we have available the input and output images but the specific

                                                                                                            transformation that produced the output image from the input is generally unknown

                                                                                                            The problem is to estimate the transformation function and then use it to register the two

                                                                                                            images

                                                                                                            - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                            same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                            - align images of a given location taken by the same instrument at different moments

                                                                                                            of time (satellite images)

                                                                                                            Solving the problem using tie points (also called control points) which are

                                                                                                            corresponding points whose locations are known precisely in the input and reference

                                                                                                            image

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            How to select tie points

                                                                                                            - interactively selecting them

                                                                                                            - use of algorithms that try to detect these points

                                                                                                            - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                            the imaging sensors These objects produce a set of known points (called reseau

                                                                                                            marks) directly on all images captured by the system which can be used as guides

                                                                                                            for establishing tie points

                                                                                                            The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                            of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                            a bilinear approximation is given by

                                                                                                            1 2 3 4

                                                                                                            5 6 7 8

                                                                                                            x c v c w c v w cy c v c w c v w c

                                                                                                            (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                            frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                            subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                            subregions marked by 4 tie points we applied the transformation model described above

                                                                                                            The number of tie points and the sophistication of the model required to solve the register

                                                                                                            problem depend on the severity of the geometrical distortion

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Probabilistic Methods

                                                                                                            zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                            p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                            ( ) kk

                                                                                                            np zM N

                                                                                                            nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                            pixels in the image) 1

                                                                                                            0( ) 1

                                                                                                            L

                                                                                                            kk

                                                                                                            p z

                                                                                                            The mean (average) intensity of an image is given by 1

                                                                                                            0( )

                                                                                                            L

                                                                                                            k kk

                                                                                                            m z p z

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            The variance of the intensities is 1

                                                                                                            2 2

                                                                                                            0( ) ( )

                                                                                                            L

                                                                                                            k kk

                                                                                                            z m p z

                                                                                                            The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                            measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                            ( ) is used

                                                                                                            The n-th moment of a random variable z about the mean is defined as 1

                                                                                                            0( ) ( ) ( )

                                                                                                            Ln

                                                                                                            n k kk

                                                                                                            z z m p z

                                                                                                            ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                            3( ) 0z the intensities are biased to values higher than the mean

                                                                                                            ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                            mean

                                                                                                            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                            Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                            Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                            Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Intensity Transformations and Spatial Filtering

                                                                                                            ( ) ( )g x y T f x y

                                                                                                            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                            neighborhood of (x y)

                                                                                                            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                            and much smaller in size than the image

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                            called spatial filter (spatial mask kernel template or window)

                                                                                                            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                            ( )s T r

                                                                                                            s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                            Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                            darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                            is called contrast stretching

                                                                                                            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                            thresholding function

                                                                                                            Some Basic Intensity Transformation Functions

                                                                                                            Image Negatives

                                                                                                            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                            - equivalent of a photographic negative

                                                                                                            - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                            image

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Original Negative image

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                            Some basic intensity transformation functions

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                            range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                            while compressing the higher-level values The opposite is true for the inverse log

                                                                                                            transformation The log functions compress the dynamic range of images with large

                                                                                                            variations in pixel values

                                                                                                            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Power-Law (Gamma) Transformations

                                                                                                            - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                            Plots of gamma transformation for different values of γ (c=1)

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                            of output values with the opposite being true for higher values of input values The

                                                                                                            curves with 1 have the opposite effect of those generated with values of 1

                                                                                                            1c - identity transformation

                                                                                                            A variety of devices used for image capture printing and display respond according to a

                                                                                                            power law The process used to correct these power-law response phenomena is called

                                                                                                            gamma correction

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Piecewise-Linear Transformations Functions

                                                                                                            Contrast stretching

                                                                                                            - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                            intensity range of the recording tool or display device

                                                                                                            a b c d Fig5

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            11

                                                                                                            1

                                                                                                            2 1 1 21 2

                                                                                                            2 1 2 1

                                                                                                            22

                                                                                                            2

                                                                                                            [0 ]

                                                                                                            ( ) ( )( ) [ ]( ) ( )

                                                                                                            ( 1 ) [ 1]( 1 )

                                                                                                            s r r rrs r r s r rT r r r r

                                                                                                            r r r rs L r r r L

                                                                                                            L r

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            1 1 2 2r s r s identity transformation (no change)

                                                                                                            1 2 1 2 0 1r r s s L thresholding function

                                                                                                            Figure 5(b) shows an 8-bit image with low contrast

                                                                                                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                            in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                            from their original range to the full range [0 L-1]

                                                                                                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                            2 2 1r s m L where m is the mean gray level in the image

                                                                                                            The original image on which these results are based is a scanning electron microscope

                                                                                                            image of pollen magnified approximately 700 times

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Intensity-level slicing

                                                                                                            - highlighting a specific range of intensities in an image

                                                                                                            There are two approaches for intensity-level slicing

                                                                                                            1 display in one value (white for example) all the values in the range of interest and in

                                                                                                            another (say black) all other intensities (Figure 311 (a))

                                                                                                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                            intensities in the image (Figure 311 (b))

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                            the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                            Highlights range [A B] and preserves all other intensities

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                            blockageshellip)

                                                                                                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                            image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                            Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Bit-plane slicing

                                                                                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                            This technique highlights the contribution made to the whole image appearances by each

                                                                                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            Digital Image Processing

                                                                                                            Week 1

                                                                                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                            • DIP 1 2017
                                                                                                            • DIP 02 (2017)

                                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                                              Week 1Week 1

                                                                                                              The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to ciliary body (60-70 water 6 fat protein) The lens is colored in slightly yellow The lens absorbs approximatively 8 of the visiblelight (infrared and ultraviolet light are absorbed by proteins in lens)

                                                                                                              The innermost membrane is the retina When the eye is proper focusedlight from an object outside the eye is imaged on the retinaVision is possible because of the distribution of discrete light receptors on the surface of the retina cones and rods (6-7 milion cones 75-150 milion rods)

                                                                                                              Cones located in the central part of the retina (fovea) they are sensitive to colors vision of detail each cone is link to its own nervecone vision = photopic or bright-light vision

                                                                                                              Fovea = the place where the image of the object of interest falls on

                                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                                              Week 1Week 1

                                                                                                              Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                                              Blind spot region without receptors

                                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                                              Week 1Week 1

                                                                                                              Image formation in the eye

                                                                                                              Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                                              Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                                              distance between lens and retina along visual axix = 17 mm

                                                                                                              range of focal length = 14 mm to 17 mm

                                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                                              Week 1Week 1

                                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                                              Week 1Week 1

                                                                                                              Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                                              Week 1Week 1

                                                                                                              All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                                              Week 1Week 1

                                                                                                              Optical illusions

                                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                                              Week 1Week 1

                                                                                                              ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                                              Week 1Week 1

                                                                                                              Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                              gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                              quantities that describe the quality of a chromatic light source radiance

                                                                                                              the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                              measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                                              Week 1Week 1

                                                                                                              For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                              brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                                              Week 1Week 1

                                                                                                              the physical meaning is determined by the source of the image

                                                                                                              ( )f D f x y

                                                                                                              Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                              f(xy) ndash characterized by two components

                                                                                                              i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                              r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                              ( ) ( ) ( )

                                                                                                              0 ( ) 0 ( ) 1

                                                                                                              f x y i x y r x y

                                                                                                              i x y r x y

                                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                                              Week 1Week 1

                                                                                                              r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                              i(xy) ndash determined by the illumination source

                                                                                                              r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                              is called gray (or intensity) scale

                                                                                                              In practice

                                                                                                              min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                              indoor values without additional illuminationmin max10 1000L L

                                                                                                              black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                              min maxL L

                                                                                                              Digital Image ProcessingDigital Image Processing

                                                                                                              Week 1Week 1

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Image Sampling and Quantization

                                                                                                              - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                              scene

                                                                                                              converting a continuous image f to digital form

                                                                                                              - digitizing (x y) is called sampling

                                                                                                              - digitizing f(x y) is called quantization

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                              (00) (01) (0 1)(10) (11) (1 1)

                                                                                                              ( )

                                                                                                              ( 10) ( 11) ( 1 1)

                                                                                                              f f f Nf f f N

                                                                                                              f x y

                                                                                                              f M f M f M N

                                                                                                              image element pixel

                                                                                                              00 01 0 1

                                                                                                              10 11 1 1

                                                                                                              10 11 1 1

                                                                                                              ( ) ( )

                                                                                                              N

                                                                                                              i jN M N

                                                                                                              i j

                                                                                                              M M M N

                                                                                                              a a aa f x i y j f i ja a a

                                                                                                              Aa

                                                                                                              a a a

                                                                                                              f(00) ndash the upper left corner of the image

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              M N ge 0 L=2k

                                                                                                              [0 1]i j i ja a L

                                                                                                              Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Number of bits required to store a digitized image

                                                                                                              for 2 b M N k M N b N k

                                                                                                              When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                              Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                              Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                              (eg 100 line pairs per mm)

                                                                                                              Dots per unit distance are commonly used in printing and publishing

                                                                                                              In US the measure is expressed in dots per inch (dpi)

                                                                                                              (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                              Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                              The number of intensity levels (L) is determined by hardware considerations

                                                                                                              L=2k ndash most common k = 8

                                                                                                              Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                              150 dpi (lower left) 72 dpi (lower right)

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Reducing the number of gray levels 256 128 64 32

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Reducing the number of gray levels 16 8 4 2

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                              Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                              Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                              Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                              750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                              same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                              image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                              Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                              Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                              intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                              This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                              straight edges

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                              where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                              be written using the 4 nearest neighbors of point (x y)

                                                                                                              Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                              modest increase in computational effort

                                                                                                              Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                              nearest neighbors of the point 3 3

                                                                                                              0 0

                                                                                                              ( ) i ji j

                                                                                                              i jv x y c x y

                                                                                                              The coefficients cij are obtained solving a 16x16 linear system

                                                                                                              intensity levels of the 16 nearest neighbors of 3 3

                                                                                                              0 0

                                                                                                              ( )i ji j

                                                                                                              i jc x y x y

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                              bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                              programs such as Adobe Photoshop and Corel Photopaint

                                                                                                              Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                              the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                              then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                              neighbor interpolation was used (both for shrinking and zooming)

                                                                                                              Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                              interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                              from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Neighbors of a Pixel

                                                                                                              A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                              This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                              The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                              and are denoted ND(p)

                                                                                                              The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                              N8 (p)

                                                                                                              If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                              fall outside the image

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Adjacency Connectivity Regions Boundaries

                                                                                                              Denote by V the set of intensity levels used to define adjacency

                                                                                                              - in a binary image V 01 (V=0 V=1)

                                                                                                              - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                              We consider 3 types of adjacency

                                                                                                              (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                              (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                              (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                              m-adjacent if

                                                                                                              4( )q N p or

                                                                                                              ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                              Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                              ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              binary image

                                                                                                              0 1 1 0 1 1 0 1 1

                                                                                                              1 0 1 0 0 1 0 0 1 0

                                                                                                              0 0 1 0 0 1 0 0 1

                                                                                                              V

                                                                                                              The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                              8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                              m-adjacency

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                              is a sequence of distinct pixels with coordinates

                                                                                                              and are adjacent 0 0 1 1

                                                                                                              1 1

                                                                                                              ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                              n n

                                                                                                              i i i i

                                                                                                              x y x y x y x y s tx y x y i n

                                                                                                              The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                              Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                              Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                              in S if there exists a path between them consisting only of pixels from S

                                                                                                              S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                              Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                              Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                              that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                              8-adjacency are considered

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                              touches the image border

                                                                                                              the complement of 1

                                                                                                              ( )K

                                                                                                              cu k u u

                                                                                                              k

                                                                                                              R R R R

                                                                                                              We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                              background of the image

                                                                                                              The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                              points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                              region that have at least one background neighbor This definition is referred to as the

                                                                                                              inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                              border in the background

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Distance measures

                                                                                                              For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                              function or metric if

                                                                                                              (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                              (b) D(p q) = D(q p)

                                                                                                              (c) D(p z) le D(p q) + D(q z)

                                                                                                              The Euclidean distance between p and q is defined as 1

                                                                                                              2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                              The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                              centered at (x y)

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                              4( ) | | | |D p q x s y t

                                                                                                              The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                              4

                                                                                                              22 1 2

                                                                                                              2 2 1 0 1 22 1 2

                                                                                                              2

                                                                                                              D

                                                                                                              The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                              The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                              8( ) max| | | |D p q x s y t

                                                                                                              The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              8

                                                                                                              2 2 2 2 22 1 1 1 2

                                                                                                              2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                              D

                                                                                                              The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                              D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                              because these distances involve only the coordinates of the point

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Array versus Matrix Operations

                                                                                                              An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                              11 12 11 12

                                                                                                              21 22 21 22

                                                                                                              a a b ba a b b

                                                                                                              Array product

                                                                                                              11 12 11 12 11 11 12 12

                                                                                                              21 22 21 22 21 21 22 21

                                                                                                              a a b b a b a ba a b b a b a b

                                                                                                              Matrix product

                                                                                                              11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                              21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                              a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                              We assume array operations unless stated otherwise

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Linear versus Nonlinear Operations

                                                                                                              One of the most important classifications of image-processing methods is whether it is

                                                                                                              linear or nonlinear

                                                                                                              ( ) ( )H f x y g x y

                                                                                                              H is said to be a linear operator if

                                                                                                              images1 2 1 2

                                                                                                              1 2

                                                                                                              ( ) ( ) ( ) ( )

                                                                                                              H a f x y b f x y a H f x y b H f x y

                                                                                                              a b f f

                                                                                                              Example of nonlinear operator

                                                                                                              the maximum value of the pixels of image max ( )H f f x y f

                                                                                                              1 2

                                                                                                              0 2 6 5 1 1

                                                                                                              2 3 4 7f f a b

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              1 2

                                                                                                              0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                              2 3 4 7 2 4a f b f

                                                                                                              0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                              2 3 4 7

                                                                                                              Arithmetic Operations in Image Processing

                                                                                                              Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                              f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                              For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                              value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                              The two random variables are uncorrelated when their covariance is 0

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                              used in image enhancement)

                                                                                                              1

                                                                                                              1( ) ( )K

                                                                                                              ii

                                                                                                              g x y g x yK

                                                                                                              If the noise satisfies the properties stated above we have

                                                                                                              2 2( ) ( )

                                                                                                              1( ) ( ) g x y x yE g x y f x yK

                                                                                                              ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                              and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                              the average image is

                                                                                                              ( ) ( )1

                                                                                                              g x y x yK

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                              the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                              means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                              averaging process increases

                                                                                                              An important application of image averaging is in the field of astronomy where imaging

                                                                                                              under very low light levels frequently causes sensor noise to render single images

                                                                                                              virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                              was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                              64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                              images respectively

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                              100 noisy images

                                                                                                              a b c d e f

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              A frequent application of image subtraction is in the enhancement of differences between

                                                                                                              images

                                                                                                              (a) (b) (c)

                                                                                                              Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                              significant bit of each pixel (c) the difference between the two images

                                                                                                              Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                              Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                              images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                              difference between images (a) and (b)

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                              h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                              intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                              source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                              bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                              anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                              images after injection of the contrast medium

                                                                                                              In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                              Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                              propagates through the various arteries in the area being observed

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              An important application of image multiplication (and division) is shading correction

                                                                                                              Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                              f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                              When the shading function is known

                                                                                                              ( )( )( )

                                                                                                              g x yf x yh x y

                                                                                                              h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                              approximation to the shading function by imaging a target of constant intensity When the

                                                                                                              sensor is not available often the shading pattern can be estimated from the image

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              (a) (b) (c)

                                                                                                              Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                              operations The process consists of multiplying a given image by a mask image that has

                                                                                                              1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                              image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                              (a) (b) (c)

                                                                                                              Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                              min( )mf f f

                                                                                                              0 ( 255)max( )

                                                                                                              ms

                                                                                                              m

                                                                                                              ff K K K

                                                                                                              f

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Spatial Operations

                                                                                                              - are performed directly on the pixels of a given image

                                                                                                              There are three categories of spatial operations

                                                                                                              single-pixel operations

                                                                                                              neighborhood operations

                                                                                                              geometric spatial transformations

                                                                                                              Single-pixel operations

                                                                                                              - change the values of intensity for the individual pixels ( )s T z

                                                                                                              where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                              corresponding pixel in the processed image

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Neighborhood operations

                                                                                                              Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                              in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                              based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                              rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                              intensity by computing the average value of the pixels in Sxy

                                                                                                              ( )

                                                                                                              1( ) ( )xyr c S

                                                                                                              g x y f r cm n

                                                                                                              The net effect is to perform local blurring in the original image This type of process is

                                                                                                              used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                              largest region of an image

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Geometric spatial transformations and image registration

                                                                                                              - modify the spatial relationship between pixels in an image

                                                                                                              - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                              printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                              predefined set of rules

                                                                                                              A geometric transformation consists of 2 basic operations

                                                                                                              1 a spatial transformation of coordinates

                                                                                                              2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                              pixels

                                                                                                              The coordinate system transformation ( ) [( )]x y T v w

                                                                                                              (v w) ndash pixel coordinates in the original image

                                                                                                              (x y) ndash pixel coordinates in the transformed image

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                              Affine transform

                                                                                                              11 1211 21 31

                                                                                                              21 2212 22 33

                                                                                                              31 32

                                                                                                              0[ 1] [ 1] [ 1] 0

                                                                                                              1

                                                                                                              t tx t v t w t

                                                                                                              x y v w T v w t ty t v t w t

                                                                                                              t t

                                                                                                              (AT)

                                                                                                              This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                              on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                              result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                              scaling rotation and translation matrices from Table 1

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Affine transformations

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                              the process we have to assign intensity values to those locations This task is done by

                                                                                                              using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                              In practice we can use equation (AT) in two basic ways

                                                                                                              forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                              location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                              Problems

                                                                                                              - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                              the same location in the output image

                                                                                                              - some output locations have no correspondent in the original image (no intensity

                                                                                                              assignment)

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                              computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                              It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                              pixel value

                                                                                                              Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                              numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Image registration ndash align two or more images of the same scene

                                                                                                              In image registration we have available the input and output images but the specific

                                                                                                              transformation that produced the output image from the input is generally unknown

                                                                                                              The problem is to estimate the transformation function and then use it to register the two

                                                                                                              images

                                                                                                              - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                              same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                              - align images of a given location taken by the same instrument at different moments

                                                                                                              of time (satellite images)

                                                                                                              Solving the problem using tie points (also called control points) which are

                                                                                                              corresponding points whose locations are known precisely in the input and reference

                                                                                                              image

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              How to select tie points

                                                                                                              - interactively selecting them

                                                                                                              - use of algorithms that try to detect these points

                                                                                                              - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                              the imaging sensors These objects produce a set of known points (called reseau

                                                                                                              marks) directly on all images captured by the system which can be used as guides

                                                                                                              for establishing tie points

                                                                                                              The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                              of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                              a bilinear approximation is given by

                                                                                                              1 2 3 4

                                                                                                              5 6 7 8

                                                                                                              x c v c w c v w cy c v c w c v w c

                                                                                                              (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                              frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                              subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                              subregions marked by 4 tie points we applied the transformation model described above

                                                                                                              The number of tie points and the sophistication of the model required to solve the register

                                                                                                              problem depend on the severity of the geometrical distortion

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Probabilistic Methods

                                                                                                              zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                              p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                              ( ) kk

                                                                                                              np zM N

                                                                                                              nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                              pixels in the image) 1

                                                                                                              0( ) 1

                                                                                                              L

                                                                                                              kk

                                                                                                              p z

                                                                                                              The mean (average) intensity of an image is given by 1

                                                                                                              0( )

                                                                                                              L

                                                                                                              k kk

                                                                                                              m z p z

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              The variance of the intensities is 1

                                                                                                              2 2

                                                                                                              0( ) ( )

                                                                                                              L

                                                                                                              k kk

                                                                                                              z m p z

                                                                                                              The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                              measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                              ( ) is used

                                                                                                              The n-th moment of a random variable z about the mean is defined as 1

                                                                                                              0( ) ( ) ( )

                                                                                                              Ln

                                                                                                              n k kk

                                                                                                              z z m p z

                                                                                                              ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                              3( ) 0z the intensities are biased to values higher than the mean

                                                                                                              ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                              mean

                                                                                                              Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                              Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                              Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                              Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              Intensity Transformations and Spatial Filtering

                                                                                                              ( ) ( )g x y T f x y

                                                                                                              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                              neighborhood of (x y)

                                                                                                              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                              and much smaller in size than the image

                                                                                                              Digital Image Processing

                                                                                                              Week 1

                                                                                                              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                              called spatial filter (spatial mask kernel template or window)

                                                                                                              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                              ( )s T r

                                                                                                              s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                              Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                              darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                              is called contrast stretching

                                                                                                              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

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                                                                                                              Week 1

                                                                                                              Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                              thresholding function

                                                                                                              Some Basic Intensity Transformation Functions

                                                                                                              Image Negatives

                                                                                                              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                              - equivalent of a photographic negative

                                                                                                              - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                              image

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                                                                                                              Original Negative image

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                                                                                                              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                              Some basic intensity transformation functions

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                                                                                                              This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                              range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                              while compressing the higher-level values The opposite is true for the inverse log

                                                                                                              transformation The log functions compress the dynamic range of images with large

                                                                                                              variations in pixel values

                                                                                                              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

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                                                                                                              Power-Law (Gamma) Transformations

                                                                                                              - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                              Plots of gamma transformation for different values of γ (c=1)

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                                                                                                              Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                              of output values with the opposite being true for higher values of input values The

                                                                                                              curves with 1 have the opposite effect of those generated with values of 1

                                                                                                              1c - identity transformation

                                                                                                              A variety of devices used for image capture printing and display respond according to a

                                                                                                              power law The process used to correct these power-law response phenomena is called

                                                                                                              gamma correction

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                                                                                                              Week 1

                                                                                                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

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                                                                                                              Piecewise-Linear Transformations Functions

                                                                                                              Contrast stretching

                                                                                                              - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                              intensity range of the recording tool or display device

                                                                                                              a b c d Fig5

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                                                                                                              11

                                                                                                              1

                                                                                                              2 1 1 21 2

                                                                                                              2 1 2 1

                                                                                                              22

                                                                                                              2

                                                                                                              [0 ]

                                                                                                              ( ) ( )( ) [ ]( ) ( )

                                                                                                              ( 1 ) [ 1]( 1 )

                                                                                                              s r r rrs r r s r rT r r r r

                                                                                                              r r r rs L r r r L

                                                                                                              L r

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                                                                                                              1 1 2 2r s r s identity transformation (no change)

                                                                                                              1 2 1 2 0 1r r s s L thresholding function

                                                                                                              Figure 5(b) shows an 8-bit image with low contrast

                                                                                                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                              in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                              from their original range to the full range [0 L-1]

                                                                                                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                              2 2 1r s m L where m is the mean gray level in the image

                                                                                                              The original image on which these results are based is a scanning electron microscope

                                                                                                              image of pollen magnified approximately 700 times

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                                                                                                              Intensity-level slicing

                                                                                                              - highlighting a specific range of intensities in an image

                                                                                                              There are two approaches for intensity-level slicing

                                                                                                              1 display in one value (white for example) all the values in the range of interest and in

                                                                                                              another (say black) all other intensities (Figure 311 (a))

                                                                                                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                              intensities in the image (Figure 311 (b))

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                                                                                                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                              the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                              Highlights range [A B] and preserves all other intensities

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                                                                                                              which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                              blockageshellip)

                                                                                                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                              image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                              Fig 6 - Aortic angiogram and intensity sliced versions

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                                                                                                              Bit-plane slicing

                                                                                                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                              This technique highlights the contribution made to the whole image appearances by each

                                                                                                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

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                                                                                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                              • DIP 1 2017
                                                                                                              • DIP 02 (2017)

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                                                                                                                Rods distributed over al the retina surface several rods are contected toa single nerve not specialized in detail vision serve to give a general overall picture of the filed of viewnot involved in color visionsensitive to low level of illumination

                                                                                                                Blind spot region without receptors

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                                                                                                                Image formation in the eye

                                                                                                                Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                                                Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                                                distance between lens and retina along visual axix = 17 mm

                                                                                                                range of focal length = 14 mm to 17 mm

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                                                                                                                Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

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                                                                                                                All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

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                                                                                                                Optical illusions

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                                                                                                                ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

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                                                                                                                Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                                gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                                quantities that describe the quality of a chromatic light source radiance

                                                                                                                the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                                measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

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                                                                                                                For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                                brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

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                                                                                                                the physical meaning is determined by the source of the image

                                                                                                                ( )f D f x y

                                                                                                                Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                                f(xy) ndash characterized by two components

                                                                                                                i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                                r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                                ( ) ( ) ( )

                                                                                                                0 ( ) 0 ( ) 1

                                                                                                                f x y i x y r x y

                                                                                                                i x y r x y

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                                                                                                                r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                                i(xy) ndash determined by the illumination source

                                                                                                                r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                                is called gray (or intensity) scale

                                                                                                                In practice

                                                                                                                min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                                indoor values without additional illuminationmin max10 1000L L

                                                                                                                black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                                min maxL L

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                                                                                                                Image Sampling and Quantization

                                                                                                                - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                                scene

                                                                                                                converting a continuous image f to digital form

                                                                                                                - digitizing (x y) is called sampling

                                                                                                                - digitizing f(x y) is called quantization

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                                                                                                                Continuous image projected onto a sensor array Result of image sampling and quantization

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                                                                                                                Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                ( )

                                                                                                                ( 10) ( 11) ( 1 1)

                                                                                                                f f f Nf f f N

                                                                                                                f x y

                                                                                                                f M f M f M N

                                                                                                                image element pixel

                                                                                                                00 01 0 1

                                                                                                                10 11 1 1

                                                                                                                10 11 1 1

                                                                                                                ( ) ( )

                                                                                                                N

                                                                                                                i jN M N

                                                                                                                i j

                                                                                                                M M M N

                                                                                                                a a aa f x i y j f i ja a a

                                                                                                                Aa

                                                                                                                a a a

                                                                                                                f(00) ndash the upper left corner of the image

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                                                                                                                M N ge 0 L=2k

                                                                                                                [0 1]i j i ja a L

                                                                                                                Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

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                                                                                                                Number of bits required to store a digitized image

                                                                                                                for 2 b M N k M N b N k

                                                                                                                When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

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                                                                                                                Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                (eg 100 line pairs per mm)

                                                                                                                Dots per unit distance are commonly used in printing and publishing

                                                                                                                In US the measure is expressed in dots per inch (dpi)

                                                                                                                (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                The number of intensity levels (L) is determined by hardware considerations

                                                                                                                L=2k ndash most common k = 8

                                                                                                                Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                                Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                150 dpi (lower left) 72 dpi (lower right)

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                                                                                                                Reducing the number of gray levels 256 128 64 32

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                                                                                                                Reducing the number of gray levels 16 8 4 2

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                                                                                                                Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                straight edges

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                                                                                                                Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                be written using the 4 nearest neighbors of point (x y)

                                                                                                                Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                modest increase in computational effort

                                                                                                                Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                nearest neighbors of the point 3 3

                                                                                                                0 0

                                                                                                                ( ) i ji j

                                                                                                                i jv x y c x y

                                                                                                                The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                0 0

                                                                                                                ( )i ji j

                                                                                                                i jc x y x y

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                                                                                                                Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                from 1250 dpi to 150 dpi (instead of 72 dpi)

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                                                                                                                Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

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                                                                                                                Neighbors of a Pixel

                                                                                                                A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                and are denoted ND(p)

                                                                                                                The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                N8 (p)

                                                                                                                If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                fall outside the image

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                                                                                                                Adjacency Connectivity Regions Boundaries

                                                                                                                Denote by V the set of intensity levels used to define adjacency

                                                                                                                - in a binary image V 01 (V=0 V=1)

                                                                                                                - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                We consider 3 types of adjacency

                                                                                                                (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                m-adjacent if

                                                                                                                4( )q N p or

                                                                                                                ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                ambiguities that often arise when 8-adjacency is used Consider the example

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                                                                                                                Week 1

                                                                                                                binary image

                                                                                                                0 1 1 0 1 1 0 1 1

                                                                                                                1 0 1 0 0 1 0 0 1 0

                                                                                                                0 0 1 0 0 1 0 0 1

                                                                                                                V

                                                                                                                The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                m-adjacency

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                                                                                                                A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                is a sequence of distinct pixels with coordinates

                                                                                                                and are adjacent 0 0 1 1

                                                                                                                1 1

                                                                                                                ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                n n

                                                                                                                i i i i

                                                                                                                x y x y x y x y s tx y x y i n

                                                                                                                The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                in S if there exists a path between them consisting only of pixels from S

                                                                                                                S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                8-adjacency are considered

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                                                                                                                Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                touches the image border

                                                                                                                the complement of 1

                                                                                                                ( )K

                                                                                                                cu k u u

                                                                                                                k

                                                                                                                R R R R

                                                                                                                We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                background of the image

                                                                                                                The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                region that have at least one background neighbor This definition is referred to as the

                                                                                                                inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                border in the background

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                                                                                                                Distance measures

                                                                                                                For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                function or metric if

                                                                                                                (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                (b) D(p q) = D(q p)

                                                                                                                (c) D(p z) le D(p q) + D(q z)

                                                                                                                The Euclidean distance between p and q is defined as 1

                                                                                                                2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                centered at (x y)

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                                                                                                                The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                4( ) | | | |D p q x s y t

                                                                                                                The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                4

                                                                                                                22 1 2

                                                                                                                2 2 1 0 1 22 1 2

                                                                                                                2

                                                                                                                D

                                                                                                                The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                8( ) max| | | |D p q x s y t

                                                                                                                The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                8

                                                                                                                2 2 2 2 22 1 1 1 2

                                                                                                                2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                D

                                                                                                                The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                because these distances involve only the coordinates of the point

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                                                                                                                Array versus Matrix Operations

                                                                                                                An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                11 12 11 12

                                                                                                                21 22 21 22

                                                                                                                a a b ba a b b

                                                                                                                Array product

                                                                                                                11 12 11 12 11 11 12 12

                                                                                                                21 22 21 22 21 21 22 21

                                                                                                                a a b b a b a ba a b b a b a b

                                                                                                                Matrix product

                                                                                                                11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                We assume array operations unless stated otherwise

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                                                                                                                Week 1

                                                                                                                Linear versus Nonlinear Operations

                                                                                                                One of the most important classifications of image-processing methods is whether it is

                                                                                                                linear or nonlinear

                                                                                                                ( ) ( )H f x y g x y

                                                                                                                H is said to be a linear operator if

                                                                                                                images1 2 1 2

                                                                                                                1 2

                                                                                                                ( ) ( ) ( ) ( )

                                                                                                                H a f x y b f x y a H f x y b H f x y

                                                                                                                a b f f

                                                                                                                Example of nonlinear operator

                                                                                                                the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                1 2

                                                                                                                0 2 6 5 1 1

                                                                                                                2 3 4 7f f a b

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                                                                                                                Week 1

                                                                                                                1 2

                                                                                                                0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                2 3 4 7 2 4a f b f

                                                                                                                0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                2 3 4 7

                                                                                                                Arithmetic Operations in Image Processing

                                                                                                                Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                The two random variables are uncorrelated when their covariance is 0

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                used in image enhancement)

                                                                                                                1

                                                                                                                1( ) ( )K

                                                                                                                ii

                                                                                                                g x y g x yK

                                                                                                                If the noise satisfies the properties stated above we have

                                                                                                                2 2( ) ( )

                                                                                                                1( ) ( ) g x y x yE g x y f x yK

                                                                                                                ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                the average image is

                                                                                                                ( ) ( )1

                                                                                                                g x y x yK

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                averaging process increases

                                                                                                                An important application of image averaging is in the field of astronomy where imaging

                                                                                                                under very low light levels frequently causes sensor noise to render single images

                                                                                                                virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                images respectively

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                100 noisy images

                                                                                                                a b c d e f

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                images

                                                                                                                (a) (b) (c)

                                                                                                                Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                significant bit of each pixel (c) the difference between the two images

                                                                                                                Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                difference between images (a) and (b)

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                images after injection of the contrast medium

                                                                                                                In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                propagates through the various arteries in the area being observed

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                An important application of image multiplication (and division) is shading correction

                                                                                                                Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                When the shading function is known

                                                                                                                ( )( )( )

                                                                                                                g x yf x yh x y

                                                                                                                h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                sensor is not available often the shading pattern can be estimated from the image

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                (a) (b) (c)

                                                                                                                Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                operations The process consists of multiplying a given image by a mask image that has

                                                                                                                1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                (a) (b) (c)

                                                                                                                Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                min( )mf f f

                                                                                                                0 ( 255)max( )

                                                                                                                ms

                                                                                                                m

                                                                                                                ff K K K

                                                                                                                f

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Spatial Operations

                                                                                                                - are performed directly on the pixels of a given image

                                                                                                                There are three categories of spatial operations

                                                                                                                single-pixel operations

                                                                                                                neighborhood operations

                                                                                                                geometric spatial transformations

                                                                                                                Single-pixel operations

                                                                                                                - change the values of intensity for the individual pixels ( )s T z

                                                                                                                where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                corresponding pixel in the processed image

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Neighborhood operations

                                                                                                                Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                intensity by computing the average value of the pixels in Sxy

                                                                                                                ( )

                                                                                                                1( ) ( )xyr c S

                                                                                                                g x y f r cm n

                                                                                                                The net effect is to perform local blurring in the original image This type of process is

                                                                                                                used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                largest region of an image

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Geometric spatial transformations and image registration

                                                                                                                - modify the spatial relationship between pixels in an image

                                                                                                                - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                predefined set of rules

                                                                                                                A geometric transformation consists of 2 basic operations

                                                                                                                1 a spatial transformation of coordinates

                                                                                                                2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                pixels

                                                                                                                The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                (v w) ndash pixel coordinates in the original image

                                                                                                                (x y) ndash pixel coordinates in the transformed image

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                Affine transform

                                                                                                                11 1211 21 31

                                                                                                                21 2212 22 33

                                                                                                                31 32

                                                                                                                0[ 1] [ 1] [ 1] 0

                                                                                                                1

                                                                                                                t tx t v t w t

                                                                                                                x y v w T v w t ty t v t w t

                                                                                                                t t

                                                                                                                (AT)

                                                                                                                This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                scaling rotation and translation matrices from Table 1

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Affine transformations

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                the process we have to assign intensity values to those locations This task is done by

                                                                                                                using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                In practice we can use equation (AT) in two basic ways

                                                                                                                forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                Problems

                                                                                                                - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                the same location in the output image

                                                                                                                - some output locations have no correspondent in the original image (no intensity

                                                                                                                assignment)

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                pixel value

                                                                                                                Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Image registration ndash align two or more images of the same scene

                                                                                                                In image registration we have available the input and output images but the specific

                                                                                                                transformation that produced the output image from the input is generally unknown

                                                                                                                The problem is to estimate the transformation function and then use it to register the two

                                                                                                                images

                                                                                                                - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                - align images of a given location taken by the same instrument at different moments

                                                                                                                of time (satellite images)

                                                                                                                Solving the problem using tie points (also called control points) which are

                                                                                                                corresponding points whose locations are known precisely in the input and reference

                                                                                                                image

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                How to select tie points

                                                                                                                - interactively selecting them

                                                                                                                - use of algorithms that try to detect these points

                                                                                                                - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                marks) directly on all images captured by the system which can be used as guides

                                                                                                                for establishing tie points

                                                                                                                The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                a bilinear approximation is given by

                                                                                                                1 2 3 4

                                                                                                                5 6 7 8

                                                                                                                x c v c w c v w cy c v c w c v w c

                                                                                                                (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                The number of tie points and the sophistication of the model required to solve the register

                                                                                                                problem depend on the severity of the geometrical distortion

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Probabilistic Methods

                                                                                                                zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                ( ) kk

                                                                                                                np zM N

                                                                                                                nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                pixels in the image) 1

                                                                                                                0( ) 1

                                                                                                                L

                                                                                                                kk

                                                                                                                p z

                                                                                                                The mean (average) intensity of an image is given by 1

                                                                                                                0( )

                                                                                                                L

                                                                                                                k kk

                                                                                                                m z p z

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                The variance of the intensities is 1

                                                                                                                2 2

                                                                                                                0( ) ( )

                                                                                                                L

                                                                                                                k kk

                                                                                                                z m p z

                                                                                                                The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                ( ) is used

                                                                                                                The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                0( ) ( ) ( )

                                                                                                                Ln

                                                                                                                n k kk

                                                                                                                z z m p z

                                                                                                                ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                mean

                                                                                                                Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Intensity Transformations and Spatial Filtering

                                                                                                                ( ) ( )g x y T f x y

                                                                                                                f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                neighborhood of (x y)

                                                                                                                - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                and much smaller in size than the image

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                called spatial filter (spatial mask kernel template or window)

                                                                                                                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                ( )s T r

                                                                                                                s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                is called contrast stretching

                                                                                                                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                thresholding function

                                                                                                                Some Basic Intensity Transformation Functions

                                                                                                                Image Negatives

                                                                                                                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                - equivalent of a photographic negative

                                                                                                                - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                image

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Original Negative image

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                Some basic intensity transformation functions

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                transformation The log functions compress the dynamic range of images with large

                                                                                                                variations in pixel values

                                                                                                                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Power-Law (Gamma) Transformations

                                                                                                                - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                Plots of gamma transformation for different values of γ (c=1)

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                of output values with the opposite being true for higher values of input values The

                                                                                                                curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                1c - identity transformation

                                                                                                                A variety of devices used for image capture printing and display respond according to a

                                                                                                                power law The process used to correct these power-law response phenomena is called

                                                                                                                gamma correction

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Piecewise-Linear Transformations Functions

                                                                                                                Contrast stretching

                                                                                                                - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                intensity range of the recording tool or display device

                                                                                                                a b c d Fig5

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                11

                                                                                                                1

                                                                                                                2 1 1 21 2

                                                                                                                2 1 2 1

                                                                                                                22

                                                                                                                2

                                                                                                                [0 ]

                                                                                                                ( ) ( )( ) [ ]( ) ( )

                                                                                                                ( 1 ) [ 1]( 1 )

                                                                                                                s r r rrs r r s r rT r r r r

                                                                                                                r r r rs L r r r L

                                                                                                                L r

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                1 1 2 2r s r s identity transformation (no change)

                                                                                                                1 2 1 2 0 1r r s s L thresholding function

                                                                                                                Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                from their original range to the full range [0 L-1]

                                                                                                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                2 2 1r s m L where m is the mean gray level in the image

                                                                                                                The original image on which these results are based is a scanning electron microscope

                                                                                                                image of pollen magnified approximately 700 times

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Intensity-level slicing

                                                                                                                - highlighting a specific range of intensities in an image

                                                                                                                There are two approaches for intensity-level slicing

                                                                                                                1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                another (say black) all other intensities (Figure 311 (a))

                                                                                                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                intensities in the image (Figure 311 (b))

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                Highlights range [A B] and preserves all other intensities

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                blockageshellip)

                                                                                                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Bit-plane slicing

                                                                                                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                This technique highlights the contribution made to the whole image appearances by each

                                                                                                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                Digital Image Processing

                                                                                                                Week 1

                                                                                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                • DIP 1 2017
                                                                                                                • DIP 02 (2017)

                                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                                  Week 1Week 1

                                                                                                                  Image formation in the eye

                                                                                                                  Ordinary photographic camera the lens has fixed focal length focusing atvarious distances is done by modifying the distance between the lens and the image plane (were the film or imaging chip are located)

                                                                                                                  Human eye the distance between the lens and the retina (the imaging region)is fixed the focal length needed to achieve proper focus is obtained by varyingthe shape of the lens (the fibers in the ciliary body accomplish this flattening or thickening the lens for distant or near objects respectively

                                                                                                                  distance between lens and retina along visual axix = 17 mm

                                                                                                                  range of focal length = 14 mm to 17 mm

                                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                                  Week 1Week 1

                                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                                  Week 1Week 1

                                                                                                                  Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                                  Week 1Week 1

                                                                                                                  All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                                  Week 1Week 1

                                                                                                                  Optical illusions

                                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                                  Week 1Week 1

                                                                                                                  ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                                  Week 1Week 1

                                                                                                                  Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                                  gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                                  quantities that describe the quality of a chromatic light source radiance

                                                                                                                  the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                                  measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                                  Week 1Week 1

                                                                                                                  For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                                  brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                                  Week 1Week 1

                                                                                                                  the physical meaning is determined by the source of the image

                                                                                                                  ( )f D f x y

                                                                                                                  Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                                  f(xy) ndash characterized by two components

                                                                                                                  i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                                  r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                                  ( ) ( ) ( )

                                                                                                                  0 ( ) 0 ( ) 1

                                                                                                                  f x y i x y r x y

                                                                                                                  i x y r x y

                                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                                  Week 1Week 1

                                                                                                                  r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                                  i(xy) ndash determined by the illumination source

                                                                                                                  r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                                  is called gray (or intensity) scale

                                                                                                                  In practice

                                                                                                                  min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                                  indoor values without additional illuminationmin max10 1000L L

                                                                                                                  black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                                  min maxL L

                                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                                  Week 1Week 1

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Image Sampling and Quantization

                                                                                                                  - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                                  scene

                                                                                                                  converting a continuous image f to digital form

                                                                                                                  - digitizing (x y) is called sampling

                                                                                                                  - digitizing f(x y) is called quantization

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                  (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                  ( )

                                                                                                                  ( 10) ( 11) ( 1 1)

                                                                                                                  f f f Nf f f N

                                                                                                                  f x y

                                                                                                                  f M f M f M N

                                                                                                                  image element pixel

                                                                                                                  00 01 0 1

                                                                                                                  10 11 1 1

                                                                                                                  10 11 1 1

                                                                                                                  ( ) ( )

                                                                                                                  N

                                                                                                                  i jN M N

                                                                                                                  i j

                                                                                                                  M M M N

                                                                                                                  a a aa f x i y j f i ja a a

                                                                                                                  Aa

                                                                                                                  a a a

                                                                                                                  f(00) ndash the upper left corner of the image

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  M N ge 0 L=2k

                                                                                                                  [0 1]i j i ja a L

                                                                                                                  Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Number of bits required to store a digitized image

                                                                                                                  for 2 b M N k M N b N k

                                                                                                                  When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                  Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                  Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                  (eg 100 line pairs per mm)

                                                                                                                  Dots per unit distance are commonly used in printing and publishing

                                                                                                                  In US the measure is expressed in dots per inch (dpi)

                                                                                                                  (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                  Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                  The number of intensity levels (L) is determined by hardware considerations

                                                                                                                  L=2k ndash most common k = 8

                                                                                                                  Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                  150 dpi (lower left) 72 dpi (lower right)

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Reducing the number of gray levels 256 128 64 32

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Reducing the number of gray levels 16 8 4 2

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                  Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                  Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                  Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                  750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                  same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                  image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                  Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                  Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                  intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                  This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                  straight edges

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                  where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                  be written using the 4 nearest neighbors of point (x y)

                                                                                                                  Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                  modest increase in computational effort

                                                                                                                  Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                  nearest neighbors of the point 3 3

                                                                                                                  0 0

                                                                                                                  ( ) i ji j

                                                                                                                  i jv x y c x y

                                                                                                                  The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                  intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                  0 0

                                                                                                                  ( )i ji j

                                                                                                                  i jc x y x y

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                  bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                  programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                  Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                  the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                  then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                  neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                  Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                  interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                  from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Neighbors of a Pixel

                                                                                                                  A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                  This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                  The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                  and are denoted ND(p)

                                                                                                                  The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                  N8 (p)

                                                                                                                  If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                  fall outside the image

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Adjacency Connectivity Regions Boundaries

                                                                                                                  Denote by V the set of intensity levels used to define adjacency

                                                                                                                  - in a binary image V 01 (V=0 V=1)

                                                                                                                  - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                  We consider 3 types of adjacency

                                                                                                                  (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                  (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                  (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                  m-adjacent if

                                                                                                                  4( )q N p or

                                                                                                                  ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                  Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                  ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  binary image

                                                                                                                  0 1 1 0 1 1 0 1 1

                                                                                                                  1 0 1 0 0 1 0 0 1 0

                                                                                                                  0 0 1 0 0 1 0 0 1

                                                                                                                  V

                                                                                                                  The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                  8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                  m-adjacency

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                  is a sequence of distinct pixels with coordinates

                                                                                                                  and are adjacent 0 0 1 1

                                                                                                                  1 1

                                                                                                                  ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                  n n

                                                                                                                  i i i i

                                                                                                                  x y x y x y x y s tx y x y i n

                                                                                                                  The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                  Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                  Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                  in S if there exists a path between them consisting only of pixels from S

                                                                                                                  S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                  Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                  Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                  that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                  8-adjacency are considered

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                  touches the image border

                                                                                                                  the complement of 1

                                                                                                                  ( )K

                                                                                                                  cu k u u

                                                                                                                  k

                                                                                                                  R R R R

                                                                                                                  We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                  background of the image

                                                                                                                  The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                  points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                  region that have at least one background neighbor This definition is referred to as the

                                                                                                                  inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                  border in the background

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Distance measures

                                                                                                                  For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                  function or metric if

                                                                                                                  (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                  (b) D(p q) = D(q p)

                                                                                                                  (c) D(p z) le D(p q) + D(q z)

                                                                                                                  The Euclidean distance between p and q is defined as 1

                                                                                                                  2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                  The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                  centered at (x y)

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                  4( ) | | | |D p q x s y t

                                                                                                                  The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                  4

                                                                                                                  22 1 2

                                                                                                                  2 2 1 0 1 22 1 2

                                                                                                                  2

                                                                                                                  D

                                                                                                                  The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                  The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                  8( ) max| | | |D p q x s y t

                                                                                                                  The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  8

                                                                                                                  2 2 2 2 22 1 1 1 2

                                                                                                                  2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                  D

                                                                                                                  The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                  D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                  because these distances involve only the coordinates of the point

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Array versus Matrix Operations

                                                                                                                  An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                  11 12 11 12

                                                                                                                  21 22 21 22

                                                                                                                  a a b ba a b b

                                                                                                                  Array product

                                                                                                                  11 12 11 12 11 11 12 12

                                                                                                                  21 22 21 22 21 21 22 21

                                                                                                                  a a b b a b a ba a b b a b a b

                                                                                                                  Matrix product

                                                                                                                  11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                  21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                  a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                  We assume array operations unless stated otherwise

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Linear versus Nonlinear Operations

                                                                                                                  One of the most important classifications of image-processing methods is whether it is

                                                                                                                  linear or nonlinear

                                                                                                                  ( ) ( )H f x y g x y

                                                                                                                  H is said to be a linear operator if

                                                                                                                  images1 2 1 2

                                                                                                                  1 2

                                                                                                                  ( ) ( ) ( ) ( )

                                                                                                                  H a f x y b f x y a H f x y b H f x y

                                                                                                                  a b f f

                                                                                                                  Example of nonlinear operator

                                                                                                                  the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                  1 2

                                                                                                                  0 2 6 5 1 1

                                                                                                                  2 3 4 7f f a b

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  1 2

                                                                                                                  0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                  2 3 4 7 2 4a f b f

                                                                                                                  0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                  2 3 4 7

                                                                                                                  Arithmetic Operations in Image Processing

                                                                                                                  Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                  f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                  For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                  value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                  The two random variables are uncorrelated when their covariance is 0

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                  used in image enhancement)

                                                                                                                  1

                                                                                                                  1( ) ( )K

                                                                                                                  ii

                                                                                                                  g x y g x yK

                                                                                                                  If the noise satisfies the properties stated above we have

                                                                                                                  2 2( ) ( )

                                                                                                                  1( ) ( ) g x y x yE g x y f x yK

                                                                                                                  ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                  and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                  the average image is

                                                                                                                  ( ) ( )1

                                                                                                                  g x y x yK

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                  the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                  means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                  averaging process increases

                                                                                                                  An important application of image averaging is in the field of astronomy where imaging

                                                                                                                  under very low light levels frequently causes sensor noise to render single images

                                                                                                                  virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                  was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                  64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                  images respectively

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                  100 noisy images

                                                                                                                  a b c d e f

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                  images

                                                                                                                  (a) (b) (c)

                                                                                                                  Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                  significant bit of each pixel (c) the difference between the two images

                                                                                                                  Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                  Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                  images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                  difference between images (a) and (b)

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                  h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                  intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                  source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                  bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                  anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                  images after injection of the contrast medium

                                                                                                                  In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                  Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                  propagates through the various arteries in the area being observed

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  An important application of image multiplication (and division) is shading correction

                                                                                                                  Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                  f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                  When the shading function is known

                                                                                                                  ( )( )( )

                                                                                                                  g x yf x yh x y

                                                                                                                  h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                  approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                  sensor is not available often the shading pattern can be estimated from the image

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  (a) (b) (c)

                                                                                                                  Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                  operations The process consists of multiplying a given image by a mask image that has

                                                                                                                  1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                  image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                  (a) (b) (c)

                                                                                                                  Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                  min( )mf f f

                                                                                                                  0 ( 255)max( )

                                                                                                                  ms

                                                                                                                  m

                                                                                                                  ff K K K

                                                                                                                  f

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Spatial Operations

                                                                                                                  - are performed directly on the pixels of a given image

                                                                                                                  There are three categories of spatial operations

                                                                                                                  single-pixel operations

                                                                                                                  neighborhood operations

                                                                                                                  geometric spatial transformations

                                                                                                                  Single-pixel operations

                                                                                                                  - change the values of intensity for the individual pixels ( )s T z

                                                                                                                  where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                  corresponding pixel in the processed image

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Neighborhood operations

                                                                                                                  Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                  in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                  based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                  rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                  intensity by computing the average value of the pixels in Sxy

                                                                                                                  ( )

                                                                                                                  1( ) ( )xyr c S

                                                                                                                  g x y f r cm n

                                                                                                                  The net effect is to perform local blurring in the original image This type of process is

                                                                                                                  used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                  largest region of an image

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Geometric spatial transformations and image registration

                                                                                                                  - modify the spatial relationship between pixels in an image

                                                                                                                  - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                  printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                  predefined set of rules

                                                                                                                  A geometric transformation consists of 2 basic operations

                                                                                                                  1 a spatial transformation of coordinates

                                                                                                                  2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                  pixels

                                                                                                                  The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                  (v w) ndash pixel coordinates in the original image

                                                                                                                  (x y) ndash pixel coordinates in the transformed image

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                  Affine transform

                                                                                                                  11 1211 21 31

                                                                                                                  21 2212 22 33

                                                                                                                  31 32

                                                                                                                  0[ 1] [ 1] [ 1] 0

                                                                                                                  1

                                                                                                                  t tx t v t w t

                                                                                                                  x y v w T v w t ty t v t w t

                                                                                                                  t t

                                                                                                                  (AT)

                                                                                                                  This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                  on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                  result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                  scaling rotation and translation matrices from Table 1

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Affine transformations

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                  the process we have to assign intensity values to those locations This task is done by

                                                                                                                  using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                  In practice we can use equation (AT) in two basic ways

                                                                                                                  forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                  location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                  Problems

                                                                                                                  - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                  the same location in the output image

                                                                                                                  - some output locations have no correspondent in the original image (no intensity

                                                                                                                  assignment)

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                  computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                  It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                  pixel value

                                                                                                                  Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                  numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Image registration ndash align two or more images of the same scene

                                                                                                                  In image registration we have available the input and output images but the specific

                                                                                                                  transformation that produced the output image from the input is generally unknown

                                                                                                                  The problem is to estimate the transformation function and then use it to register the two

                                                                                                                  images

                                                                                                                  - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                  same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                  - align images of a given location taken by the same instrument at different moments

                                                                                                                  of time (satellite images)

                                                                                                                  Solving the problem using tie points (also called control points) which are

                                                                                                                  corresponding points whose locations are known precisely in the input and reference

                                                                                                                  image

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  How to select tie points

                                                                                                                  - interactively selecting them

                                                                                                                  - use of algorithms that try to detect these points

                                                                                                                  - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                  the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                  marks) directly on all images captured by the system which can be used as guides

                                                                                                                  for establishing tie points

                                                                                                                  The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                  of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                  a bilinear approximation is given by

                                                                                                                  1 2 3 4

                                                                                                                  5 6 7 8

                                                                                                                  x c v c w c v w cy c v c w c v w c

                                                                                                                  (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                  frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                  subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                  subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                  The number of tie points and the sophistication of the model required to solve the register

                                                                                                                  problem depend on the severity of the geometrical distortion

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Probabilistic Methods

                                                                                                                  zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                  p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                  ( ) kk

                                                                                                                  np zM N

                                                                                                                  nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                  pixels in the image) 1

                                                                                                                  0( ) 1

                                                                                                                  L

                                                                                                                  kk

                                                                                                                  p z

                                                                                                                  The mean (average) intensity of an image is given by 1

                                                                                                                  0( )

                                                                                                                  L

                                                                                                                  k kk

                                                                                                                  m z p z

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                                                                                                                  Week 1

                                                                                                                  The variance of the intensities is 1

                                                                                                                  2 2

                                                                                                                  0( ) ( )

                                                                                                                  L

                                                                                                                  k kk

                                                                                                                  z m p z

                                                                                                                  The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                  measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                  ( ) is used

                                                                                                                  The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                  0( ) ( ) ( )

                                                                                                                  Ln

                                                                                                                  n k kk

                                                                                                                  z z m p z

                                                                                                                  ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                  3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                  ( 3( ) 0z the intensities are biased to values lower than the mean

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                                                                                                                  Week 1

                                                                                                                  3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                  mean

                                                                                                                  Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                  Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                  Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                  Figure 1(c) ndash standard deviation 492 (variance = 24206)

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                                                                                                                  Week 1

                                                                                                                  Intensity Transformations and Spatial Filtering

                                                                                                                  ( ) ( )g x y T f x y

                                                                                                                  f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                  neighborhood of (x y)

                                                                                                                  - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                  and much smaller in size than the image

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                                                                                                                  - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                  called spatial filter (spatial mask kernel template or window)

                                                                                                                  ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                  ( )s T r

                                                                                                                  s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                  Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                  darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                  is called contrast stretching

                                                                                                                  Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

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                                                                                                                  Week 1

                                                                                                                  Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                  thresholding function

                                                                                                                  Some Basic Intensity Transformation Functions

                                                                                                                  Image Negatives

                                                                                                                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                  - equivalent of a photographic negative

                                                                                                                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                  image

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                                                                                                                  Week 1

                                                                                                                  Original Negative image

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                                                                                                                  Week 1

                                                                                                                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                  Some basic intensity transformation functions

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                                                                                                                  This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                  range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                  while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                  transformation The log functions compress the dynamic range of images with large

                                                                                                                  variations in pixel values

                                                                                                                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                  Digital Image Processing

                                                                                                                  Week 1

                                                                                                                  Power-Law (Gamma) Transformations

                                                                                                                  - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                  Plots of gamma transformation for different values of γ (c=1)

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                                                                                                                  Week 1

                                                                                                                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                  of output values with the opposite being true for higher values of input values The

                                                                                                                  curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                  1c - identity transformation

                                                                                                                  A variety of devices used for image capture printing and display respond according to a

                                                                                                                  power law The process used to correct these power-law response phenomena is called

                                                                                                                  gamma correction

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                                                                                                                  Week 1

                                                                                                                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

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                                                                                                                  Week 1

                                                                                                                  Piecewise-Linear Transformations Functions

                                                                                                                  Contrast stretching

                                                                                                                  - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                  intensity range of the recording tool or display device

                                                                                                                  a b c d Fig5

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                                                                                                                  Week 1

                                                                                                                  11

                                                                                                                  1

                                                                                                                  2 1 1 21 2

                                                                                                                  2 1 2 1

                                                                                                                  22

                                                                                                                  2

                                                                                                                  [0 ]

                                                                                                                  ( ) ( )( ) [ ]( ) ( )

                                                                                                                  ( 1 ) [ 1]( 1 )

                                                                                                                  s r r rrs r r s r rT r r r r

                                                                                                                  r r r rs L r r r L

                                                                                                                  L r

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                                                                                                                  Week 1

                                                                                                                  1 1 2 2r s r s identity transformation (no change)

                                                                                                                  1 2 1 2 0 1r r s s L thresholding function

                                                                                                                  Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                  in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                  from their original range to the full range [0 L-1]

                                                                                                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                  2 2 1r s m L where m is the mean gray level in the image

                                                                                                                  The original image on which these results are based is a scanning electron microscope

                                                                                                                  image of pollen magnified approximately 700 times

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                                                                                                                  Intensity-level slicing

                                                                                                                  - highlighting a specific range of intensities in an image

                                                                                                                  There are two approaches for intensity-level slicing

                                                                                                                  1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                  another (say black) all other intensities (Figure 311 (a))

                                                                                                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                  intensities in the image (Figure 311 (b))

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                                                                                                                  Week 1

                                                                                                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                  the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                  Highlights range [A B] and preserves all other intensities

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                                                                                                                  Week 1

                                                                                                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                  blockageshellip)

                                                                                                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                  image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                  Fig 6 - Aortic angiogram and intensity sliced versions

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                                                                                                                  Bit-plane slicing

                                                                                                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                  This technique highlights the contribution made to the whole image appearances by each

                                                                                                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

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                                                                                                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                  • DIP 1 2017
                                                                                                                  • DIP 02 (2017)

                                                                                                                    Digital Image ProcessingDigital Image Processing

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                                                                                                                    Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

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                                                                                                                    All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

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                                                                                                                    Optical illusions

                                                                                                                    Digital Image ProcessingDigital Image Processing

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                                                                                                                    ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

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                                                                                                                    Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                                    gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                                    quantities that describe the quality of a chromatic light source radiance

                                                                                                                    the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                                    measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

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                                                                                                                    For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                                    brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

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                                                                                                                    the physical meaning is determined by the source of the image

                                                                                                                    ( )f D f x y

                                                                                                                    Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                                    f(xy) ndash characterized by two components

                                                                                                                    i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                                    r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                                    ( ) ( ) ( )

                                                                                                                    0 ( ) 0 ( ) 1

                                                                                                                    f x y i x y r x y

                                                                                                                    i x y r x y

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                                                                                                                    r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                                    i(xy) ndash determined by the illumination source

                                                                                                                    r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                                    is called gray (or intensity) scale

                                                                                                                    In practice

                                                                                                                    min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                                    indoor values without additional illuminationmin max10 1000L L

                                                                                                                    black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                                    min maxL L

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                                                                                                                    Digital Image Processing

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                                                                                                                    Image Sampling and Quantization

                                                                                                                    - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                                    scene

                                                                                                                    converting a continuous image f to digital form

                                                                                                                    - digitizing (x y) is called sampling

                                                                                                                    - digitizing f(x y) is called quantization

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                                                                                                                    Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                                    Digital Image Processing

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                                                                                                                    Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                    (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                    ( )

                                                                                                                    ( 10) ( 11) ( 1 1)

                                                                                                                    f f f Nf f f N

                                                                                                                    f x y

                                                                                                                    f M f M f M N

                                                                                                                    image element pixel

                                                                                                                    00 01 0 1

                                                                                                                    10 11 1 1

                                                                                                                    10 11 1 1

                                                                                                                    ( ) ( )

                                                                                                                    N

                                                                                                                    i jN M N

                                                                                                                    i j

                                                                                                                    M M M N

                                                                                                                    a a aa f x i y j f i ja a a

                                                                                                                    Aa

                                                                                                                    a a a

                                                                                                                    f(00) ndash the upper left corner of the image

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                                                                                                                    M N ge 0 L=2k

                                                                                                                    [0 1]i j i ja a L

                                                                                                                    Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

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                                                                                                                    Number of bits required to store a digitized image

                                                                                                                    for 2 b M N k M N b N k

                                                                                                                    When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                                    Digital Image Processing

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                                                                                                                    Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                    Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                    Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                    (eg 100 line pairs per mm)

                                                                                                                    Dots per unit distance are commonly used in printing and publishing

                                                                                                                    In US the measure is expressed in dots per inch (dpi)

                                                                                                                    (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                    Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                    The number of intensity levels (L) is determined by hardware considerations

                                                                                                                    L=2k ndash most common k = 8

                                                                                                                    Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                                    Week 1

                                                                                                                    Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                    150 dpi (lower left) 72 dpi (lower right)

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                                                                                                                    Week 1

                                                                                                                    Reducing the number of gray levels 256 128 64 32

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                                                                                                                    Reducing the number of gray levels 16 8 4 2

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                                                                                                                    Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                    Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                    Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                    Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                    750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                    same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                    image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                    Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                    Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                    intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                    This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                    straight edges

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                                                                                                                    Week 1

                                                                                                                    Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                    where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                    be written using the 4 nearest neighbors of point (x y)

                                                                                                                    Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                    modest increase in computational effort

                                                                                                                    Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                    nearest neighbors of the point 3 3

                                                                                                                    0 0

                                                                                                                    ( ) i ji j

                                                                                                                    i jv x y c x y

                                                                                                                    The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                    intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                    0 0

                                                                                                                    ( )i ji j

                                                                                                                    i jc x y x y

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                                                                                                                    Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                    bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                    programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                    Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                    the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                    then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                    neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                    Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                    interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                    from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                    Digital Image Processing

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                                                                                                                    Neighbors of a Pixel

                                                                                                                    A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                    This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                    The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                    and are denoted ND(p)

                                                                                                                    The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                    N8 (p)

                                                                                                                    If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                    fall outside the image

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                                                                                                                    Adjacency Connectivity Regions Boundaries

                                                                                                                    Denote by V the set of intensity levels used to define adjacency

                                                                                                                    - in a binary image V 01 (V=0 V=1)

                                                                                                                    - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                    We consider 3 types of adjacency

                                                                                                                    (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                    (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                    (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                    m-adjacent if

                                                                                                                    4( )q N p or

                                                                                                                    ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                    Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                    ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    binary image

                                                                                                                    0 1 1 0 1 1 0 1 1

                                                                                                                    1 0 1 0 0 1 0 0 1 0

                                                                                                                    0 0 1 0 0 1 0 0 1

                                                                                                                    V

                                                                                                                    The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                    8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                    m-adjacency

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                                                                                                                    A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                    is a sequence of distinct pixels with coordinates

                                                                                                                    and are adjacent 0 0 1 1

                                                                                                                    1 1

                                                                                                                    ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                    n n

                                                                                                                    i i i i

                                                                                                                    x y x y x y x y s tx y x y i n

                                                                                                                    The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                    Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                    Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                    in S if there exists a path between them consisting only of pixels from S

                                                                                                                    S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                    Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                    Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                    that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                    8-adjacency are considered

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                                                                                                                    Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                    touches the image border

                                                                                                                    the complement of 1

                                                                                                                    ( )K

                                                                                                                    cu k u u

                                                                                                                    k

                                                                                                                    R R R R

                                                                                                                    We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                    background of the image

                                                                                                                    The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                    points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                    region that have at least one background neighbor This definition is referred to as the

                                                                                                                    inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                    border in the background

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Distance measures

                                                                                                                    For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                    function or metric if

                                                                                                                    (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                    (b) D(p q) = D(q p)

                                                                                                                    (c) D(p z) le D(p q) + D(q z)

                                                                                                                    The Euclidean distance between p and q is defined as 1

                                                                                                                    2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                    The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                    centered at (x y)

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                    4( ) | | | |D p q x s y t

                                                                                                                    The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                    4

                                                                                                                    22 1 2

                                                                                                                    2 2 1 0 1 22 1 2

                                                                                                                    2

                                                                                                                    D

                                                                                                                    The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                    The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                    8( ) max| | | |D p q x s y t

                                                                                                                    The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    8

                                                                                                                    2 2 2 2 22 1 1 1 2

                                                                                                                    2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                    D

                                                                                                                    The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                    D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                    because these distances involve only the coordinates of the point

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Array versus Matrix Operations

                                                                                                                    An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                    11 12 11 12

                                                                                                                    21 22 21 22

                                                                                                                    a a b ba a b b

                                                                                                                    Array product

                                                                                                                    11 12 11 12 11 11 12 12

                                                                                                                    21 22 21 22 21 21 22 21

                                                                                                                    a a b b a b a ba a b b a b a b

                                                                                                                    Matrix product

                                                                                                                    11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                    21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                    a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                    We assume array operations unless stated otherwise

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Linear versus Nonlinear Operations

                                                                                                                    One of the most important classifications of image-processing methods is whether it is

                                                                                                                    linear or nonlinear

                                                                                                                    ( ) ( )H f x y g x y

                                                                                                                    H is said to be a linear operator if

                                                                                                                    images1 2 1 2

                                                                                                                    1 2

                                                                                                                    ( ) ( ) ( ) ( )

                                                                                                                    H a f x y b f x y a H f x y b H f x y

                                                                                                                    a b f f

                                                                                                                    Example of nonlinear operator

                                                                                                                    the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                    1 2

                                                                                                                    0 2 6 5 1 1

                                                                                                                    2 3 4 7f f a b

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    1 2

                                                                                                                    0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                    2 3 4 7 2 4a f b f

                                                                                                                    0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                    2 3 4 7

                                                                                                                    Arithmetic Operations in Image Processing

                                                                                                                    Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                    f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                    For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                    value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                    The two random variables are uncorrelated when their covariance is 0

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                    used in image enhancement)

                                                                                                                    1

                                                                                                                    1( ) ( )K

                                                                                                                    ii

                                                                                                                    g x y g x yK

                                                                                                                    If the noise satisfies the properties stated above we have

                                                                                                                    2 2( ) ( )

                                                                                                                    1( ) ( ) g x y x yE g x y f x yK

                                                                                                                    ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                    and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                    the average image is

                                                                                                                    ( ) ( )1

                                                                                                                    g x y x yK

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                    the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                    means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                    averaging process increases

                                                                                                                    An important application of image averaging is in the field of astronomy where imaging

                                                                                                                    under very low light levels frequently causes sensor noise to render single images

                                                                                                                    virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                    was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                    64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                    images respectively

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                    100 noisy images

                                                                                                                    a b c d e f

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                    images

                                                                                                                    (a) (b) (c)

                                                                                                                    Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                    significant bit of each pixel (c) the difference between the two images

                                                                                                                    Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                    Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                    images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                    difference between images (a) and (b)

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                    h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                    intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                    source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                    bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                    anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                    images after injection of the contrast medium

                                                                                                                    In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                    Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                    propagates through the various arteries in the area being observed

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    An important application of image multiplication (and division) is shading correction

                                                                                                                    Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                    f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                    When the shading function is known

                                                                                                                    ( )( )( )

                                                                                                                    g x yf x yh x y

                                                                                                                    h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                    approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                    sensor is not available often the shading pattern can be estimated from the image

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    (a) (b) (c)

                                                                                                                    Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                    operations The process consists of multiplying a given image by a mask image that has

                                                                                                                    1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                    image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                    (a) (b) (c)

                                                                                                                    Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                    min( )mf f f

                                                                                                                    0 ( 255)max( )

                                                                                                                    ms

                                                                                                                    m

                                                                                                                    ff K K K

                                                                                                                    f

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Spatial Operations

                                                                                                                    - are performed directly on the pixels of a given image

                                                                                                                    There are three categories of spatial operations

                                                                                                                    single-pixel operations

                                                                                                                    neighborhood operations

                                                                                                                    geometric spatial transformations

                                                                                                                    Single-pixel operations

                                                                                                                    - change the values of intensity for the individual pixels ( )s T z

                                                                                                                    where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                    corresponding pixel in the processed image

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Neighborhood operations

                                                                                                                    Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                    in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                    based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                    rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                    intensity by computing the average value of the pixels in Sxy

                                                                                                                    ( )

                                                                                                                    1( ) ( )xyr c S

                                                                                                                    g x y f r cm n

                                                                                                                    The net effect is to perform local blurring in the original image This type of process is

                                                                                                                    used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                    largest region of an image

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Geometric spatial transformations and image registration

                                                                                                                    - modify the spatial relationship between pixels in an image

                                                                                                                    - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                    printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                    predefined set of rules

                                                                                                                    A geometric transformation consists of 2 basic operations

                                                                                                                    1 a spatial transformation of coordinates

                                                                                                                    2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                    pixels

                                                                                                                    The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                    (v w) ndash pixel coordinates in the original image

                                                                                                                    (x y) ndash pixel coordinates in the transformed image

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                    Affine transform

                                                                                                                    11 1211 21 31

                                                                                                                    21 2212 22 33

                                                                                                                    31 32

                                                                                                                    0[ 1] [ 1] [ 1] 0

                                                                                                                    1

                                                                                                                    t tx t v t w t

                                                                                                                    x y v w T v w t ty t v t w t

                                                                                                                    t t

                                                                                                                    (AT)

                                                                                                                    This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                    on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                    result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                    scaling rotation and translation matrices from Table 1

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Affine transformations

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                    the process we have to assign intensity values to those locations This task is done by

                                                                                                                    using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                    In practice we can use equation (AT) in two basic ways

                                                                                                                    forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                    location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                    Problems

                                                                                                                    - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                    the same location in the output image

                                                                                                                    - some output locations have no correspondent in the original image (no intensity

                                                                                                                    assignment)

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                    computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                    It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                    pixel value

                                                                                                                    Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                    numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Image registration ndash align two or more images of the same scene

                                                                                                                    In image registration we have available the input and output images but the specific

                                                                                                                    transformation that produced the output image from the input is generally unknown

                                                                                                                    The problem is to estimate the transformation function and then use it to register the two

                                                                                                                    images

                                                                                                                    - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                    same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                    - align images of a given location taken by the same instrument at different moments

                                                                                                                    of time (satellite images)

                                                                                                                    Solving the problem using tie points (also called control points) which are

                                                                                                                    corresponding points whose locations are known precisely in the input and reference

                                                                                                                    image

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    How to select tie points

                                                                                                                    - interactively selecting them

                                                                                                                    - use of algorithms that try to detect these points

                                                                                                                    - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                    the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                    marks) directly on all images captured by the system which can be used as guides

                                                                                                                    for establishing tie points

                                                                                                                    The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                    of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                    a bilinear approximation is given by

                                                                                                                    1 2 3 4

                                                                                                                    5 6 7 8

                                                                                                                    x c v c w c v w cy c v c w c v w c

                                                                                                                    (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                    frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                    subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                    subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                    The number of tie points and the sophistication of the model required to solve the register

                                                                                                                    problem depend on the severity of the geometrical distortion

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Probabilistic Methods

                                                                                                                    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                    p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                    ( ) kk

                                                                                                                    np zM N

                                                                                                                    nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                    pixels in the image) 1

                                                                                                                    0( ) 1

                                                                                                                    L

                                                                                                                    kk

                                                                                                                    p z

                                                                                                                    The mean (average) intensity of an image is given by 1

                                                                                                                    0( )

                                                                                                                    L

                                                                                                                    k kk

                                                                                                                    m z p z

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    The variance of the intensities is 1

                                                                                                                    2 2

                                                                                                                    0( ) ( )

                                                                                                                    L

                                                                                                                    k kk

                                                                                                                    z m p z

                                                                                                                    The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                    measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                    ( ) is used

                                                                                                                    The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                    0( ) ( ) ( )

                                                                                                                    Ln

                                                                                                                    n k kk

                                                                                                                    z z m p z

                                                                                                                    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                    3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                    ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                    mean

                                                                                                                    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                    Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                    Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                    Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Intensity Transformations and Spatial Filtering

                                                                                                                    ( ) ( )g x y T f x y

                                                                                                                    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                    neighborhood of (x y)

                                                                                                                    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                    and much smaller in size than the image

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                    called spatial filter (spatial mask kernel template or window)

                                                                                                                    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                    ( )s T r

                                                                                                                    s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                    Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                    darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                    is called contrast stretching

                                                                                                                    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                    thresholding function

                                                                                                                    Some Basic Intensity Transformation Functions

                                                                                                                    Image Negatives

                                                                                                                    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                    - equivalent of a photographic negative

                                                                                                                    - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                    image

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Original Negative image

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                    Some basic intensity transformation functions

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                    range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                    while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                    transformation The log functions compress the dynamic range of images with large

                                                                                                                    variations in pixel values

                                                                                                                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Power-Law (Gamma) Transformations

                                                                                                                    - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                    Plots of gamma transformation for different values of γ (c=1)

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                    of output values with the opposite being true for higher values of input values The

                                                                                                                    curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                    1c - identity transformation

                                                                                                                    A variety of devices used for image capture printing and display respond according to a

                                                                                                                    power law The process used to correct these power-law response phenomena is called

                                                                                                                    gamma correction

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Piecewise-Linear Transformations Functions

                                                                                                                    Contrast stretching

                                                                                                                    - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                    intensity range of the recording tool or display device

                                                                                                                    a b c d Fig5

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    11

                                                                                                                    1

                                                                                                                    2 1 1 21 2

                                                                                                                    2 1 2 1

                                                                                                                    22

                                                                                                                    2

                                                                                                                    [0 ]

                                                                                                                    ( ) ( )( ) [ ]( ) ( )

                                                                                                                    ( 1 ) [ 1]( 1 )

                                                                                                                    s r r rrs r r s r rT r r r r

                                                                                                                    r r r rs L r r r L

                                                                                                                    L r

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    1 1 2 2r s r s identity transformation (no change)

                                                                                                                    1 2 1 2 0 1r r s s L thresholding function

                                                                                                                    Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                    in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                    from their original range to the full range [0 L-1]

                                                                                                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                    2 2 1r s m L where m is the mean gray level in the image

                                                                                                                    The original image on which these results are based is a scanning electron microscope

                                                                                                                    image of pollen magnified approximately 700 times

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Intensity-level slicing

                                                                                                                    - highlighting a specific range of intensities in an image

                                                                                                                    There are two approaches for intensity-level slicing

                                                                                                                    1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                    another (say black) all other intensities (Figure 311 (a))

                                                                                                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                    intensities in the image (Figure 311 (b))

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                    the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                    Highlights range [A B] and preserves all other intensities

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                    blockageshellip)

                                                                                                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                    image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                    Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Bit-plane slicing

                                                                                                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                    This technique highlights the contribution made to the whole image appearances by each

                                                                                                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    Digital Image Processing

                                                                                                                    Week 1

                                                                                                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                    • DIP 1 2017
                                                                                                                    • DIP 02 (2017)

                                                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                                                      Week 1Week 1

                                                                                                                      Illustration of Mach band effectPercieved intensityis not a simple function of actual intensity

                                                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                                                      Week 1Week 1

                                                                                                                      All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                                                      Week 1Week 1

                                                                                                                      Optical illusions

                                                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                                                      Week 1Week 1

                                                                                                                      ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                                                      Week 1Week 1

                                                                                                                      Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                                      gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                                      quantities that describe the quality of a chromatic light source radiance

                                                                                                                      the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                                      measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                                                      Week 1Week 1

                                                                                                                      For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                                      brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                                                      Week 1Week 1

                                                                                                                      the physical meaning is determined by the source of the image

                                                                                                                      ( )f D f x y

                                                                                                                      Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                                      f(xy) ndash characterized by two components

                                                                                                                      i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                                      r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                                      ( ) ( ) ( )

                                                                                                                      0 ( ) 0 ( ) 1

                                                                                                                      f x y i x y r x y

                                                                                                                      i x y r x y

                                                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                                                      Week 1Week 1

                                                                                                                      r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                                      i(xy) ndash determined by the illumination source

                                                                                                                      r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                                      is called gray (or intensity) scale

                                                                                                                      In practice

                                                                                                                      min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                                      indoor values without additional illuminationmin max10 1000L L

                                                                                                                      black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                                      min maxL L

                                                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                                                      Week 1Week 1

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Image Sampling and Quantization

                                                                                                                      - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                                      scene

                                                                                                                      converting a continuous image f to digital form

                                                                                                                      - digitizing (x y) is called sampling

                                                                                                                      - digitizing f(x y) is called quantization

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                      (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                      ( )

                                                                                                                      ( 10) ( 11) ( 1 1)

                                                                                                                      f f f Nf f f N

                                                                                                                      f x y

                                                                                                                      f M f M f M N

                                                                                                                      image element pixel

                                                                                                                      00 01 0 1

                                                                                                                      10 11 1 1

                                                                                                                      10 11 1 1

                                                                                                                      ( ) ( )

                                                                                                                      N

                                                                                                                      i jN M N

                                                                                                                      i j

                                                                                                                      M M M N

                                                                                                                      a a aa f x i y j f i ja a a

                                                                                                                      Aa

                                                                                                                      a a a

                                                                                                                      f(00) ndash the upper left corner of the image

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      M N ge 0 L=2k

                                                                                                                      [0 1]i j i ja a L

                                                                                                                      Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Number of bits required to store a digitized image

                                                                                                                      for 2 b M N k M N b N k

                                                                                                                      When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                      Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                      Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                      (eg 100 line pairs per mm)

                                                                                                                      Dots per unit distance are commonly used in printing and publishing

                                                                                                                      In US the measure is expressed in dots per inch (dpi)

                                                                                                                      (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                      Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                      The number of intensity levels (L) is determined by hardware considerations

                                                                                                                      L=2k ndash most common k = 8

                                                                                                                      Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                      150 dpi (lower left) 72 dpi (lower right)

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Reducing the number of gray levels 256 128 64 32

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Reducing the number of gray levels 16 8 4 2

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                      Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                      Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                      Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                      750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                      same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                      image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                      Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                      Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                      intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                      This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                      straight edges

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                      where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                      be written using the 4 nearest neighbors of point (x y)

                                                                                                                      Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                      modest increase in computational effort

                                                                                                                      Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                      nearest neighbors of the point 3 3

                                                                                                                      0 0

                                                                                                                      ( ) i ji j

                                                                                                                      i jv x y c x y

                                                                                                                      The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                      intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                      0 0

                                                                                                                      ( )i ji j

                                                                                                                      i jc x y x y

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                      bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                      programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                      Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                      the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                      then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                      neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                      Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                      interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                      from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Neighbors of a Pixel

                                                                                                                      A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                      This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                      The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                      and are denoted ND(p)

                                                                                                                      The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                      N8 (p)

                                                                                                                      If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                      fall outside the image

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Adjacency Connectivity Regions Boundaries

                                                                                                                      Denote by V the set of intensity levels used to define adjacency

                                                                                                                      - in a binary image V 01 (V=0 V=1)

                                                                                                                      - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                      We consider 3 types of adjacency

                                                                                                                      (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                      (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                      (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                      m-adjacent if

                                                                                                                      4( )q N p or

                                                                                                                      ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                      Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                      ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      binary image

                                                                                                                      0 1 1 0 1 1 0 1 1

                                                                                                                      1 0 1 0 0 1 0 0 1 0

                                                                                                                      0 0 1 0 0 1 0 0 1

                                                                                                                      V

                                                                                                                      The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                      8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                      m-adjacency

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                      is a sequence of distinct pixels with coordinates

                                                                                                                      and are adjacent 0 0 1 1

                                                                                                                      1 1

                                                                                                                      ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                      n n

                                                                                                                      i i i i

                                                                                                                      x y x y x y x y s tx y x y i n

                                                                                                                      The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                      Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                      Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                      in S if there exists a path between them consisting only of pixels from S

                                                                                                                      S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                      Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                      Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                      that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                      8-adjacency are considered

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                      touches the image border

                                                                                                                      the complement of 1

                                                                                                                      ( )K

                                                                                                                      cu k u u

                                                                                                                      k

                                                                                                                      R R R R

                                                                                                                      We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                      background of the image

                                                                                                                      The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                      points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                      region that have at least one background neighbor This definition is referred to as the

                                                                                                                      inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                      border in the background

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Distance measures

                                                                                                                      For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                      function or metric if

                                                                                                                      (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                      (b) D(p q) = D(q p)

                                                                                                                      (c) D(p z) le D(p q) + D(q z)

                                                                                                                      The Euclidean distance between p and q is defined as 1

                                                                                                                      2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                      The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                      centered at (x y)

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                      4( ) | | | |D p q x s y t

                                                                                                                      The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                      4

                                                                                                                      22 1 2

                                                                                                                      2 2 1 0 1 22 1 2

                                                                                                                      2

                                                                                                                      D

                                                                                                                      The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                      The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                      8( ) max| | | |D p q x s y t

                                                                                                                      The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      8

                                                                                                                      2 2 2 2 22 1 1 1 2

                                                                                                                      2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                      D

                                                                                                                      The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                      D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                      because these distances involve only the coordinates of the point

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Array versus Matrix Operations

                                                                                                                      An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                      11 12 11 12

                                                                                                                      21 22 21 22

                                                                                                                      a a b ba a b b

                                                                                                                      Array product

                                                                                                                      11 12 11 12 11 11 12 12

                                                                                                                      21 22 21 22 21 21 22 21

                                                                                                                      a a b b a b a ba a b b a b a b

                                                                                                                      Matrix product

                                                                                                                      11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                      21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                      a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                      We assume array operations unless stated otherwise

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Linear versus Nonlinear Operations

                                                                                                                      One of the most important classifications of image-processing methods is whether it is

                                                                                                                      linear or nonlinear

                                                                                                                      ( ) ( )H f x y g x y

                                                                                                                      H is said to be a linear operator if

                                                                                                                      images1 2 1 2

                                                                                                                      1 2

                                                                                                                      ( ) ( ) ( ) ( )

                                                                                                                      H a f x y b f x y a H f x y b H f x y

                                                                                                                      a b f f

                                                                                                                      Example of nonlinear operator

                                                                                                                      the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                      1 2

                                                                                                                      0 2 6 5 1 1

                                                                                                                      2 3 4 7f f a b

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      1 2

                                                                                                                      0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                      2 3 4 7 2 4a f b f

                                                                                                                      0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                      2 3 4 7

                                                                                                                      Arithmetic Operations in Image Processing

                                                                                                                      Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                      f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                      For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                      value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                      The two random variables are uncorrelated when their covariance is 0

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                      used in image enhancement)

                                                                                                                      1

                                                                                                                      1( ) ( )K

                                                                                                                      ii

                                                                                                                      g x y g x yK

                                                                                                                      If the noise satisfies the properties stated above we have

                                                                                                                      2 2( ) ( )

                                                                                                                      1( ) ( ) g x y x yE g x y f x yK

                                                                                                                      ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                      and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                      the average image is

                                                                                                                      ( ) ( )1

                                                                                                                      g x y x yK

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                      the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                      means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                      averaging process increases

                                                                                                                      An important application of image averaging is in the field of astronomy where imaging

                                                                                                                      under very low light levels frequently causes sensor noise to render single images

                                                                                                                      virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                      was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                      64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                      images respectively

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                      100 noisy images

                                                                                                                      a b c d e f

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                      images

                                                                                                                      (a) (b) (c)

                                                                                                                      Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                      significant bit of each pixel (c) the difference between the two images

                                                                                                                      Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                      Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                      images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                      difference between images (a) and (b)

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                      h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                      intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                      source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                      bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                      anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                      images after injection of the contrast medium

                                                                                                                      In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                      Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                      propagates through the various arteries in the area being observed

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      An important application of image multiplication (and division) is shading correction

                                                                                                                      Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                      f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                      When the shading function is known

                                                                                                                      ( )( )( )

                                                                                                                      g x yf x yh x y

                                                                                                                      h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                      approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                      sensor is not available often the shading pattern can be estimated from the image

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      (a) (b) (c)

                                                                                                                      Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                      operations The process consists of multiplying a given image by a mask image that has

                                                                                                                      1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                      image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                      (a) (b) (c)

                                                                                                                      Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                      min( )mf f f

                                                                                                                      0 ( 255)max( )

                                                                                                                      ms

                                                                                                                      m

                                                                                                                      ff K K K

                                                                                                                      f

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Spatial Operations

                                                                                                                      - are performed directly on the pixels of a given image

                                                                                                                      There are three categories of spatial operations

                                                                                                                      single-pixel operations

                                                                                                                      neighborhood operations

                                                                                                                      geometric spatial transformations

                                                                                                                      Single-pixel operations

                                                                                                                      - change the values of intensity for the individual pixels ( )s T z

                                                                                                                      where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                      corresponding pixel in the processed image

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Neighborhood operations

                                                                                                                      Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                      in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                      based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                      rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                      intensity by computing the average value of the pixels in Sxy

                                                                                                                      ( )

                                                                                                                      1( ) ( )xyr c S

                                                                                                                      g x y f r cm n

                                                                                                                      The net effect is to perform local blurring in the original image This type of process is

                                                                                                                      used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                      largest region of an image

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Geometric spatial transformations and image registration

                                                                                                                      - modify the spatial relationship between pixels in an image

                                                                                                                      - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                      printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                      predefined set of rules

                                                                                                                      A geometric transformation consists of 2 basic operations

                                                                                                                      1 a spatial transformation of coordinates

                                                                                                                      2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                      pixels

                                                                                                                      The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                      (v w) ndash pixel coordinates in the original image

                                                                                                                      (x y) ndash pixel coordinates in the transformed image

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                      Affine transform

                                                                                                                      11 1211 21 31

                                                                                                                      21 2212 22 33

                                                                                                                      31 32

                                                                                                                      0[ 1] [ 1] [ 1] 0

                                                                                                                      1

                                                                                                                      t tx t v t w t

                                                                                                                      x y v w T v w t ty t v t w t

                                                                                                                      t t

                                                                                                                      (AT)

                                                                                                                      This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                      on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                      result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                      scaling rotation and translation matrices from Table 1

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Affine transformations

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                      the process we have to assign intensity values to those locations This task is done by

                                                                                                                      using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                      In practice we can use equation (AT) in two basic ways

                                                                                                                      forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                      location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                      Problems

                                                                                                                      - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                      the same location in the output image

                                                                                                                      - some output locations have no correspondent in the original image (no intensity

                                                                                                                      assignment)

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                      computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                      It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                      pixel value

                                                                                                                      Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                      numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      Image registration ndash align two or more images of the same scene

                                                                                                                      In image registration we have available the input and output images but the specific

                                                                                                                      transformation that produced the output image from the input is generally unknown

                                                                                                                      The problem is to estimate the transformation function and then use it to register the two

                                                                                                                      images

                                                                                                                      - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                      same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                      - align images of a given location taken by the same instrument at different moments

                                                                                                                      of time (satellite images)

                                                                                                                      Solving the problem using tie points (also called control points) which are

                                                                                                                      corresponding points whose locations are known precisely in the input and reference

                                                                                                                      image

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                                                                                                                      How to select tie points

                                                                                                                      - interactively selecting them

                                                                                                                      - use of algorithms that try to detect these points

                                                                                                                      - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                      the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                      marks) directly on all images captured by the system which can be used as guides

                                                                                                                      for establishing tie points

                                                                                                                      The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                      of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                      a bilinear approximation is given by

                                                                                                                      1 2 3 4

                                                                                                                      5 6 7 8

                                                                                                                      x c v c w c v w cy c v c w c v w c

                                                                                                                      (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

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                                                                                                                      When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                      frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                      subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                      subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                      The number of tie points and the sophistication of the model required to solve the register

                                                                                                                      problem depend on the severity of the geometrical distortion

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                                                                                                                      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                      Digital Image Processing

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                                                                                                                      Probabilistic Methods

                                                                                                                      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                      p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                      ( ) kk

                                                                                                                      np zM N

                                                                                                                      nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                      pixels in the image) 1

                                                                                                                      0( ) 1

                                                                                                                      L

                                                                                                                      kk

                                                                                                                      p z

                                                                                                                      The mean (average) intensity of an image is given by 1

                                                                                                                      0( )

                                                                                                                      L

                                                                                                                      k kk

                                                                                                                      m z p z

                                                                                                                      Digital Image Processing

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                                                                                                                      The variance of the intensities is 1

                                                                                                                      2 2

                                                                                                                      0( ) ( )

                                                                                                                      L

                                                                                                                      k kk

                                                                                                                      z m p z

                                                                                                                      The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                      measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                      ( ) is used

                                                                                                                      The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                      0( ) ( ) ( )

                                                                                                                      Ln

                                                                                                                      n k kk

                                                                                                                      z z m p z

                                                                                                                      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                      3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                      ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                      Digital Image Processing

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                                                                                                                      3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                      mean

                                                                                                                      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                      Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                      Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                      Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                      Digital Image Processing

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                                                                                                                      Intensity Transformations and Spatial Filtering

                                                                                                                      ( ) ( )g x y T f x y

                                                                                                                      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                      neighborhood of (x y)

                                                                                                                      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                      and much smaller in size than the image

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                                                                                                                      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                      called spatial filter (spatial mask kernel template or window)

                                                                                                                      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                      ( )s T r

                                                                                                                      s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                      Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                      darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                      is called contrast stretching

                                                                                                                      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

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                                                                                                                      Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                      thresholding function

                                                                                                                      Some Basic Intensity Transformation Functions

                                                                                                                      Image Negatives

                                                                                                                      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                      - equivalent of a photographic negative

                                                                                                                      - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                      image

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                                                                                                                      Original Negative image

                                                                                                                      Digital Image Processing

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                                                                                                                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                      Some basic intensity transformation functions

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                                                                                                                      This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                      range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                      while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                      transformation The log functions compress the dynamic range of images with large

                                                                                                                      variations in pixel values

                                                                                                                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

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                                                                                                                      Power-Law (Gamma) Transformations

                                                                                                                      - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                      Plots of gamma transformation for different values of γ (c=1)

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                                                                                                                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                      of output values with the opposite being true for higher values of input values The

                                                                                                                      curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                      1c - identity transformation

                                                                                                                      A variety of devices used for image capture printing and display respond according to a

                                                                                                                      power law The process used to correct these power-law response phenomena is called

                                                                                                                      gamma correction

                                                                                                                      Digital Image Processing

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                                                                                                                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

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                                                                                                                      Piecewise-Linear Transformations Functions

                                                                                                                      Contrast stretching

                                                                                                                      - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                      intensity range of the recording tool or display device

                                                                                                                      a b c d Fig5

                                                                                                                      Digital Image Processing

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                                                                                                                      11

                                                                                                                      1

                                                                                                                      2 1 1 21 2

                                                                                                                      2 1 2 1

                                                                                                                      22

                                                                                                                      2

                                                                                                                      [0 ]

                                                                                                                      ( ) ( )( ) [ ]( ) ( )

                                                                                                                      ( 1 ) [ 1]( 1 )

                                                                                                                      s r r rrs r r s r rT r r r r

                                                                                                                      r r r rs L r r r L

                                                                                                                      L r

                                                                                                                      Digital Image Processing

                                                                                                                      Week 1

                                                                                                                      1 1 2 2r s r s identity transformation (no change)

                                                                                                                      1 2 1 2 0 1r r s s L thresholding function

                                                                                                                      Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                      in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                      from their original range to the full range [0 L-1]

                                                                                                                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                      2 2 1r s m L where m is the mean gray level in the image

                                                                                                                      The original image on which these results are based is a scanning electron microscope

                                                                                                                      image of pollen magnified approximately 700 times

                                                                                                                      Digital Image Processing

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                                                                                                                      Intensity-level slicing

                                                                                                                      - highlighting a specific range of intensities in an image

                                                                                                                      There are two approaches for intensity-level slicing

                                                                                                                      1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                      another (say black) all other intensities (Figure 311 (a))

                                                                                                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                      intensities in the image (Figure 311 (b))

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                                                                                                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                      the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                      Highlights range [A B] and preserves all other intensities

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                                                                                                                      Week 1

                                                                                                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                      blockageshellip)

                                                                                                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                      image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                      Fig 6 - Aortic angiogram and intensity sliced versions

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                                                                                                                      Bit-plane slicing

                                                                                                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                      This technique highlights the contribution made to the whole image appearances by each

                                                                                                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

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                                                                                                                      Digital Image Processing

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                                                                                                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                      • DIP 1 2017
                                                                                                                      • DIP 02 (2017)

                                                                                                                        Digital Image ProcessingDigital Image Processing

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                                                                                                                        All the inner squares have the same intensity but they appear progressively darker as the background becomes lighter

                                                                                                                        Digital Image ProcessingDigital Image Processing

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                                                                                                                        Optical illusions

                                                                                                                        Digital Image ProcessingDigital Image Processing

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                                                                                                                        ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                                                        Digital Image ProcessingDigital Image Processing

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                                                                                                                        Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                                        gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                                        quantities that describe the quality of a chromatic light source radiance

                                                                                                                        the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                                        measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

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                                                                                                                        For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                                        brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                                                        Digital Image ProcessingDigital Image Processing

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                                                                                                                        the physical meaning is determined by the source of the image

                                                                                                                        ( )f D f x y

                                                                                                                        Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                                        f(xy) ndash characterized by two components

                                                                                                                        i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                                        r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                                        ( ) ( ) ( )

                                                                                                                        0 ( ) 0 ( ) 1

                                                                                                                        f x y i x y r x y

                                                                                                                        i x y r x y

                                                                                                                        Digital Image ProcessingDigital Image Processing

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                                                                                                                        r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                                        i(xy) ndash determined by the illumination source

                                                                                                                        r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                                        is called gray (or intensity) scale

                                                                                                                        In practice

                                                                                                                        min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                                        indoor values without additional illuminationmin max10 1000L L

                                                                                                                        black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                                        min maxL L

                                                                                                                        Digital Image ProcessingDigital Image Processing

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                                                                                                                        Digital Image Processing

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                                                                                                                        Image Sampling and Quantization

                                                                                                                        - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                                        scene

                                                                                                                        converting a continuous image f to digital form

                                                                                                                        - digitizing (x y) is called sampling

                                                                                                                        - digitizing f(x y) is called quantization

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                                                                                                                        Digital Image Processing

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                                                                                                                        Continuous image projected onto a sensor array Result of image sampling and quantization

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                                                                                                                        Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                        (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                        ( )

                                                                                                                        ( 10) ( 11) ( 1 1)

                                                                                                                        f f f Nf f f N

                                                                                                                        f x y

                                                                                                                        f M f M f M N

                                                                                                                        image element pixel

                                                                                                                        00 01 0 1

                                                                                                                        10 11 1 1

                                                                                                                        10 11 1 1

                                                                                                                        ( ) ( )

                                                                                                                        N

                                                                                                                        i jN M N

                                                                                                                        i j

                                                                                                                        M M M N

                                                                                                                        a a aa f x i y j f i ja a a

                                                                                                                        Aa

                                                                                                                        a a a

                                                                                                                        f(00) ndash the upper left corner of the image

                                                                                                                        Digital Image Processing

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                                                                                                                        M N ge 0 L=2k

                                                                                                                        [0 1]i j i ja a L

                                                                                                                        Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

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                                                                                                                        Number of bits required to store a digitized image

                                                                                                                        for 2 b M N k M N b N k

                                                                                                                        When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                                        Digital Image Processing

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                                                                                                                        Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                        Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                        Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                        (eg 100 line pairs per mm)

                                                                                                                        Dots per unit distance are commonly used in printing and publishing

                                                                                                                        In US the measure is expressed in dots per inch (dpi)

                                                                                                                        (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                        Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                        The number of intensity levels (L) is determined by hardware considerations

                                                                                                                        L=2k ndash most common k = 8

                                                                                                                        Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                                        Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                        150 dpi (lower left) 72 dpi (lower right)

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                                                                                                                        Reducing the number of gray levels 256 128 64 32

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                                                                                                                        Reducing the number of gray levels 16 8 4 2

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                                                                                                                        Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                        Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                        Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                        Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                        750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                        same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                        image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                        Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                        Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                        intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                        This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                        straight edges

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                                                                                                                        Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                        where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                        be written using the 4 nearest neighbors of point (x y)

                                                                                                                        Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                        modest increase in computational effort

                                                                                                                        Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                        nearest neighbors of the point 3 3

                                                                                                                        0 0

                                                                                                                        ( ) i ji j

                                                                                                                        i jv x y c x y

                                                                                                                        The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                        intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                        0 0

                                                                                                                        ( )i ji j

                                                                                                                        i jc x y x y

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                                                                                                                        Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                        bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                        programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                        Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                        the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                        then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                        neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                        Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                        interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                        from 1250 dpi to 150 dpi (instead of 72 dpi)

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                                                                                                                        Week 1

                                                                                                                        Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

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                                                                                                                        Neighbors of a Pixel

                                                                                                                        A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                        This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                        The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                        and are denoted ND(p)

                                                                                                                        The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                        N8 (p)

                                                                                                                        If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                        fall outside the image

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                                                                                                                        Adjacency Connectivity Regions Boundaries

                                                                                                                        Denote by V the set of intensity levels used to define adjacency

                                                                                                                        - in a binary image V 01 (V=0 V=1)

                                                                                                                        - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                        We consider 3 types of adjacency

                                                                                                                        (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                        (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                        (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                        m-adjacent if

                                                                                                                        4( )q N p or

                                                                                                                        ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                        Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                        ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        binary image

                                                                                                                        0 1 1 0 1 1 0 1 1

                                                                                                                        1 0 1 0 0 1 0 0 1 0

                                                                                                                        0 0 1 0 0 1 0 0 1

                                                                                                                        V

                                                                                                                        The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                        8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                        m-adjacency

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                                                                                                                        Week 1

                                                                                                                        A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                        is a sequence of distinct pixels with coordinates

                                                                                                                        and are adjacent 0 0 1 1

                                                                                                                        1 1

                                                                                                                        ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                        n n

                                                                                                                        i i i i

                                                                                                                        x y x y x y x y s tx y x y i n

                                                                                                                        The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                        Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                        Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                        in S if there exists a path between them consisting only of pixels from S

                                                                                                                        S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                        Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                        Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                        that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                        8-adjacency are considered

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                                                                                                                        Week 1

                                                                                                                        Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                        touches the image border

                                                                                                                        the complement of 1

                                                                                                                        ( )K

                                                                                                                        cu k u u

                                                                                                                        k

                                                                                                                        R R R R

                                                                                                                        We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                        background of the image

                                                                                                                        The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                        points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                        region that have at least one background neighbor This definition is referred to as the

                                                                                                                        inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                        border in the background

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Distance measures

                                                                                                                        For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                        function or metric if

                                                                                                                        (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                        (b) D(p q) = D(q p)

                                                                                                                        (c) D(p z) le D(p q) + D(q z)

                                                                                                                        The Euclidean distance between p and q is defined as 1

                                                                                                                        2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                        The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                        centered at (x y)

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                        4( ) | | | |D p q x s y t

                                                                                                                        The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                        4

                                                                                                                        22 1 2

                                                                                                                        2 2 1 0 1 22 1 2

                                                                                                                        2

                                                                                                                        D

                                                                                                                        The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                        The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                        8( ) max| | | |D p q x s y t

                                                                                                                        The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        8

                                                                                                                        2 2 2 2 22 1 1 1 2

                                                                                                                        2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                        D

                                                                                                                        The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                        D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                        because these distances involve only the coordinates of the point

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Array versus Matrix Operations

                                                                                                                        An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                        11 12 11 12

                                                                                                                        21 22 21 22

                                                                                                                        a a b ba a b b

                                                                                                                        Array product

                                                                                                                        11 12 11 12 11 11 12 12

                                                                                                                        21 22 21 22 21 21 22 21

                                                                                                                        a a b b a b a ba a b b a b a b

                                                                                                                        Matrix product

                                                                                                                        11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                        21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                        a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                        We assume array operations unless stated otherwise

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Linear versus Nonlinear Operations

                                                                                                                        One of the most important classifications of image-processing methods is whether it is

                                                                                                                        linear or nonlinear

                                                                                                                        ( ) ( )H f x y g x y

                                                                                                                        H is said to be a linear operator if

                                                                                                                        images1 2 1 2

                                                                                                                        1 2

                                                                                                                        ( ) ( ) ( ) ( )

                                                                                                                        H a f x y b f x y a H f x y b H f x y

                                                                                                                        a b f f

                                                                                                                        Example of nonlinear operator

                                                                                                                        the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                        1 2

                                                                                                                        0 2 6 5 1 1

                                                                                                                        2 3 4 7f f a b

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        1 2

                                                                                                                        0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                        2 3 4 7 2 4a f b f

                                                                                                                        0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                        2 3 4 7

                                                                                                                        Arithmetic Operations in Image Processing

                                                                                                                        Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                        f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                        For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                        value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                        The two random variables are uncorrelated when their covariance is 0

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                        used in image enhancement)

                                                                                                                        1

                                                                                                                        1( ) ( )K

                                                                                                                        ii

                                                                                                                        g x y g x yK

                                                                                                                        If the noise satisfies the properties stated above we have

                                                                                                                        2 2( ) ( )

                                                                                                                        1( ) ( ) g x y x yE g x y f x yK

                                                                                                                        ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                        and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                        the average image is

                                                                                                                        ( ) ( )1

                                                                                                                        g x y x yK

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                        the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                        means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                        averaging process increases

                                                                                                                        An important application of image averaging is in the field of astronomy where imaging

                                                                                                                        under very low light levels frequently causes sensor noise to render single images

                                                                                                                        virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                        was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                        64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                        images respectively

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                        100 noisy images

                                                                                                                        a b c d e f

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                        images

                                                                                                                        (a) (b) (c)

                                                                                                                        Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                        significant bit of each pixel (c) the difference between the two images

                                                                                                                        Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                        Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                        images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                        difference between images (a) and (b)

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                        h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                        intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                        source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                        bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                        anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                        images after injection of the contrast medium

                                                                                                                        In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                        Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                        propagates through the various arteries in the area being observed

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        An important application of image multiplication (and division) is shading correction

                                                                                                                        Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                        f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                        When the shading function is known

                                                                                                                        ( )( )( )

                                                                                                                        g x yf x yh x y

                                                                                                                        h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                        approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                        sensor is not available often the shading pattern can be estimated from the image

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        (a) (b) (c)

                                                                                                                        Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                        operations The process consists of multiplying a given image by a mask image that has

                                                                                                                        1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                        image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                        (a) (b) (c)

                                                                                                                        Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                        min( )mf f f

                                                                                                                        0 ( 255)max( )

                                                                                                                        ms

                                                                                                                        m

                                                                                                                        ff K K K

                                                                                                                        f

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Spatial Operations

                                                                                                                        - are performed directly on the pixels of a given image

                                                                                                                        There are three categories of spatial operations

                                                                                                                        single-pixel operations

                                                                                                                        neighborhood operations

                                                                                                                        geometric spatial transformations

                                                                                                                        Single-pixel operations

                                                                                                                        - change the values of intensity for the individual pixels ( )s T z

                                                                                                                        where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                        corresponding pixel in the processed image

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Neighborhood operations

                                                                                                                        Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                        in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                        based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                        rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                        intensity by computing the average value of the pixels in Sxy

                                                                                                                        ( )

                                                                                                                        1( ) ( )xyr c S

                                                                                                                        g x y f r cm n

                                                                                                                        The net effect is to perform local blurring in the original image This type of process is

                                                                                                                        used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                        largest region of an image

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Geometric spatial transformations and image registration

                                                                                                                        - modify the spatial relationship between pixels in an image

                                                                                                                        - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                        printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                        predefined set of rules

                                                                                                                        A geometric transformation consists of 2 basic operations

                                                                                                                        1 a spatial transformation of coordinates

                                                                                                                        2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                        pixels

                                                                                                                        The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                        (v w) ndash pixel coordinates in the original image

                                                                                                                        (x y) ndash pixel coordinates in the transformed image

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                        Affine transform

                                                                                                                        11 1211 21 31

                                                                                                                        21 2212 22 33

                                                                                                                        31 32

                                                                                                                        0[ 1] [ 1] [ 1] 0

                                                                                                                        1

                                                                                                                        t tx t v t w t

                                                                                                                        x y v w T v w t ty t v t w t

                                                                                                                        t t

                                                                                                                        (AT)

                                                                                                                        This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                        on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                        result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                        scaling rotation and translation matrices from Table 1

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Affine transformations

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                        the process we have to assign intensity values to those locations This task is done by

                                                                                                                        using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                        In practice we can use equation (AT) in two basic ways

                                                                                                                        forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                        location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                        Problems

                                                                                                                        - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                        the same location in the output image

                                                                                                                        - some output locations have no correspondent in the original image (no intensity

                                                                                                                        assignment)

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                        computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                        It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                        pixel value

                                                                                                                        Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                        numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Image registration ndash align two or more images of the same scene

                                                                                                                        In image registration we have available the input and output images but the specific

                                                                                                                        transformation that produced the output image from the input is generally unknown

                                                                                                                        The problem is to estimate the transformation function and then use it to register the two

                                                                                                                        images

                                                                                                                        - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                        same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                        - align images of a given location taken by the same instrument at different moments

                                                                                                                        of time (satellite images)

                                                                                                                        Solving the problem using tie points (also called control points) which are

                                                                                                                        corresponding points whose locations are known precisely in the input and reference

                                                                                                                        image

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        How to select tie points

                                                                                                                        - interactively selecting them

                                                                                                                        - use of algorithms that try to detect these points

                                                                                                                        - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                        the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                        marks) directly on all images captured by the system which can be used as guides

                                                                                                                        for establishing tie points

                                                                                                                        The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                        of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                        a bilinear approximation is given by

                                                                                                                        1 2 3 4

                                                                                                                        5 6 7 8

                                                                                                                        x c v c w c v w cy c v c w c v w c

                                                                                                                        (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                        frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                        subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                        subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                        The number of tie points and the sophistication of the model required to solve the register

                                                                                                                        problem depend on the severity of the geometrical distortion

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Probabilistic Methods

                                                                                                                        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                        p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                        ( ) kk

                                                                                                                        np zM N

                                                                                                                        nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                        pixels in the image) 1

                                                                                                                        0( ) 1

                                                                                                                        L

                                                                                                                        kk

                                                                                                                        p z

                                                                                                                        The mean (average) intensity of an image is given by 1

                                                                                                                        0( )

                                                                                                                        L

                                                                                                                        k kk

                                                                                                                        m z p z

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        The variance of the intensities is 1

                                                                                                                        2 2

                                                                                                                        0( ) ( )

                                                                                                                        L

                                                                                                                        k kk

                                                                                                                        z m p z

                                                                                                                        The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                        measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                        ( ) is used

                                                                                                                        The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                        0( ) ( ) ( )

                                                                                                                        Ln

                                                                                                                        n k kk

                                                                                                                        z z m p z

                                                                                                                        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                        3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                        ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                        mean

                                                                                                                        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                        Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                        Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                        Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Intensity Transformations and Spatial Filtering

                                                                                                                        ( ) ( )g x y T f x y

                                                                                                                        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                        neighborhood of (x y)

                                                                                                                        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                        and much smaller in size than the image

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                        called spatial filter (spatial mask kernel template or window)

                                                                                                                        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                        ( )s T r

                                                                                                                        s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                        Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                        darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                        is called contrast stretching

                                                                                                                        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                        thresholding function

                                                                                                                        Some Basic Intensity Transformation Functions

                                                                                                                        Image Negatives

                                                                                                                        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                        - equivalent of a photographic negative

                                                                                                                        - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                        image

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Original Negative image

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                        Some basic intensity transformation functions

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                        range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                        while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                        transformation The log functions compress the dynamic range of images with large

                                                                                                                        variations in pixel values

                                                                                                                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Power-Law (Gamma) Transformations

                                                                                                                        - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                        Plots of gamma transformation for different values of γ (c=1)

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                        of output values with the opposite being true for higher values of input values The

                                                                                                                        curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                        1c - identity transformation

                                                                                                                        A variety of devices used for image capture printing and display respond according to a

                                                                                                                        power law The process used to correct these power-law response phenomena is called

                                                                                                                        gamma correction

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Piecewise-Linear Transformations Functions

                                                                                                                        Contrast stretching

                                                                                                                        - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                        intensity range of the recording tool or display device

                                                                                                                        a b c d Fig5

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        11

                                                                                                                        1

                                                                                                                        2 1 1 21 2

                                                                                                                        2 1 2 1

                                                                                                                        22

                                                                                                                        2

                                                                                                                        [0 ]

                                                                                                                        ( ) ( )( ) [ ]( ) ( )

                                                                                                                        ( 1 ) [ 1]( 1 )

                                                                                                                        s r r rrs r r s r rT r r r r

                                                                                                                        r r r rs L r r r L

                                                                                                                        L r

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        1 1 2 2r s r s identity transformation (no change)

                                                                                                                        1 2 1 2 0 1r r s s L thresholding function

                                                                                                                        Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                        in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                        from their original range to the full range [0 L-1]

                                                                                                                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                        2 2 1r s m L where m is the mean gray level in the image

                                                                                                                        The original image on which these results are based is a scanning electron microscope

                                                                                                                        image of pollen magnified approximately 700 times

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Intensity-level slicing

                                                                                                                        - highlighting a specific range of intensities in an image

                                                                                                                        There are two approaches for intensity-level slicing

                                                                                                                        1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                        another (say black) all other intensities (Figure 311 (a))

                                                                                                                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                        intensities in the image (Figure 311 (b))

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                        the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                        Highlights range [A B] and preserves all other intensities

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                        blockageshellip)

                                                                                                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                        image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                        Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Bit-plane slicing

                                                                                                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                        This technique highlights the contribution made to the whole image appearances by each

                                                                                                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        Digital Image Processing

                                                                                                                        Week 1

                                                                                                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                        • DIP 1 2017
                                                                                                                        • DIP 02 (2017)

                                                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                                                          Week 1Week 1

                                                                                                                          Optical illusions

                                                                                                                          Digital Image ProcessingDigital Image Processing

                                                                                                                          Week 1Week 1

                                                                                                                          ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                                                          Digital Image ProcessingDigital Image Processing

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                                                                                                                          Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                                          gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                                          quantities that describe the quality of a chromatic light source radiance

                                                                                                                          the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                                          measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

                                                                                                                          Digital Image ProcessingDigital Image Processing

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                                                                                                                          For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                                          brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                                                          Digital Image ProcessingDigital Image Processing

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                                                                                                                          the physical meaning is determined by the source of the image

                                                                                                                          ( )f D f x y

                                                                                                                          Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                                          f(xy) ndash characterized by two components

                                                                                                                          i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                                          r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                                          ( ) ( ) ( )

                                                                                                                          0 ( ) 0 ( ) 1

                                                                                                                          f x y i x y r x y

                                                                                                                          i x y r x y

                                                                                                                          Digital Image ProcessingDigital Image Processing

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                                                                                                                          r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                                          i(xy) ndash determined by the illumination source

                                                                                                                          r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                                          is called gray (or intensity) scale

                                                                                                                          In practice

                                                                                                                          min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                                          indoor values without additional illuminationmin max10 1000L L

                                                                                                                          black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                                          min maxL L

                                                                                                                          Digital Image ProcessingDigital Image Processing

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                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          Image Sampling and Quantization

                                                                                                                          - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                                          scene

                                                                                                                          converting a continuous image f to digital form

                                                                                                                          - digitizing (x y) is called sampling

                                                                                                                          - digitizing f(x y) is called quantization

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                                                                                                                          Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                                          Digital Image Processing

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                                                                                                                          Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                          (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                          ( )

                                                                                                                          ( 10) ( 11) ( 1 1)

                                                                                                                          f f f Nf f f N

                                                                                                                          f x y

                                                                                                                          f M f M f M N

                                                                                                                          image element pixel

                                                                                                                          00 01 0 1

                                                                                                                          10 11 1 1

                                                                                                                          10 11 1 1

                                                                                                                          ( ) ( )

                                                                                                                          N

                                                                                                                          i jN M N

                                                                                                                          i j

                                                                                                                          M M M N

                                                                                                                          a a aa f x i y j f i ja a a

                                                                                                                          Aa

                                                                                                                          a a a

                                                                                                                          f(00) ndash the upper left corner of the image

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          M N ge 0 L=2k

                                                                                                                          [0 1]i j i ja a L

                                                                                                                          Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          Digital Image Processing

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                                                                                                                          Number of bits required to store a digitized image

                                                                                                                          for 2 b M N k M N b N k

                                                                                                                          When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                          Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                          Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                          (eg 100 line pairs per mm)

                                                                                                                          Dots per unit distance are commonly used in printing and publishing

                                                                                                                          In US the measure is expressed in dots per inch (dpi)

                                                                                                                          (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                          Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                          The number of intensity levels (L) is determined by hardware considerations

                                                                                                                          L=2k ndash most common k = 8

                                                                                                                          Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                                          Week 1

                                                                                                                          Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                          150 dpi (lower left) 72 dpi (lower right)

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                                                                                                                          Reducing the number of gray levels 256 128 64 32

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                                                                                                                          Reducing the number of gray levels 16 8 4 2

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                                                                                                                          Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                          Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                          Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                          Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                          750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                          same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                          image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                          Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                          Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                          intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                          This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                          straight edges

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                                                                                                                          Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                          where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                          be written using the 4 nearest neighbors of point (x y)

                                                                                                                          Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                          modest increase in computational effort

                                                                                                                          Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                          nearest neighbors of the point 3 3

                                                                                                                          0 0

                                                                                                                          ( ) i ji j

                                                                                                                          i jv x y c x y

                                                                                                                          The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                          intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                          0 0

                                                                                                                          ( )i ji j

                                                                                                                          i jc x y x y

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                                                                                                                          Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                          bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                          programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                          Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                          the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                          then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                          neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                          Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                          interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                          from 1250 dpi to 150 dpi (instead of 72 dpi)

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                                                                                                                          Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

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                                                                                                                          Neighbors of a Pixel

                                                                                                                          A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                          This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                          The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                          and are denoted ND(p)

                                                                                                                          The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                          N8 (p)

                                                                                                                          If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                          fall outside the image

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                                                                                                                          Adjacency Connectivity Regions Boundaries

                                                                                                                          Denote by V the set of intensity levels used to define adjacency

                                                                                                                          - in a binary image V 01 (V=0 V=1)

                                                                                                                          - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                          We consider 3 types of adjacency

                                                                                                                          (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                          (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                          (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                          m-adjacent if

                                                                                                                          4( )q N p or

                                                                                                                          ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                          Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                          ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          binary image

                                                                                                                          0 1 1 0 1 1 0 1 1

                                                                                                                          1 0 1 0 0 1 0 0 1 0

                                                                                                                          0 0 1 0 0 1 0 0 1

                                                                                                                          V

                                                                                                                          The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                          8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                          m-adjacency

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                          is a sequence of distinct pixels with coordinates

                                                                                                                          and are adjacent 0 0 1 1

                                                                                                                          1 1

                                                                                                                          ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                          n n

                                                                                                                          i i i i

                                                                                                                          x y x y x y x y s tx y x y i n

                                                                                                                          The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                          Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                          Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                          in S if there exists a path between them consisting only of pixels from S

                                                                                                                          S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                          Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                          Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                          that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                          8-adjacency are considered

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                                                                                                                          Week 1

                                                                                                                          Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                          touches the image border

                                                                                                                          the complement of 1

                                                                                                                          ( )K

                                                                                                                          cu k u u

                                                                                                                          k

                                                                                                                          R R R R

                                                                                                                          We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                          background of the image

                                                                                                                          The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                          points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                          region that have at least one background neighbor This definition is referred to as the

                                                                                                                          inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                          border in the background

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          Distance measures

                                                                                                                          For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                          function or metric if

                                                                                                                          (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                          (b) D(p q) = D(q p)

                                                                                                                          (c) D(p z) le D(p q) + D(q z)

                                                                                                                          The Euclidean distance between p and q is defined as 1

                                                                                                                          2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                          The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                          centered at (x y)

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                          4( ) | | | |D p q x s y t

                                                                                                                          The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                          4

                                                                                                                          22 1 2

                                                                                                                          2 2 1 0 1 22 1 2

                                                                                                                          2

                                                                                                                          D

                                                                                                                          The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                          The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                          8( ) max| | | |D p q x s y t

                                                                                                                          The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          8

                                                                                                                          2 2 2 2 22 1 1 1 2

                                                                                                                          2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                          D

                                                                                                                          The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                          D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                          because these distances involve only the coordinates of the point

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          Array versus Matrix Operations

                                                                                                                          An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                          11 12 11 12

                                                                                                                          21 22 21 22

                                                                                                                          a a b ba a b b

                                                                                                                          Array product

                                                                                                                          11 12 11 12 11 11 12 12

                                                                                                                          21 22 21 22 21 21 22 21

                                                                                                                          a a b b a b a ba a b b a b a b

                                                                                                                          Matrix product

                                                                                                                          11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                          21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                          a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                          We assume array operations unless stated otherwise

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                                                                                                                          Week 1

                                                                                                                          Linear versus Nonlinear Operations

                                                                                                                          One of the most important classifications of image-processing methods is whether it is

                                                                                                                          linear or nonlinear

                                                                                                                          ( ) ( )H f x y g x y

                                                                                                                          H is said to be a linear operator if

                                                                                                                          images1 2 1 2

                                                                                                                          1 2

                                                                                                                          ( ) ( ) ( ) ( )

                                                                                                                          H a f x y b f x y a H f x y b H f x y

                                                                                                                          a b f f

                                                                                                                          Example of nonlinear operator

                                                                                                                          the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                          1 2

                                                                                                                          0 2 6 5 1 1

                                                                                                                          2 3 4 7f f a b

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          1 2

                                                                                                                          0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                          2 3 4 7 2 4a f b f

                                                                                                                          0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                          2 3 4 7

                                                                                                                          Arithmetic Operations in Image Processing

                                                                                                                          Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                          f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                          For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                          value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                          The two random variables are uncorrelated when their covariance is 0

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                          used in image enhancement)

                                                                                                                          1

                                                                                                                          1( ) ( )K

                                                                                                                          ii

                                                                                                                          g x y g x yK

                                                                                                                          If the noise satisfies the properties stated above we have

                                                                                                                          2 2( ) ( )

                                                                                                                          1( ) ( ) g x y x yE g x y f x yK

                                                                                                                          ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                          and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                          the average image is

                                                                                                                          ( ) ( )1

                                                                                                                          g x y x yK

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                          the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                          means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                          averaging process increases

                                                                                                                          An important application of image averaging is in the field of astronomy where imaging

                                                                                                                          under very low light levels frequently causes sensor noise to render single images

                                                                                                                          virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                          was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                          64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                          images respectively

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                          100 noisy images

                                                                                                                          a b c d e f

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                          images

                                                                                                                          (a) (b) (c)

                                                                                                                          Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                          significant bit of each pixel (c) the difference between the two images

                                                                                                                          Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                          Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                          images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                          difference between images (a) and (b)

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                          h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                          intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                          source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                          bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                          anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                          images after injection of the contrast medium

                                                                                                                          In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                          Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                          propagates through the various arteries in the area being observed

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          An important application of image multiplication (and division) is shading correction

                                                                                                                          Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                          f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                          When the shading function is known

                                                                                                                          ( )( )( )

                                                                                                                          g x yf x yh x y

                                                                                                                          h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                          approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                          sensor is not available often the shading pattern can be estimated from the image

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          (a) (b) (c)

                                                                                                                          Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                          operations The process consists of multiplying a given image by a mask image that has

                                                                                                                          1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                          image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                          (a) (b) (c)

                                                                                                                          Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                          min( )mf f f

                                                                                                                          0 ( 255)max( )

                                                                                                                          ms

                                                                                                                          m

                                                                                                                          ff K K K

                                                                                                                          f

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          Spatial Operations

                                                                                                                          - are performed directly on the pixels of a given image

                                                                                                                          There are three categories of spatial operations

                                                                                                                          single-pixel operations

                                                                                                                          neighborhood operations

                                                                                                                          geometric spatial transformations

                                                                                                                          Single-pixel operations

                                                                                                                          - change the values of intensity for the individual pixels ( )s T z

                                                                                                                          where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                          corresponding pixel in the processed image

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          Neighborhood operations

                                                                                                                          Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                          in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                          based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                          rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                          intensity by computing the average value of the pixels in Sxy

                                                                                                                          ( )

                                                                                                                          1( ) ( )xyr c S

                                                                                                                          g x y f r cm n

                                                                                                                          The net effect is to perform local blurring in the original image This type of process is

                                                                                                                          used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                          largest region of an image

                                                                                                                          Digital Image Processing

                                                                                                                          Week 1

                                                                                                                          Geometric spatial transformations and image registration

                                                                                                                          - modify the spatial relationship between pixels in an image

                                                                                                                          - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                          printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                          predefined set of rules

                                                                                                                          A geometric transformation consists of 2 basic operations

                                                                                                                          1 a spatial transformation of coordinates

                                                                                                                          2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                          pixels

                                                                                                                          The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                          (v w) ndash pixel coordinates in the original image

                                                                                                                          (x y) ndash pixel coordinates in the transformed image

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                                                                                                                          [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                          Affine transform

                                                                                                                          11 1211 21 31

                                                                                                                          21 2212 22 33

                                                                                                                          31 32

                                                                                                                          0[ 1] [ 1] [ 1] 0

                                                                                                                          1

                                                                                                                          t tx t v t w t

                                                                                                                          x y v w T v w t ty t v t w t

                                                                                                                          t t

                                                                                                                          (AT)

                                                                                                                          This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                          on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                          result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                          scaling rotation and translation matrices from Table 1

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                                                                                                                          Week 1

                                                                                                                          Affine transformations

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                                                                                                                          The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                          the process we have to assign intensity values to those locations This task is done by

                                                                                                                          using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                          In practice we can use equation (AT) in two basic ways

                                                                                                                          forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                          location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                          Problems

                                                                                                                          - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                          the same location in the output image

                                                                                                                          - some output locations have no correspondent in the original image (no intensity

                                                                                                                          assignment)

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                                                                                                                          inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                          computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                          It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                          pixel value

                                                                                                                          Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                          numerous commercial implementations of spatial transformations (MATLAB for ex)

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                                                                                                                          Image registration ndash align two or more images of the same scene

                                                                                                                          In image registration we have available the input and output images but the specific

                                                                                                                          transformation that produced the output image from the input is generally unknown

                                                                                                                          The problem is to estimate the transformation function and then use it to register the two

                                                                                                                          images

                                                                                                                          - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                          same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                          - align images of a given location taken by the same instrument at different moments

                                                                                                                          of time (satellite images)

                                                                                                                          Solving the problem using tie points (also called control points) which are

                                                                                                                          corresponding points whose locations are known precisely in the input and reference

                                                                                                                          image

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                                                                                                                          How to select tie points

                                                                                                                          - interactively selecting them

                                                                                                                          - use of algorithms that try to detect these points

                                                                                                                          - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                          the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                          marks) directly on all images captured by the system which can be used as guides

                                                                                                                          for establishing tie points

                                                                                                                          The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                          of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                          a bilinear approximation is given by

                                                                                                                          1 2 3 4

                                                                                                                          5 6 7 8

                                                                                                                          x c v c w c v w cy c v c w c v w c

                                                                                                                          (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

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                                                                                                                          When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                          frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                          subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                          subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                          The number of tie points and the sophistication of the model required to solve the register

                                                                                                                          problem depend on the severity of the geometrical distortion

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                                                                                                                          a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

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                                                                                                                          Probabilistic Methods

                                                                                                                          zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                          p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                          ( ) kk

                                                                                                                          np zM N

                                                                                                                          nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                          pixels in the image) 1

                                                                                                                          0( ) 1

                                                                                                                          L

                                                                                                                          kk

                                                                                                                          p z

                                                                                                                          The mean (average) intensity of an image is given by 1

                                                                                                                          0( )

                                                                                                                          L

                                                                                                                          k kk

                                                                                                                          m z p z

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                                                                                                                          The variance of the intensities is 1

                                                                                                                          2 2

                                                                                                                          0( ) ( )

                                                                                                                          L

                                                                                                                          k kk

                                                                                                                          z m p z

                                                                                                                          The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                          measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                          ( ) is used

                                                                                                                          The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                          0( ) ( ) ( )

                                                                                                                          Ln

                                                                                                                          n k kk

                                                                                                                          z z m p z

                                                                                                                          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                          3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                          ( 3( ) 0z the intensities are biased to values lower than the mean

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                                                                                                                          3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                          mean

                                                                                                                          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                          Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                          Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                          Figure 1(c) ndash standard deviation 492 (variance = 24206)

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                                                                                                                          Intensity Transformations and Spatial Filtering

                                                                                                                          ( ) ( )g x y T f x y

                                                                                                                          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                          neighborhood of (x y)

                                                                                                                          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                          and much smaller in size than the image

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                                                                                                                          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                          called spatial filter (spatial mask kernel template or window)

                                                                                                                          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                          ( )s T r

                                                                                                                          s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                          Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                          darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                          is called contrast stretching

                                                                                                                          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

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                                                                                                                          Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                          thresholding function

                                                                                                                          Some Basic Intensity Transformation Functions

                                                                                                                          Image Negatives

                                                                                                                          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                          - equivalent of a photographic negative

                                                                                                                          - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                          image

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                                                                                                                          Original Negative image

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                                                                                                                          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                          Some basic intensity transformation functions

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                                                                                                                          This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                          range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                          while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                          transformation The log functions compress the dynamic range of images with large

                                                                                                                          variations in pixel values

                                                                                                                          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

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                                                                                                                          Power-Law (Gamma) Transformations

                                                                                                                          - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                          Plots of gamma transformation for different values of γ (c=1)

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                                                                                                                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                          of output values with the opposite being true for higher values of input values The

                                                                                                                          curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                          1c - identity transformation

                                                                                                                          A variety of devices used for image capture printing and display respond according to a

                                                                                                                          power law The process used to correct these power-law response phenomena is called

                                                                                                                          gamma correction

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                                                                                                                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

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                                                                                                                          Piecewise-Linear Transformations Functions

                                                                                                                          Contrast stretching

                                                                                                                          - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                          intensity range of the recording tool or display device

                                                                                                                          a b c d Fig5

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                                                                                                                          11

                                                                                                                          1

                                                                                                                          2 1 1 21 2

                                                                                                                          2 1 2 1

                                                                                                                          22

                                                                                                                          2

                                                                                                                          [0 ]

                                                                                                                          ( ) ( )( ) [ ]( ) ( )

                                                                                                                          ( 1 ) [ 1]( 1 )

                                                                                                                          s r r rrs r r s r rT r r r r

                                                                                                                          r r r rs L r r r L

                                                                                                                          L r

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                                                                                                                          Week 1

                                                                                                                          1 1 2 2r s r s identity transformation (no change)

                                                                                                                          1 2 1 2 0 1r r s s L thresholding function

                                                                                                                          Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                          in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                          from their original range to the full range [0 L-1]

                                                                                                                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                          2 2 1r s m L where m is the mean gray level in the image

                                                                                                                          The original image on which these results are based is a scanning electron microscope

                                                                                                                          image of pollen magnified approximately 700 times

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                                                                                                                          Intensity-level slicing

                                                                                                                          - highlighting a specific range of intensities in an image

                                                                                                                          There are two approaches for intensity-level slicing

                                                                                                                          1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                          another (say black) all other intensities (Figure 311 (a))

                                                                                                                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                          intensities in the image (Figure 311 (b))

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                                                                                                                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                          the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                          Highlights range [A B] and preserves all other intensities

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                                                                                                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                          blockageshellip)

                                                                                                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                          image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                          Fig 6 - Aortic angiogram and intensity sliced versions

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                                                                                                                          Bit-plane slicing

                                                                                                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                          This technique highlights the contribution made to the whole image appearances by each

                                                                                                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

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                                                                                                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                          • DIP 1 2017
                                                                                                                          • DIP 02 (2017)

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                                                                                                                            ALBASTRU BLUEVERDE GREENGALBEN YELLOW ROSU REDPORTOCALIU ORANGEGALBEN YELLOWVISINIU BURGUNDYALB WHITE

                                                                                                                            Digital Image ProcessingDigital Image Processing

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                                                                                                                            Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                                            gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                                            quantities that describe the quality of a chromatic light source radiance

                                                                                                                            the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                                            measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

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                                                                                                                            For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                                            brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

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                                                                                                                            the physical meaning is determined by the source of the image

                                                                                                                            ( )f D f x y

                                                                                                                            Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                                            f(xy) ndash characterized by two components

                                                                                                                            i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                                            r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                                            ( ) ( ) ( )

                                                                                                                            0 ( ) 0 ( ) 1

                                                                                                                            f x y i x y r x y

                                                                                                                            i x y r x y

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                                                                                                                            r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                                            i(xy) ndash determined by the illumination source

                                                                                                                            r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                                            is called gray (or intensity) scale

                                                                                                                            In practice

                                                                                                                            min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                                            indoor values without additional illuminationmin max10 1000L L

                                                                                                                            black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                                            min maxL L

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                                                                                                                            Digital Image Processing

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                                                                                                                            Image Sampling and Quantization

                                                                                                                            - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                                            scene

                                                                                                                            converting a continuous image f to digital form

                                                                                                                            - digitizing (x y) is called sampling

                                                                                                                            - digitizing f(x y) is called quantization

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                                                                                                                            Continuous image projected onto a sensor array Result of image sampling and quantization

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                                                                                                                            Week 1

                                                                                                                            Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                            (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                            ( )

                                                                                                                            ( 10) ( 11) ( 1 1)

                                                                                                                            f f f Nf f f N

                                                                                                                            f x y

                                                                                                                            f M f M f M N

                                                                                                                            image element pixel

                                                                                                                            00 01 0 1

                                                                                                                            10 11 1 1

                                                                                                                            10 11 1 1

                                                                                                                            ( ) ( )

                                                                                                                            N

                                                                                                                            i jN M N

                                                                                                                            i j

                                                                                                                            M M M N

                                                                                                                            a a aa f x i y j f i ja a a

                                                                                                                            Aa

                                                                                                                            a a a

                                                                                                                            f(00) ndash the upper left corner of the image

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                                                                                                                            M N ge 0 L=2k

                                                                                                                            [0 1]i j i ja a L

                                                                                                                            Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

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                                                                                                                            Number of bits required to store a digitized image

                                                                                                                            for 2 b M N k M N b N k

                                                                                                                            When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

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                                                                                                                            Week 1

                                                                                                                            Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                            Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                            Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                            (eg 100 line pairs per mm)

                                                                                                                            Dots per unit distance are commonly used in printing and publishing

                                                                                                                            In US the measure is expressed in dots per inch (dpi)

                                                                                                                            (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                            Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                            The number of intensity levels (L) is determined by hardware considerations

                                                                                                                            L=2k ndash most common k = 8

                                                                                                                            Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                                            Week 1

                                                                                                                            Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                            150 dpi (lower left) 72 dpi (lower right)

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                                                                                                                            Week 1

                                                                                                                            Reducing the number of gray levels 256 128 64 32

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                                                                                                                            Reducing the number of gray levels 16 8 4 2

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                                                                                                                            Week 1

                                                                                                                            Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                            Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                            Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                            Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                            750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                            same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                            image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                            Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                            Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                            intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                            This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                            straight edges

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                                                                                                                            Week 1

                                                                                                                            Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                            where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                            be written using the 4 nearest neighbors of point (x y)

                                                                                                                            Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                            modest increase in computational effort

                                                                                                                            Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                            nearest neighbors of the point 3 3

                                                                                                                            0 0

                                                                                                                            ( ) i ji j

                                                                                                                            i jv x y c x y

                                                                                                                            The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                            intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                            0 0

                                                                                                                            ( )i ji j

                                                                                                                            i jc x y x y

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                                                                                                                            Week 1

                                                                                                                            Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                            bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                            programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                            Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                            the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                            then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                            neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                            Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                            interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                            from 1250 dpi to 150 dpi (instead of 72 dpi)

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                                                                                                                            Week 1

                                                                                                                            Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

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                                                                                                                            Week 1

                                                                                                                            Neighbors of a Pixel

                                                                                                                            A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                            This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                            The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                            and are denoted ND(p)

                                                                                                                            The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                            N8 (p)

                                                                                                                            If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                            fall outside the image

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                                                                                                                            Adjacency Connectivity Regions Boundaries

                                                                                                                            Denote by V the set of intensity levels used to define adjacency

                                                                                                                            - in a binary image V 01 (V=0 V=1)

                                                                                                                            - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                            We consider 3 types of adjacency

                                                                                                                            (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                            (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                            (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                            m-adjacent if

                                                                                                                            4( )q N p or

                                                                                                                            ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                            Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                            ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            binary image

                                                                                                                            0 1 1 0 1 1 0 1 1

                                                                                                                            1 0 1 0 0 1 0 0 1 0

                                                                                                                            0 0 1 0 0 1 0 0 1

                                                                                                                            V

                                                                                                                            The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                            8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                            m-adjacency

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                            is a sequence of distinct pixels with coordinates

                                                                                                                            and are adjacent 0 0 1 1

                                                                                                                            1 1

                                                                                                                            ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                            n n

                                                                                                                            i i i i

                                                                                                                            x y x y x y x y s tx y x y i n

                                                                                                                            The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                            Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                            Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                            in S if there exists a path between them consisting only of pixels from S

                                                                                                                            S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                            Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                            Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                            that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                            8-adjacency are considered

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                            touches the image border

                                                                                                                            the complement of 1

                                                                                                                            ( )K

                                                                                                                            cu k u u

                                                                                                                            k

                                                                                                                            R R R R

                                                                                                                            We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                            background of the image

                                                                                                                            The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                            points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                            region that have at least one background neighbor This definition is referred to as the

                                                                                                                            inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                            border in the background

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Distance measures

                                                                                                                            For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                            function or metric if

                                                                                                                            (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                            (b) D(p q) = D(q p)

                                                                                                                            (c) D(p z) le D(p q) + D(q z)

                                                                                                                            The Euclidean distance between p and q is defined as 1

                                                                                                                            2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                            The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                            centered at (x y)

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                            4( ) | | | |D p q x s y t

                                                                                                                            The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                            4

                                                                                                                            22 1 2

                                                                                                                            2 2 1 0 1 22 1 2

                                                                                                                            2

                                                                                                                            D

                                                                                                                            The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                            The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                            8( ) max| | | |D p q x s y t

                                                                                                                            The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            8

                                                                                                                            2 2 2 2 22 1 1 1 2

                                                                                                                            2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                            D

                                                                                                                            The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                            D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                            because these distances involve only the coordinates of the point

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Array versus Matrix Operations

                                                                                                                            An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                            11 12 11 12

                                                                                                                            21 22 21 22

                                                                                                                            a a b ba a b b

                                                                                                                            Array product

                                                                                                                            11 12 11 12 11 11 12 12

                                                                                                                            21 22 21 22 21 21 22 21

                                                                                                                            a a b b a b a ba a b b a b a b

                                                                                                                            Matrix product

                                                                                                                            11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                            21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                            a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                            We assume array operations unless stated otherwise

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Linear versus Nonlinear Operations

                                                                                                                            One of the most important classifications of image-processing methods is whether it is

                                                                                                                            linear or nonlinear

                                                                                                                            ( ) ( )H f x y g x y

                                                                                                                            H is said to be a linear operator if

                                                                                                                            images1 2 1 2

                                                                                                                            1 2

                                                                                                                            ( ) ( ) ( ) ( )

                                                                                                                            H a f x y b f x y a H f x y b H f x y

                                                                                                                            a b f f

                                                                                                                            Example of nonlinear operator

                                                                                                                            the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                            1 2

                                                                                                                            0 2 6 5 1 1

                                                                                                                            2 3 4 7f f a b

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            1 2

                                                                                                                            0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                            2 3 4 7 2 4a f b f

                                                                                                                            0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                            2 3 4 7

                                                                                                                            Arithmetic Operations in Image Processing

                                                                                                                            Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                            f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                            For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                            value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                            The two random variables are uncorrelated when their covariance is 0

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                            used in image enhancement)

                                                                                                                            1

                                                                                                                            1( ) ( )K

                                                                                                                            ii

                                                                                                                            g x y g x yK

                                                                                                                            If the noise satisfies the properties stated above we have

                                                                                                                            2 2( ) ( )

                                                                                                                            1( ) ( ) g x y x yE g x y f x yK

                                                                                                                            ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                            and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                            the average image is

                                                                                                                            ( ) ( )1

                                                                                                                            g x y x yK

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                                                                                                                            Week 1

                                                                                                                            As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                            the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                            means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                            averaging process increases

                                                                                                                            An important application of image averaging is in the field of astronomy where imaging

                                                                                                                            under very low light levels frequently causes sensor noise to render single images

                                                                                                                            virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                            was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                            64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                            images respectively

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                                                                                                                            Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                            100 noisy images

                                                                                                                            a b c d e f

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                                                                                                                            Week 1

                                                                                                                            A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                            images

                                                                                                                            (a) (b) (c)

                                                                                                                            Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                            significant bit of each pixel (c) the difference between the two images

                                                                                                                            Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                            Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                            images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                            difference between images (a) and (b)

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                            h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                            intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                            source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                            bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                            anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                            images after injection of the contrast medium

                                                                                                                            In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                            Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                            propagates through the various arteries in the area being observed

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            An important application of image multiplication (and division) is shading correction

                                                                                                                            Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                            f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                            When the shading function is known

                                                                                                                            ( )( )( )

                                                                                                                            g x yf x yh x y

                                                                                                                            h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                            approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                            sensor is not available often the shading pattern can be estimated from the image

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                                                                                                                            Week 1

                                                                                                                            (a) (b) (c)

                                                                                                                            Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                            operations The process consists of multiplying a given image by a mask image that has

                                                                                                                            1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                            image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                            (a) (b) (c)

                                                                                                                            Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                            min( )mf f f

                                                                                                                            0 ( 255)max( )

                                                                                                                            ms

                                                                                                                            m

                                                                                                                            ff K K K

                                                                                                                            f

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Spatial Operations

                                                                                                                            - are performed directly on the pixels of a given image

                                                                                                                            There are three categories of spatial operations

                                                                                                                            single-pixel operations

                                                                                                                            neighborhood operations

                                                                                                                            geometric spatial transformations

                                                                                                                            Single-pixel operations

                                                                                                                            - change the values of intensity for the individual pixels ( )s T z

                                                                                                                            where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                            corresponding pixel in the processed image

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Neighborhood operations

                                                                                                                            Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                            in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                            based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                            rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                            intensity by computing the average value of the pixels in Sxy

                                                                                                                            ( )

                                                                                                                            1( ) ( )xyr c S

                                                                                                                            g x y f r cm n

                                                                                                                            The net effect is to perform local blurring in the original image This type of process is

                                                                                                                            used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                            largest region of an image

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Geometric spatial transformations and image registration

                                                                                                                            - modify the spatial relationship between pixels in an image

                                                                                                                            - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                            printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                            predefined set of rules

                                                                                                                            A geometric transformation consists of 2 basic operations

                                                                                                                            1 a spatial transformation of coordinates

                                                                                                                            2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                            pixels

                                                                                                                            The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                            (v w) ndash pixel coordinates in the original image

                                                                                                                            (x y) ndash pixel coordinates in the transformed image

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                            Affine transform

                                                                                                                            11 1211 21 31

                                                                                                                            21 2212 22 33

                                                                                                                            31 32

                                                                                                                            0[ 1] [ 1] [ 1] 0

                                                                                                                            1

                                                                                                                            t tx t v t w t

                                                                                                                            x y v w T v w t ty t v t w t

                                                                                                                            t t

                                                                                                                            (AT)

                                                                                                                            This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                            on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                            result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                            scaling rotation and translation matrices from Table 1

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Affine transformations

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                            the process we have to assign intensity values to those locations This task is done by

                                                                                                                            using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                            In practice we can use equation (AT) in two basic ways

                                                                                                                            forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                            location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                            Problems

                                                                                                                            - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                            the same location in the output image

                                                                                                                            - some output locations have no correspondent in the original image (no intensity

                                                                                                                            assignment)

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                            computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                            It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                            pixel value

                                                                                                                            Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                            numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

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                                                                                                                            Image registration ndash align two or more images of the same scene

                                                                                                                            In image registration we have available the input and output images but the specific

                                                                                                                            transformation that produced the output image from the input is generally unknown

                                                                                                                            The problem is to estimate the transformation function and then use it to register the two

                                                                                                                            images

                                                                                                                            - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                            same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                            - align images of a given location taken by the same instrument at different moments

                                                                                                                            of time (satellite images)

                                                                                                                            Solving the problem using tie points (also called control points) which are

                                                                                                                            corresponding points whose locations are known precisely in the input and reference

                                                                                                                            image

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            How to select tie points

                                                                                                                            - interactively selecting them

                                                                                                                            - use of algorithms that try to detect these points

                                                                                                                            - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                            the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                            marks) directly on all images captured by the system which can be used as guides

                                                                                                                            for establishing tie points

                                                                                                                            The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                            of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                            a bilinear approximation is given by

                                                                                                                            1 2 3 4

                                                                                                                            5 6 7 8

                                                                                                                            x c v c w c v w cy c v c w c v w c

                                                                                                                            (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                            frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                            subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                            subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                            The number of tie points and the sophistication of the model required to solve the register

                                                                                                                            problem depend on the severity of the geometrical distortion

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Probabilistic Methods

                                                                                                                            zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                            p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                            ( ) kk

                                                                                                                            np zM N

                                                                                                                            nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                            pixels in the image) 1

                                                                                                                            0( ) 1

                                                                                                                            L

                                                                                                                            kk

                                                                                                                            p z

                                                                                                                            The mean (average) intensity of an image is given by 1

                                                                                                                            0( )

                                                                                                                            L

                                                                                                                            k kk

                                                                                                                            m z p z

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            The variance of the intensities is 1

                                                                                                                            2 2

                                                                                                                            0( ) ( )

                                                                                                                            L

                                                                                                                            k kk

                                                                                                                            z m p z

                                                                                                                            The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                            measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                            ( ) is used

                                                                                                                            The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                            0( ) ( ) ( )

                                                                                                                            Ln

                                                                                                                            n k kk

                                                                                                                            z z m p z

                                                                                                                            ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                            3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                            ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                            mean

                                                                                                                            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                            Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                            Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                            Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Intensity Transformations and Spatial Filtering

                                                                                                                            ( ) ( )g x y T f x y

                                                                                                                            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                            neighborhood of (x y)

                                                                                                                            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                            and much smaller in size than the image

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                            called spatial filter (spatial mask kernel template or window)

                                                                                                                            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                            ( )s T r

                                                                                                                            s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                            Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                            darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                            is called contrast stretching

                                                                                                                            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                            thresholding function

                                                                                                                            Some Basic Intensity Transformation Functions

                                                                                                                            Image Negatives

                                                                                                                            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                            - equivalent of a photographic negative

                                                                                                                            - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                            image

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Original Negative image

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                            Some basic intensity transformation functions

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                            range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                            while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                            transformation The log functions compress the dynamic range of images with large

                                                                                                                            variations in pixel values

                                                                                                                            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Power-Law (Gamma) Transformations

                                                                                                                            - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                            Plots of gamma transformation for different values of γ (c=1)

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                                                                                                                            Week 1

                                                                                                                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                            of output values with the opposite being true for higher values of input values The

                                                                                                                            curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                            1c - identity transformation

                                                                                                                            A variety of devices used for image capture printing and display respond according to a

                                                                                                                            power law The process used to correct these power-law response phenomena is called

                                                                                                                            gamma correction

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                                                                                                                            Week 1

                                                                                                                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

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                                                                                                                            Week 1

                                                                                                                            Piecewise-Linear Transformations Functions

                                                                                                                            Contrast stretching

                                                                                                                            - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                            intensity range of the recording tool or display device

                                                                                                                            a b c d Fig5

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                                                                                                                            Week 1

                                                                                                                            11

                                                                                                                            1

                                                                                                                            2 1 1 21 2

                                                                                                                            2 1 2 1

                                                                                                                            22

                                                                                                                            2

                                                                                                                            [0 ]

                                                                                                                            ( ) ( )( ) [ ]( ) ( )

                                                                                                                            ( 1 ) [ 1]( 1 )

                                                                                                                            s r r rrs r r s r rT r r r r

                                                                                                                            r r r rs L r r r L

                                                                                                                            L r

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                                                                                                                            Week 1

                                                                                                                            1 1 2 2r s r s identity transformation (no change)

                                                                                                                            1 2 1 2 0 1r r s s L thresholding function

                                                                                                                            Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                            in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                            from their original range to the full range [0 L-1]

                                                                                                                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                            2 2 1r s m L where m is the mean gray level in the image

                                                                                                                            The original image on which these results are based is a scanning electron microscope

                                                                                                                            image of pollen magnified approximately 700 times

                                                                                                                            Digital Image Processing

                                                                                                                            Week 1

                                                                                                                            Intensity-level slicing

                                                                                                                            - highlighting a specific range of intensities in an image

                                                                                                                            There are two approaches for intensity-level slicing

                                                                                                                            1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                            another (say black) all other intensities (Figure 311 (a))

                                                                                                                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                            intensities in the image (Figure 311 (b))

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                                                                                                                            Week 1

                                                                                                                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                            the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                            Highlights range [A B] and preserves all other intensities

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                                                                                                                            Week 1

                                                                                                                            which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                            blockageshellip)

                                                                                                                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                            image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                            Fig 6 - Aortic angiogram and intensity sliced versions

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                                                                                                                            Bit-plane slicing

                                                                                                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                            This technique highlights the contribution made to the whole image appearances by each

                                                                                                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

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                                                                                                                            Week 1

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                                                                                                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                            • DIP 1 2017
                                                                                                                            • DIP 02 (2017)

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                                                                                                                              Lightbullachromatic or monochromatic light - light that is void of color the attribute of such light is its intensity or amount

                                                                                                                              gray level is used to describe monochromatic intensitybecause it ranges from black to grays and to white bull chromatic light spans the electromagnetic energy spectrumfrom approximately 043 to 079 μm

                                                                                                                              quantities that describe the quality of a chromatic light source radiance

                                                                                                                              the total amount of energy that flows from the light source and it isusually measured in watts (W) luminance

                                                                                                                              measured in lumens (lm) gives a measure of the amount of energy an observer perceives from a light source

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                                                                                                                              For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                                              brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

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                                                                                                                              the physical meaning is determined by the source of the image

                                                                                                                              ( )f D f x y

                                                                                                                              Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                                              f(xy) ndash characterized by two components

                                                                                                                              i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                                              r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                                              ( ) ( ) ( )

                                                                                                                              0 ( ) 0 ( ) 1

                                                                                                                              f x y i x y r x y

                                                                                                                              i x y r x y

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                                                                                                                              r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                                              i(xy) ndash determined by the illumination source

                                                                                                                              r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                                              is called gray (or intensity) scale

                                                                                                                              In practice

                                                                                                                              min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                                              indoor values without additional illuminationmin max10 1000L L

                                                                                                                              black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                                              min maxL L

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                                                                                                                              Digital Image Processing

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                                                                                                                              Image Sampling and Quantization

                                                                                                                              - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                                              scene

                                                                                                                              converting a continuous image f to digital form

                                                                                                                              - digitizing (x y) is called sampling

                                                                                                                              - digitizing f(x y) is called quantization

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                                                                                                                              Continuous image projected onto a sensor array Result of image sampling and quantization

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                                                                                                                              Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                              (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                              ( )

                                                                                                                              ( 10) ( 11) ( 1 1)

                                                                                                                              f f f Nf f f N

                                                                                                                              f x y

                                                                                                                              f M f M f M N

                                                                                                                              image element pixel

                                                                                                                              00 01 0 1

                                                                                                                              10 11 1 1

                                                                                                                              10 11 1 1

                                                                                                                              ( ) ( )

                                                                                                                              N

                                                                                                                              i jN M N

                                                                                                                              i j

                                                                                                                              M M M N

                                                                                                                              a a aa f x i y j f i ja a a

                                                                                                                              Aa

                                                                                                                              a a a

                                                                                                                              f(00) ndash the upper left corner of the image

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                                                                                                                              M N ge 0 L=2k

                                                                                                                              [0 1]i j i ja a L

                                                                                                                              Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

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                                                                                                                              Number of bits required to store a digitized image

                                                                                                                              for 2 b M N k M N b N k

                                                                                                                              When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

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                                                                                                                              Week 1

                                                                                                                              Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                              Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                              Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                              (eg 100 line pairs per mm)

                                                                                                                              Dots per unit distance are commonly used in printing and publishing

                                                                                                                              In US the measure is expressed in dots per inch (dpi)

                                                                                                                              (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                              Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                              The number of intensity levels (L) is determined by hardware considerations

                                                                                                                              L=2k ndash most common k = 8

                                                                                                                              Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                                              Week 1

                                                                                                                              Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                              150 dpi (lower left) 72 dpi (lower right)

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                                                                                                                              Week 1

                                                                                                                              Reducing the number of gray levels 256 128 64 32

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                                                                                                                              Reducing the number of gray levels 16 8 4 2

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                                                                                                                              Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                              Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                              Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                              Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                              750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                              same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                              image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                              Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                              Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                              intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                              This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                              straight edges

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                                                                                                                              Week 1

                                                                                                                              Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                              where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                              be written using the 4 nearest neighbors of point (x y)

                                                                                                                              Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                              modest increase in computational effort

                                                                                                                              Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                              nearest neighbors of the point 3 3

                                                                                                                              0 0

                                                                                                                              ( ) i ji j

                                                                                                                              i jv x y c x y

                                                                                                                              The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                              intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                              0 0

                                                                                                                              ( )i ji j

                                                                                                                              i jc x y x y

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                                                                                                                              Week 1

                                                                                                                              Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                              bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                              programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                              Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                              the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                              then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                              neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                              Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                              interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                              from 1250 dpi to 150 dpi (instead of 72 dpi)

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                                                                                                                              Week 1

                                                                                                                              Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

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                                                                                                                              Neighbors of a Pixel

                                                                                                                              A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                              This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                              The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                              and are denoted ND(p)

                                                                                                                              The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                              N8 (p)

                                                                                                                              If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                              fall outside the image

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                                                                                                                              Adjacency Connectivity Regions Boundaries

                                                                                                                              Denote by V the set of intensity levels used to define adjacency

                                                                                                                              - in a binary image V 01 (V=0 V=1)

                                                                                                                              - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                              We consider 3 types of adjacency

                                                                                                                              (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                              (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                              (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                              m-adjacent if

                                                                                                                              4( )q N p or

                                                                                                                              ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                              Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                              ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              binary image

                                                                                                                              0 1 1 0 1 1 0 1 1

                                                                                                                              1 0 1 0 0 1 0 0 1 0

                                                                                                                              0 0 1 0 0 1 0 0 1

                                                                                                                              V

                                                                                                                              The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                              8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                              m-adjacency

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                                                                                                                              A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                              is a sequence of distinct pixels with coordinates

                                                                                                                              and are adjacent 0 0 1 1

                                                                                                                              1 1

                                                                                                                              ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                              n n

                                                                                                                              i i i i

                                                                                                                              x y x y x y x y s tx y x y i n

                                                                                                                              The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                              Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                              Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                              in S if there exists a path between them consisting only of pixels from S

                                                                                                                              S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                              Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                              Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                              that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                              8-adjacency are considered

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                                                                                                                              Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                              touches the image border

                                                                                                                              the complement of 1

                                                                                                                              ( )K

                                                                                                                              cu k u u

                                                                                                                              k

                                                                                                                              R R R R

                                                                                                                              We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                              background of the image

                                                                                                                              The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                              points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                              region that have at least one background neighbor This definition is referred to as the

                                                                                                                              inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                              border in the background

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                                                                                                                              Distance measures

                                                                                                                              For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                              function or metric if

                                                                                                                              (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                              (b) D(p q) = D(q p)

                                                                                                                              (c) D(p z) le D(p q) + D(q z)

                                                                                                                              The Euclidean distance between p and q is defined as 1

                                                                                                                              2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                              The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                              centered at (x y)

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                              4( ) | | | |D p q x s y t

                                                                                                                              The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                              4

                                                                                                                              22 1 2

                                                                                                                              2 2 1 0 1 22 1 2

                                                                                                                              2

                                                                                                                              D

                                                                                                                              The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                              The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                              8( ) max| | | |D p q x s y t

                                                                                                                              The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              8

                                                                                                                              2 2 2 2 22 1 1 1 2

                                                                                                                              2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                              D

                                                                                                                              The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                              D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                              because these distances involve only the coordinates of the point

                                                                                                                              Digital Image Processing

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                                                                                                                              Array versus Matrix Operations

                                                                                                                              An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                              11 12 11 12

                                                                                                                              21 22 21 22

                                                                                                                              a a b ba a b b

                                                                                                                              Array product

                                                                                                                              11 12 11 12 11 11 12 12

                                                                                                                              21 22 21 22 21 21 22 21

                                                                                                                              a a b b a b a ba a b b a b a b

                                                                                                                              Matrix product

                                                                                                                              11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                              21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                              a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                              We assume array operations unless stated otherwise

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                                                                                                                              Week 1

                                                                                                                              Linear versus Nonlinear Operations

                                                                                                                              One of the most important classifications of image-processing methods is whether it is

                                                                                                                              linear or nonlinear

                                                                                                                              ( ) ( )H f x y g x y

                                                                                                                              H is said to be a linear operator if

                                                                                                                              images1 2 1 2

                                                                                                                              1 2

                                                                                                                              ( ) ( ) ( ) ( )

                                                                                                                              H a f x y b f x y a H f x y b H f x y

                                                                                                                              a b f f

                                                                                                                              Example of nonlinear operator

                                                                                                                              the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                              1 2

                                                                                                                              0 2 6 5 1 1

                                                                                                                              2 3 4 7f f a b

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              1 2

                                                                                                                              0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                              2 3 4 7 2 4a f b f

                                                                                                                              0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                              2 3 4 7

                                                                                                                              Arithmetic Operations in Image Processing

                                                                                                                              Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                              f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                              For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                              value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                              The two random variables are uncorrelated when their covariance is 0

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                              used in image enhancement)

                                                                                                                              1

                                                                                                                              1( ) ( )K

                                                                                                                              ii

                                                                                                                              g x y g x yK

                                                                                                                              If the noise satisfies the properties stated above we have

                                                                                                                              2 2( ) ( )

                                                                                                                              1( ) ( ) g x y x yE g x y f x yK

                                                                                                                              ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                              and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                              the average image is

                                                                                                                              ( ) ( )1

                                                                                                                              g x y x yK

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                              the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                              means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                              averaging process increases

                                                                                                                              An important application of image averaging is in the field of astronomy where imaging

                                                                                                                              under very low light levels frequently causes sensor noise to render single images

                                                                                                                              virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                              was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                              64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                              images respectively

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                              100 noisy images

                                                                                                                              a b c d e f

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                              images

                                                                                                                              (a) (b) (c)

                                                                                                                              Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                              significant bit of each pixel (c) the difference between the two images

                                                                                                                              Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                              Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                              images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                              difference between images (a) and (b)

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                              h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                              intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                              source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                              bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                              anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                              images after injection of the contrast medium

                                                                                                                              In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                              Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                              propagates through the various arteries in the area being observed

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              An important application of image multiplication (and division) is shading correction

                                                                                                                              Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                              f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                              When the shading function is known

                                                                                                                              ( )( )( )

                                                                                                                              g x yf x yh x y

                                                                                                                              h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                              approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                              sensor is not available often the shading pattern can be estimated from the image

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              (a) (b) (c)

                                                                                                                              Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                              operations The process consists of multiplying a given image by a mask image that has

                                                                                                                              1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                              image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                              (a) (b) (c)

                                                                                                                              Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                              min( )mf f f

                                                                                                                              0 ( 255)max( )

                                                                                                                              ms

                                                                                                                              m

                                                                                                                              ff K K K

                                                                                                                              f

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Spatial Operations

                                                                                                                              - are performed directly on the pixels of a given image

                                                                                                                              There are three categories of spatial operations

                                                                                                                              single-pixel operations

                                                                                                                              neighborhood operations

                                                                                                                              geometric spatial transformations

                                                                                                                              Single-pixel operations

                                                                                                                              - change the values of intensity for the individual pixels ( )s T z

                                                                                                                              where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                              corresponding pixel in the processed image

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Neighborhood operations

                                                                                                                              Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                              in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                              based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                              rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                              intensity by computing the average value of the pixels in Sxy

                                                                                                                              ( )

                                                                                                                              1( ) ( )xyr c S

                                                                                                                              g x y f r cm n

                                                                                                                              The net effect is to perform local blurring in the original image This type of process is

                                                                                                                              used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                              largest region of an image

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Geometric spatial transformations and image registration

                                                                                                                              - modify the spatial relationship between pixels in an image

                                                                                                                              - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                              printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                              predefined set of rules

                                                                                                                              A geometric transformation consists of 2 basic operations

                                                                                                                              1 a spatial transformation of coordinates

                                                                                                                              2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                              pixels

                                                                                                                              The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                              (v w) ndash pixel coordinates in the original image

                                                                                                                              (x y) ndash pixel coordinates in the transformed image

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                              Affine transform

                                                                                                                              11 1211 21 31

                                                                                                                              21 2212 22 33

                                                                                                                              31 32

                                                                                                                              0[ 1] [ 1] [ 1] 0

                                                                                                                              1

                                                                                                                              t tx t v t w t

                                                                                                                              x y v w T v w t ty t v t w t

                                                                                                                              t t

                                                                                                                              (AT)

                                                                                                                              This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                              on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                              result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                              scaling rotation and translation matrices from Table 1

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Affine transformations

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                              the process we have to assign intensity values to those locations This task is done by

                                                                                                                              using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                              In practice we can use equation (AT) in two basic ways

                                                                                                                              forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                              location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                              Problems

                                                                                                                              - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                              the same location in the output image

                                                                                                                              - some output locations have no correspondent in the original image (no intensity

                                                                                                                              assignment)

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                              computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                              It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                              pixel value

                                                                                                                              Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                              numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Image registration ndash align two or more images of the same scene

                                                                                                                              In image registration we have available the input and output images but the specific

                                                                                                                              transformation that produced the output image from the input is generally unknown

                                                                                                                              The problem is to estimate the transformation function and then use it to register the two

                                                                                                                              images

                                                                                                                              - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                              same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                              - align images of a given location taken by the same instrument at different moments

                                                                                                                              of time (satellite images)

                                                                                                                              Solving the problem using tie points (also called control points) which are

                                                                                                                              corresponding points whose locations are known precisely in the input and reference

                                                                                                                              image

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              How to select tie points

                                                                                                                              - interactively selecting them

                                                                                                                              - use of algorithms that try to detect these points

                                                                                                                              - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                              the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                              marks) directly on all images captured by the system which can be used as guides

                                                                                                                              for establishing tie points

                                                                                                                              The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                              of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                              a bilinear approximation is given by

                                                                                                                              1 2 3 4

                                                                                                                              5 6 7 8

                                                                                                                              x c v c w c v w cy c v c w c v w c

                                                                                                                              (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                              frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                              subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                              subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                              The number of tie points and the sophistication of the model required to solve the register

                                                                                                                              problem depend on the severity of the geometrical distortion

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Probabilistic Methods

                                                                                                                              zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                              p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                              ( ) kk

                                                                                                                              np zM N

                                                                                                                              nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                              pixels in the image) 1

                                                                                                                              0( ) 1

                                                                                                                              L

                                                                                                                              kk

                                                                                                                              p z

                                                                                                                              The mean (average) intensity of an image is given by 1

                                                                                                                              0( )

                                                                                                                              L

                                                                                                                              k kk

                                                                                                                              m z p z

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              The variance of the intensities is 1

                                                                                                                              2 2

                                                                                                                              0( ) ( )

                                                                                                                              L

                                                                                                                              k kk

                                                                                                                              z m p z

                                                                                                                              The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                              measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                              ( ) is used

                                                                                                                              The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                              0( ) ( ) ( )

                                                                                                                              Ln

                                                                                                                              n k kk

                                                                                                                              z z m p z

                                                                                                                              ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                              3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                              ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                              mean

                                                                                                                              Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                              Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                              Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                              Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Intensity Transformations and Spatial Filtering

                                                                                                                              ( ) ( )g x y T f x y

                                                                                                                              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                              neighborhood of (x y)

                                                                                                                              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                              and much smaller in size than the image

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                              called spatial filter (spatial mask kernel template or window)

                                                                                                                              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                              ( )s T r

                                                                                                                              s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                              Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                              darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                              is called contrast stretching

                                                                                                                              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                              thresholding function

                                                                                                                              Some Basic Intensity Transformation Functions

                                                                                                                              Image Negatives

                                                                                                                              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                              - equivalent of a photographic negative

                                                                                                                              - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                              image

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Original Negative image

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                              Some basic intensity transformation functions

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                              range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                              while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                              transformation The log functions compress the dynamic range of images with large

                                                                                                                              variations in pixel values

                                                                                                                              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Power-Law (Gamma) Transformations

                                                                                                                              - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                              Plots of gamma transformation for different values of γ (c=1)

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                              of output values with the opposite being true for higher values of input values The

                                                                                                                              curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                              1c - identity transformation

                                                                                                                              A variety of devices used for image capture printing and display respond according to a

                                                                                                                              power law The process used to correct these power-law response phenomena is called

                                                                                                                              gamma correction

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Piecewise-Linear Transformations Functions

                                                                                                                              Contrast stretching

                                                                                                                              - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                              intensity range of the recording tool or display device

                                                                                                                              a b c d Fig5

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              11

                                                                                                                              1

                                                                                                                              2 1 1 21 2

                                                                                                                              2 1 2 1

                                                                                                                              22

                                                                                                                              2

                                                                                                                              [0 ]

                                                                                                                              ( ) ( )( ) [ ]( ) ( )

                                                                                                                              ( 1 ) [ 1]( 1 )

                                                                                                                              s r r rrs r r s r rT r r r r

                                                                                                                              r r r rs L r r r L

                                                                                                                              L r

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              1 1 2 2r s r s identity transformation (no change)

                                                                                                                              1 2 1 2 0 1r r s s L thresholding function

                                                                                                                              Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                              in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                              from their original range to the full range [0 L-1]

                                                                                                                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                              2 2 1r s m L where m is the mean gray level in the image

                                                                                                                              The original image on which these results are based is a scanning electron microscope

                                                                                                                              image of pollen magnified approximately 700 times

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Intensity-level slicing

                                                                                                                              - highlighting a specific range of intensities in an image

                                                                                                                              There are two approaches for intensity-level slicing

                                                                                                                              1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                              another (say black) all other intensities (Figure 311 (a))

                                                                                                                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                              intensities in the image (Figure 311 (b))

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                              the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                              Highlights range [A B] and preserves all other intensities

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                              blockageshellip)

                                                                                                                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                              image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                              Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Bit-plane slicing

                                                                                                                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                              This technique highlights the contribution made to the whole image appearances by each

                                                                                                                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              Digital Image Processing

                                                                                                                              Week 1

                                                                                                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                              • DIP 1 2017
                                                                                                                              • DIP 02 (2017)

                                                                                                                                Digital Image ProcessingDigital Image Processing

                                                                                                                                Week 1Week 1

                                                                                                                                For example light emitted from a source operating in the far infraredregion of the spectrum could have significant energy (radiance) butan observer would hardly perceive it its luminance would be almostzero

                                                                                                                                brightnessa subjective descriptor of light perception that is practicallyimpossible to measure It embodies the achromatic notion ofintensity and is one of the key factors in describing color sensation

                                                                                                                                Digital Image ProcessingDigital Image Processing

                                                                                                                                Week 1Week 1

                                                                                                                                the physical meaning is determined by the source of the image

                                                                                                                                ( )f D f x y

                                                                                                                                Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                                                f(xy) ndash characterized by two components

                                                                                                                                i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                                                r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                                                ( ) ( ) ( )

                                                                                                                                0 ( ) 0 ( ) 1

                                                                                                                                f x y i x y r x y

                                                                                                                                i x y r x y

                                                                                                                                Digital Image ProcessingDigital Image Processing

                                                                                                                                Week 1Week 1

                                                                                                                                r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                                                i(xy) ndash determined by the illumination source

                                                                                                                                r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                                                is called gray (or intensity) scale

                                                                                                                                In practice

                                                                                                                                min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                                                indoor values without additional illuminationmin max10 1000L L

                                                                                                                                black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                                                min maxL L

                                                                                                                                Digital Image ProcessingDigital Image Processing

                                                                                                                                Week 1Week 1

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Image Sampling and Quantization

                                                                                                                                - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                                                scene

                                                                                                                                converting a continuous image f to digital form

                                                                                                                                - digitizing (x y) is called sampling

                                                                                                                                - digitizing f(x y) is called quantization

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                                (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                                ( )

                                                                                                                                ( 10) ( 11) ( 1 1)

                                                                                                                                f f f Nf f f N

                                                                                                                                f x y

                                                                                                                                f M f M f M N

                                                                                                                                image element pixel

                                                                                                                                00 01 0 1

                                                                                                                                10 11 1 1

                                                                                                                                10 11 1 1

                                                                                                                                ( ) ( )

                                                                                                                                N

                                                                                                                                i jN M N

                                                                                                                                i j

                                                                                                                                M M M N

                                                                                                                                a a aa f x i y j f i ja a a

                                                                                                                                Aa

                                                                                                                                a a a

                                                                                                                                f(00) ndash the upper left corner of the image

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                M N ge 0 L=2k

                                                                                                                                [0 1]i j i ja a L

                                                                                                                                Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Number of bits required to store a digitized image

                                                                                                                                for 2 b M N k M N b N k

                                                                                                                                When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                                Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                                Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                                (eg 100 line pairs per mm)

                                                                                                                                Dots per unit distance are commonly used in printing and publishing

                                                                                                                                In US the measure is expressed in dots per inch (dpi)

                                                                                                                                (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                                Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                                The number of intensity levels (L) is determined by hardware considerations

                                                                                                                                L=2k ndash most common k = 8

                                                                                                                                Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                                150 dpi (lower left) 72 dpi (lower right)

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                                                                                                                                Week 1

                                                                                                                                Reducing the number of gray levels 256 128 64 32

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                                                                                                                                Week 1

                                                                                                                                Reducing the number of gray levels 16 8 4 2

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                straight edges

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                be written using the 4 nearest neighbors of point (x y)

                                                                                                                                Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                modest increase in computational effort

                                                                                                                                Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                nearest neighbors of the point 3 3

                                                                                                                                0 0

                                                                                                                                ( ) i ji j

                                                                                                                                i jv x y c x y

                                                                                                                                The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                0 0

                                                                                                                                ( )i ji j

                                                                                                                                i jc x y x y

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Neighbors of a Pixel

                                                                                                                                A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                and are denoted ND(p)

                                                                                                                                The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                N8 (p)

                                                                                                                                If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                fall outside the image

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Adjacency Connectivity Regions Boundaries

                                                                                                                                Denote by V the set of intensity levels used to define adjacency

                                                                                                                                - in a binary image V 01 (V=0 V=1)

                                                                                                                                - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                We consider 3 types of adjacency

                                                                                                                                (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                m-adjacent if

                                                                                                                                4( )q N p or

                                                                                                                                ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                binary image

                                                                                                                                0 1 1 0 1 1 0 1 1

                                                                                                                                1 0 1 0 0 1 0 0 1 0

                                                                                                                                0 0 1 0 0 1 0 0 1

                                                                                                                                V

                                                                                                                                The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                m-adjacency

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                is a sequence of distinct pixels with coordinates

                                                                                                                                and are adjacent 0 0 1 1

                                                                                                                                1 1

                                                                                                                                ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                n n

                                                                                                                                i i i i

                                                                                                                                x y x y x y x y s tx y x y i n

                                                                                                                                The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                in S if there exists a path between them consisting only of pixels from S

                                                                                                                                S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                8-adjacency are considered

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                touches the image border

                                                                                                                                the complement of 1

                                                                                                                                ( )K

                                                                                                                                cu k u u

                                                                                                                                k

                                                                                                                                R R R R

                                                                                                                                We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                background of the image

                                                                                                                                The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                region that have at least one background neighbor This definition is referred to as the

                                                                                                                                inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                border in the background

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Distance measures

                                                                                                                                For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                function or metric if

                                                                                                                                (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                (b) D(p q) = D(q p)

                                                                                                                                (c) D(p z) le D(p q) + D(q z)

                                                                                                                                The Euclidean distance between p and q is defined as 1

                                                                                                                                2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                centered at (x y)

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                4( ) | | | |D p q x s y t

                                                                                                                                The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                4

                                                                                                                                22 1 2

                                                                                                                                2 2 1 0 1 22 1 2

                                                                                                                                2

                                                                                                                                D

                                                                                                                                The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                8( ) max| | | |D p q x s y t

                                                                                                                                The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                8

                                                                                                                                2 2 2 2 22 1 1 1 2

                                                                                                                                2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                D

                                                                                                                                The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                because these distances involve only the coordinates of the point

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Array versus Matrix Operations

                                                                                                                                An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                11 12 11 12

                                                                                                                                21 22 21 22

                                                                                                                                a a b ba a b b

                                                                                                                                Array product

                                                                                                                                11 12 11 12 11 11 12 12

                                                                                                                                21 22 21 22 21 21 22 21

                                                                                                                                a a b b a b a ba a b b a b a b

                                                                                                                                Matrix product

                                                                                                                                11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                We assume array operations unless stated otherwise

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                                                                                                                                Week 1

                                                                                                                                Linear versus Nonlinear Operations

                                                                                                                                One of the most important classifications of image-processing methods is whether it is

                                                                                                                                linear or nonlinear

                                                                                                                                ( ) ( )H f x y g x y

                                                                                                                                H is said to be a linear operator if

                                                                                                                                images1 2 1 2

                                                                                                                                1 2

                                                                                                                                ( ) ( ) ( ) ( )

                                                                                                                                H a f x y b f x y a H f x y b H f x y

                                                                                                                                a b f f

                                                                                                                                Example of nonlinear operator

                                                                                                                                the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                1 2

                                                                                                                                0 2 6 5 1 1

                                                                                                                                2 3 4 7f f a b

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                1 2

                                                                                                                                0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                2 3 4 7 2 4a f b f

                                                                                                                                0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                2 3 4 7

                                                                                                                                Arithmetic Operations in Image Processing

                                                                                                                                Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                The two random variables are uncorrelated when their covariance is 0

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                used in image enhancement)

                                                                                                                                1

                                                                                                                                1( ) ( )K

                                                                                                                                ii

                                                                                                                                g x y g x yK

                                                                                                                                If the noise satisfies the properties stated above we have

                                                                                                                                2 2( ) ( )

                                                                                                                                1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                the average image is

                                                                                                                                ( ) ( )1

                                                                                                                                g x y x yK

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                                                                                                                                Week 1

                                                                                                                                As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                averaging process increases

                                                                                                                                An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                under very low light levels frequently causes sensor noise to render single images

                                                                                                                                virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                images respectively

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                                                                                                                                Week 1

                                                                                                                                Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                100 noisy images

                                                                                                                                a b c d e f

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                images

                                                                                                                                (a) (b) (c)

                                                                                                                                Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                significant bit of each pixel (c) the difference between the two images

                                                                                                                                Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                difference between images (a) and (b)

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                images after injection of the contrast medium

                                                                                                                                In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                propagates through the various arteries in the area being observed

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                An important application of image multiplication (and division) is shading correction

                                                                                                                                Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                When the shading function is known

                                                                                                                                ( )( )( )

                                                                                                                                g x yf x yh x y

                                                                                                                                h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                (a) (b) (c)

                                                                                                                                Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                (a) (b) (c)

                                                                                                                                Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                min( )mf f f

                                                                                                                                0 ( 255)max( )

                                                                                                                                ms

                                                                                                                                m

                                                                                                                                ff K K K

                                                                                                                                f

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Spatial Operations

                                                                                                                                - are performed directly on the pixels of a given image

                                                                                                                                There are three categories of spatial operations

                                                                                                                                single-pixel operations

                                                                                                                                neighborhood operations

                                                                                                                                geometric spatial transformations

                                                                                                                                Single-pixel operations

                                                                                                                                - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                corresponding pixel in the processed image

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Neighborhood operations

                                                                                                                                Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                intensity by computing the average value of the pixels in Sxy

                                                                                                                                ( )

                                                                                                                                1( ) ( )xyr c S

                                                                                                                                g x y f r cm n

                                                                                                                                The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                largest region of an image

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Geometric spatial transformations and image registration

                                                                                                                                - modify the spatial relationship between pixels in an image

                                                                                                                                - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                predefined set of rules

                                                                                                                                A geometric transformation consists of 2 basic operations

                                                                                                                                1 a spatial transformation of coordinates

                                                                                                                                2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                pixels

                                                                                                                                The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                (v w) ndash pixel coordinates in the original image

                                                                                                                                (x y) ndash pixel coordinates in the transformed image

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                Affine transform

                                                                                                                                11 1211 21 31

                                                                                                                                21 2212 22 33

                                                                                                                                31 32

                                                                                                                                0[ 1] [ 1] [ 1] 0

                                                                                                                                1

                                                                                                                                t tx t v t w t

                                                                                                                                x y v w T v w t ty t v t w t

                                                                                                                                t t

                                                                                                                                (AT)

                                                                                                                                This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                scaling rotation and translation matrices from Table 1

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Affine transformations

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                the process we have to assign intensity values to those locations This task is done by

                                                                                                                                using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                In practice we can use equation (AT) in two basic ways

                                                                                                                                forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                Problems

                                                                                                                                - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                the same location in the output image

                                                                                                                                - some output locations have no correspondent in the original image (no intensity

                                                                                                                                assignment)

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                pixel value

                                                                                                                                Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Image registration ndash align two or more images of the same scene

                                                                                                                                In image registration we have available the input and output images but the specific

                                                                                                                                transformation that produced the output image from the input is generally unknown

                                                                                                                                The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                images

                                                                                                                                - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                - align images of a given location taken by the same instrument at different moments

                                                                                                                                of time (satellite images)

                                                                                                                                Solving the problem using tie points (also called control points) which are

                                                                                                                                corresponding points whose locations are known precisely in the input and reference

                                                                                                                                image

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                How to select tie points

                                                                                                                                - interactively selecting them

                                                                                                                                - use of algorithms that try to detect these points

                                                                                                                                - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                marks) directly on all images captured by the system which can be used as guides

                                                                                                                                for establishing tie points

                                                                                                                                The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                a bilinear approximation is given by

                                                                                                                                1 2 3 4

                                                                                                                                5 6 7 8

                                                                                                                                x c v c w c v w cy c v c w c v w c

                                                                                                                                (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                problem depend on the severity of the geometrical distortion

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Probabilistic Methods

                                                                                                                                zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                ( ) kk

                                                                                                                                np zM N

                                                                                                                                nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                pixels in the image) 1

                                                                                                                                0( ) 1

                                                                                                                                L

                                                                                                                                kk

                                                                                                                                p z

                                                                                                                                The mean (average) intensity of an image is given by 1

                                                                                                                                0( )

                                                                                                                                L

                                                                                                                                k kk

                                                                                                                                m z p z

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                The variance of the intensities is 1

                                                                                                                                2 2

                                                                                                                                0( ) ( )

                                                                                                                                L

                                                                                                                                k kk

                                                                                                                                z m p z

                                                                                                                                The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                ( ) is used

                                                                                                                                The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                0( ) ( ) ( )

                                                                                                                                Ln

                                                                                                                                n k kk

                                                                                                                                z z m p z

                                                                                                                                ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                mean

                                                                                                                                Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Intensity Transformations and Spatial Filtering

                                                                                                                                ( ) ( )g x y T f x y

                                                                                                                                f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                neighborhood of (x y)

                                                                                                                                - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                and much smaller in size than the image

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                called spatial filter (spatial mask kernel template or window)

                                                                                                                                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                ( )s T r

                                                                                                                                s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                is called contrast stretching

                                                                                                                                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                thresholding function

                                                                                                                                Some Basic Intensity Transformation Functions

                                                                                                                                Image Negatives

                                                                                                                                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                - equivalent of a photographic negative

                                                                                                                                - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                image

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                                                                                                                                Week 1

                                                                                                                                Original Negative image

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                Some basic intensity transformation functions

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                transformation The log functions compress the dynamic range of images with large

                                                                                                                                variations in pixel values

                                                                                                                                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

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                                                                                                                                Week 1

                                                                                                                                Power-Law (Gamma) Transformations

                                                                                                                                - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                of output values with the opposite being true for higher values of input values The

                                                                                                                                curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                1c - identity transformation

                                                                                                                                A variety of devices used for image capture printing and display respond according to a

                                                                                                                                power law The process used to correct these power-law response phenomena is called

                                                                                                                                gamma correction

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Piecewise-Linear Transformations Functions

                                                                                                                                Contrast stretching

                                                                                                                                - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                intensity range of the recording tool or display device

                                                                                                                                a b c d Fig5

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                11

                                                                                                                                1

                                                                                                                                2 1 1 21 2

                                                                                                                                2 1 2 1

                                                                                                                                22

                                                                                                                                2

                                                                                                                                [0 ]

                                                                                                                                ( ) ( )( ) [ ]( ) ( )

                                                                                                                                ( 1 ) [ 1]( 1 )

                                                                                                                                s r r rrs r r s r rT r r r r

                                                                                                                                r r r rs L r r r L

                                                                                                                                L r

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                1 1 2 2r s r s identity transformation (no change)

                                                                                                                                1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                from their original range to the full range [0 L-1]

                                                                                                                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                The original image on which these results are based is a scanning electron microscope

                                                                                                                                image of pollen magnified approximately 700 times

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Intensity-level slicing

                                                                                                                                - highlighting a specific range of intensities in an image

                                                                                                                                There are two approaches for intensity-level slicing

                                                                                                                                1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                another (say black) all other intensities (Figure 311 (a))

                                                                                                                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                intensities in the image (Figure 311 (b))

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                                                                                                                                Week 1

                                                                                                                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                Highlights range [A B] and preserves all other intensities

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                                                                                                                                Week 1

                                                                                                                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                blockageshellip)

                                                                                                                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                Fig 6 - Aortic angiogram and intensity sliced versions

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                                                                                                                                Week 1

                                                                                                                                Bit-plane slicing

                                                                                                                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                Digital Image Processing

                                                                                                                                Week 1

                                                                                                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                • DIP 1 2017
                                                                                                                                • DIP 02 (2017)

                                                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                                                  Week 1Week 1

                                                                                                                                  the physical meaning is determined by the source of the image

                                                                                                                                  ( )f D f x y

                                                                                                                                  Image generated from a physical process f(xy) ndash proportional to the energy radiated by the physical source

                                                                                                                                  f(xy) ndash characterized by two components

                                                                                                                                  i(xy) = illumination component the amount of source illumination incident on the scene being viewed

                                                                                                                                  r(xy) = reflectance component the amount of illumination reflected by the objects in the scene

                                                                                                                                  ( ) ( ) ( )

                                                                                                                                  0 ( ) 0 ( ) 1

                                                                                                                                  f x y i x y r x y

                                                                                                                                  i x y r x y

                                                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                                                  Week 1Week 1

                                                                                                                                  r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                                                  i(xy) ndash determined by the illumination source

                                                                                                                                  r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                                                  is called gray (or intensity) scale

                                                                                                                                  In practice

                                                                                                                                  min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                                                  indoor values without additional illuminationmin max10 1000L L

                                                                                                                                  black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                                                  min maxL L

                                                                                                                                  Digital Image ProcessingDigital Image Processing

                                                                                                                                  Week 1Week 1

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Image Sampling and Quantization

                                                                                                                                  - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                                                  scene

                                                                                                                                  converting a continuous image f to digital form

                                                                                                                                  - digitizing (x y) is called sampling

                                                                                                                                  - digitizing f(x y) is called quantization

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                                  (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                                  ( )

                                                                                                                                  ( 10) ( 11) ( 1 1)

                                                                                                                                  f f f Nf f f N

                                                                                                                                  f x y

                                                                                                                                  f M f M f M N

                                                                                                                                  image element pixel

                                                                                                                                  00 01 0 1

                                                                                                                                  10 11 1 1

                                                                                                                                  10 11 1 1

                                                                                                                                  ( ) ( )

                                                                                                                                  N

                                                                                                                                  i jN M N

                                                                                                                                  i j

                                                                                                                                  M M M N

                                                                                                                                  a a aa f x i y j f i ja a a

                                                                                                                                  Aa

                                                                                                                                  a a a

                                                                                                                                  f(00) ndash the upper left corner of the image

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                                                                                                                                  Week 1

                                                                                                                                  M N ge 0 L=2k

                                                                                                                                  [0 1]i j i ja a L

                                                                                                                                  Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Number of bits required to store a digitized image

                                                                                                                                  for 2 b M N k M N b N k

                                                                                                                                  When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                                  Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                                  Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                                  (eg 100 line pairs per mm)

                                                                                                                                  Dots per unit distance are commonly used in printing and publishing

                                                                                                                                  In US the measure is expressed in dots per inch (dpi)

                                                                                                                                  (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                                  Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                                  The number of intensity levels (L) is determined by hardware considerations

                                                                                                                                  L=2k ndash most common k = 8

                                                                                                                                  Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                                                  Week 1

                                                                                                                                  Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                                  150 dpi (lower left) 72 dpi (lower right)

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Reducing the number of gray levels 256 128 64 32

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Reducing the number of gray levels 16 8 4 2

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                  Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                  Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                  Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                  750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                  same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                  image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                  Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                  Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                  intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                  This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                  straight edges

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                  where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                  be written using the 4 nearest neighbors of point (x y)

                                                                                                                                  Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                  modest increase in computational effort

                                                                                                                                  Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                  nearest neighbors of the point 3 3

                                                                                                                                  0 0

                                                                                                                                  ( ) i ji j

                                                                                                                                  i jv x y c x y

                                                                                                                                  The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                  intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                  0 0

                                                                                                                                  ( )i ji j

                                                                                                                                  i jc x y x y

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                  bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                  programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                  Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                  the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                  then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                  neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                  Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                  interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                  from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Neighbors of a Pixel

                                                                                                                                  A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                  This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                  The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                  and are denoted ND(p)

                                                                                                                                  The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                  N8 (p)

                                                                                                                                  If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                  fall outside the image

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Adjacency Connectivity Regions Boundaries

                                                                                                                                  Denote by V the set of intensity levels used to define adjacency

                                                                                                                                  - in a binary image V 01 (V=0 V=1)

                                                                                                                                  - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                  We consider 3 types of adjacency

                                                                                                                                  (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                  (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                  (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                  m-adjacent if

                                                                                                                                  4( )q N p or

                                                                                                                                  ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                  Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                  ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  binary image

                                                                                                                                  0 1 1 0 1 1 0 1 1

                                                                                                                                  1 0 1 0 0 1 0 0 1 0

                                                                                                                                  0 0 1 0 0 1 0 0 1

                                                                                                                                  V

                                                                                                                                  The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                  8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                  m-adjacency

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                  is a sequence of distinct pixels with coordinates

                                                                                                                                  and are adjacent 0 0 1 1

                                                                                                                                  1 1

                                                                                                                                  ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                  n n

                                                                                                                                  i i i i

                                                                                                                                  x y x y x y x y s tx y x y i n

                                                                                                                                  The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                  Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                  Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                  in S if there exists a path between them consisting only of pixels from S

                                                                                                                                  S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                  Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                  Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                  that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                  8-adjacency are considered

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                  touches the image border

                                                                                                                                  the complement of 1

                                                                                                                                  ( )K

                                                                                                                                  cu k u u

                                                                                                                                  k

                                                                                                                                  R R R R

                                                                                                                                  We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                  background of the image

                                                                                                                                  The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                  points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                  region that have at least one background neighbor This definition is referred to as the

                                                                                                                                  inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                  border in the background

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Distance measures

                                                                                                                                  For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                  function or metric if

                                                                                                                                  (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                  (b) D(p q) = D(q p)

                                                                                                                                  (c) D(p z) le D(p q) + D(q z)

                                                                                                                                  The Euclidean distance between p and q is defined as 1

                                                                                                                                  2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                  The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                  centered at (x y)

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                  4( ) | | | |D p q x s y t

                                                                                                                                  The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                  4

                                                                                                                                  22 1 2

                                                                                                                                  2 2 1 0 1 22 1 2

                                                                                                                                  2

                                                                                                                                  D

                                                                                                                                  The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                  The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                  8( ) max| | | |D p q x s y t

                                                                                                                                  The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  8

                                                                                                                                  2 2 2 2 22 1 1 1 2

                                                                                                                                  2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                  D

                                                                                                                                  The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                  D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                  because these distances involve only the coordinates of the point

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Array versus Matrix Operations

                                                                                                                                  An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                  11 12 11 12

                                                                                                                                  21 22 21 22

                                                                                                                                  a a b ba a b b

                                                                                                                                  Array product

                                                                                                                                  11 12 11 12 11 11 12 12

                                                                                                                                  21 22 21 22 21 21 22 21

                                                                                                                                  a a b b a b a ba a b b a b a b

                                                                                                                                  Matrix product

                                                                                                                                  11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                  21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                  a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                  We assume array operations unless stated otherwise

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Linear versus Nonlinear Operations

                                                                                                                                  One of the most important classifications of image-processing methods is whether it is

                                                                                                                                  linear or nonlinear

                                                                                                                                  ( ) ( )H f x y g x y

                                                                                                                                  H is said to be a linear operator if

                                                                                                                                  images1 2 1 2

                                                                                                                                  1 2

                                                                                                                                  ( ) ( ) ( ) ( )

                                                                                                                                  H a f x y b f x y a H f x y b H f x y

                                                                                                                                  a b f f

                                                                                                                                  Example of nonlinear operator

                                                                                                                                  the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                  1 2

                                                                                                                                  0 2 6 5 1 1

                                                                                                                                  2 3 4 7f f a b

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  1 2

                                                                                                                                  0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                  2 3 4 7 2 4a f b f

                                                                                                                                  0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                  2 3 4 7

                                                                                                                                  Arithmetic Operations in Image Processing

                                                                                                                                  Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                  f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                  For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                  value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                  The two random variables are uncorrelated when their covariance is 0

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                  used in image enhancement)

                                                                                                                                  1

                                                                                                                                  1( ) ( )K

                                                                                                                                  ii

                                                                                                                                  g x y g x yK

                                                                                                                                  If the noise satisfies the properties stated above we have

                                                                                                                                  2 2( ) ( )

                                                                                                                                  1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                  ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                  and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                  the average image is

                                                                                                                                  ( ) ( )1

                                                                                                                                  g x y x yK

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                  the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                  means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                  averaging process increases

                                                                                                                                  An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                  under very low light levels frequently causes sensor noise to render single images

                                                                                                                                  virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                  was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                  64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                  images respectively

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                  100 noisy images

                                                                                                                                  a b c d e f

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                  images

                                                                                                                                  (a) (b) (c)

                                                                                                                                  Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                  significant bit of each pixel (c) the difference between the two images

                                                                                                                                  Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                  Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                  images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                  difference between images (a) and (b)

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                  h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                  intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                  source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                  bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                  anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                  images after injection of the contrast medium

                                                                                                                                  In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                  Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                  propagates through the various arteries in the area being observed

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  An important application of image multiplication (and division) is shading correction

                                                                                                                                  Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                  f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                  When the shading function is known

                                                                                                                                  ( )( )( )

                                                                                                                                  g x yf x yh x y

                                                                                                                                  h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                  approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                  sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  (a) (b) (c)

                                                                                                                                  Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                  operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                  1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                  image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                  (a) (b) (c)

                                                                                                                                  Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                  min( )mf f f

                                                                                                                                  0 ( 255)max( )

                                                                                                                                  ms

                                                                                                                                  m

                                                                                                                                  ff K K K

                                                                                                                                  f

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Spatial Operations

                                                                                                                                  - are performed directly on the pixels of a given image

                                                                                                                                  There are three categories of spatial operations

                                                                                                                                  single-pixel operations

                                                                                                                                  neighborhood operations

                                                                                                                                  geometric spatial transformations

                                                                                                                                  Single-pixel operations

                                                                                                                                  - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                  where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                  corresponding pixel in the processed image

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Neighborhood operations

                                                                                                                                  Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                  in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                  based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                  rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                  intensity by computing the average value of the pixels in Sxy

                                                                                                                                  ( )

                                                                                                                                  1( ) ( )xyr c S

                                                                                                                                  g x y f r cm n

                                                                                                                                  The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                  used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                  largest region of an image

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Geometric spatial transformations and image registration

                                                                                                                                  - modify the spatial relationship between pixels in an image

                                                                                                                                  - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                  printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                  predefined set of rules

                                                                                                                                  A geometric transformation consists of 2 basic operations

                                                                                                                                  1 a spatial transformation of coordinates

                                                                                                                                  2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                  pixels

                                                                                                                                  The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                  (v w) ndash pixel coordinates in the original image

                                                                                                                                  (x y) ndash pixel coordinates in the transformed image

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                  Affine transform

                                                                                                                                  11 1211 21 31

                                                                                                                                  21 2212 22 33

                                                                                                                                  31 32

                                                                                                                                  0[ 1] [ 1] [ 1] 0

                                                                                                                                  1

                                                                                                                                  t tx t v t w t

                                                                                                                                  x y v w T v w t ty t v t w t

                                                                                                                                  t t

                                                                                                                                  (AT)

                                                                                                                                  This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                  on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                  result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                  scaling rotation and translation matrices from Table 1

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Affine transformations

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                  the process we have to assign intensity values to those locations This task is done by

                                                                                                                                  using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                  In practice we can use equation (AT) in two basic ways

                                                                                                                                  forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                  location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                  Problems

                                                                                                                                  - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                  the same location in the output image

                                                                                                                                  - some output locations have no correspondent in the original image (no intensity

                                                                                                                                  assignment)

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                  computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                  It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                  pixel value

                                                                                                                                  Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                  numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Image registration ndash align two or more images of the same scene

                                                                                                                                  In image registration we have available the input and output images but the specific

                                                                                                                                  transformation that produced the output image from the input is generally unknown

                                                                                                                                  The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                  images

                                                                                                                                  - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                  same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                  - align images of a given location taken by the same instrument at different moments

                                                                                                                                  of time (satellite images)

                                                                                                                                  Solving the problem using tie points (also called control points) which are

                                                                                                                                  corresponding points whose locations are known precisely in the input and reference

                                                                                                                                  image

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  How to select tie points

                                                                                                                                  - interactively selecting them

                                                                                                                                  - use of algorithms that try to detect these points

                                                                                                                                  - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                  the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                  marks) directly on all images captured by the system which can be used as guides

                                                                                                                                  for establishing tie points

                                                                                                                                  The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                  of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                  a bilinear approximation is given by

                                                                                                                                  1 2 3 4

                                                                                                                                  5 6 7 8

                                                                                                                                  x c v c w c v w cy c v c w c v w c

                                                                                                                                  (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                  frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                  subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                  subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                  The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                  problem depend on the severity of the geometrical distortion

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Probabilistic Methods

                                                                                                                                  zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                  p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                  ( ) kk

                                                                                                                                  np zM N

                                                                                                                                  nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                  pixels in the image) 1

                                                                                                                                  0( ) 1

                                                                                                                                  L

                                                                                                                                  kk

                                                                                                                                  p z

                                                                                                                                  The mean (average) intensity of an image is given by 1

                                                                                                                                  0( )

                                                                                                                                  L

                                                                                                                                  k kk

                                                                                                                                  m z p z

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  The variance of the intensities is 1

                                                                                                                                  2 2

                                                                                                                                  0( ) ( )

                                                                                                                                  L

                                                                                                                                  k kk

                                                                                                                                  z m p z

                                                                                                                                  The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                  measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                  ( ) is used

                                                                                                                                  The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                  0( ) ( ) ( )

                                                                                                                                  Ln

                                                                                                                                  n k kk

                                                                                                                                  z z m p z

                                                                                                                                  ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                  3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                  ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                  mean

                                                                                                                                  Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                  Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                  Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                  Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Intensity Transformations and Spatial Filtering

                                                                                                                                  ( ) ( )g x y T f x y

                                                                                                                                  f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                  neighborhood of (x y)

                                                                                                                                  - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                  and much smaller in size than the image

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                  called spatial filter (spatial mask kernel template or window)

                                                                                                                                  ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                  ( )s T r

                                                                                                                                  s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                  Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                  darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                  is called contrast stretching

                                                                                                                                  Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                  thresholding function

                                                                                                                                  Some Basic Intensity Transformation Functions

                                                                                                                                  Image Negatives

                                                                                                                                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                  - equivalent of a photographic negative

                                                                                                                                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                  image

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Original Negative image

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                  Some basic intensity transformation functions

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                  range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                  while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                  transformation The log functions compress the dynamic range of images with large

                                                                                                                                  variations in pixel values

                                                                                                                                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Power-Law (Gamma) Transformations

                                                                                                                                  - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                  Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                  of output values with the opposite being true for higher values of input values The

                                                                                                                                  curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                  1c - identity transformation

                                                                                                                                  A variety of devices used for image capture printing and display respond according to a

                                                                                                                                  power law The process used to correct these power-law response phenomena is called

                                                                                                                                  gamma correction

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Piecewise-Linear Transformations Functions

                                                                                                                                  Contrast stretching

                                                                                                                                  - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                  intensity range of the recording tool or display device

                                                                                                                                  a b c d Fig5

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  11

                                                                                                                                  1

                                                                                                                                  2 1 1 21 2

                                                                                                                                  2 1 2 1

                                                                                                                                  22

                                                                                                                                  2

                                                                                                                                  [0 ]

                                                                                                                                  ( ) ( )( ) [ ]( ) ( )

                                                                                                                                  ( 1 ) [ 1]( 1 )

                                                                                                                                  s r r rrs r r s r rT r r r r

                                                                                                                                  r r r rs L r r r L

                                                                                                                                  L r

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  1 1 2 2r s r s identity transformation (no change)

                                                                                                                                  1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                  Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                  in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                  from their original range to the full range [0 L-1]

                                                                                                                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                  2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                  The original image on which these results are based is a scanning electron microscope

                                                                                                                                  image of pollen magnified approximately 700 times

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Intensity-level slicing

                                                                                                                                  - highlighting a specific range of intensities in an image

                                                                                                                                  There are two approaches for intensity-level slicing

                                                                                                                                  1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                  another (say black) all other intensities (Figure 311 (a))

                                                                                                                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                  intensities in the image (Figure 311 (b))

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                  the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                  Highlights range [A B] and preserves all other intensities

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                  blockageshellip)

                                                                                                                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                  image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                  Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Bit-plane slicing

                                                                                                                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                  This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  Digital Image Processing

                                                                                                                                  Week 1

                                                                                                                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                  • DIP 1 2017
                                                                                                                                  • DIP 02 (2017)

                                                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                                                    Week 1Week 1

                                                                                                                                    r(xy)=0 - total absorption r(xy)=1 - total reflectance

                                                                                                                                    i(xy) ndash determined by the illumination source

                                                                                                                                    r(xy) ndash determined by the characteristics of the imaged objects

                                                                                                                                    is called gray (or intensity) scale

                                                                                                                                    In practice

                                                                                                                                    min 0 0 max min min min max max max( ) L l f x y L L i r L i r

                                                                                                                                    indoor values without additional illuminationmin max10 1000L L

                                                                                                                                    black whitemin max0 1 0 1 0 1L L L L l l L

                                                                                                                                    min maxL L

                                                                                                                                    Digital Image ProcessingDigital Image Processing

                                                                                                                                    Week 1Week 1

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Image Sampling and Quantization

                                                                                                                                    - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                                                    scene

                                                                                                                                    converting a continuous image f to digital form

                                                                                                                                    - digitizing (x y) is called sampling

                                                                                                                                    - digitizing f(x y) is called quantization

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                                    (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                                    ( )

                                                                                                                                    ( 10) ( 11) ( 1 1)

                                                                                                                                    f f f Nf f f N

                                                                                                                                    f x y

                                                                                                                                    f M f M f M N

                                                                                                                                    image element pixel

                                                                                                                                    00 01 0 1

                                                                                                                                    10 11 1 1

                                                                                                                                    10 11 1 1

                                                                                                                                    ( ) ( )

                                                                                                                                    N

                                                                                                                                    i jN M N

                                                                                                                                    i j

                                                                                                                                    M M M N

                                                                                                                                    a a aa f x i y j f i ja a a

                                                                                                                                    Aa

                                                                                                                                    a a a

                                                                                                                                    f(00) ndash the upper left corner of the image

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    M N ge 0 L=2k

                                                                                                                                    [0 1]i j i ja a L

                                                                                                                                    Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Number of bits required to store a digitized image

                                                                                                                                    for 2 b M N k M N b N k

                                                                                                                                    When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                                    Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                                    Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                                    (eg 100 line pairs per mm)

                                                                                                                                    Dots per unit distance are commonly used in printing and publishing

                                                                                                                                    In US the measure is expressed in dots per inch (dpi)

                                                                                                                                    (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                                    Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                                    The number of intensity levels (L) is determined by hardware considerations

                                                                                                                                    L=2k ndash most common k = 8

                                                                                                                                    Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                                                    Week 1

                                                                                                                                    Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                                    150 dpi (lower left) 72 dpi (lower right)

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Reducing the number of gray levels 256 128 64 32

                                                                                                                                    Digital Image Processing

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                                                                                                                                    Reducing the number of gray levels 16 8 4 2

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                    Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                    Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                    Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                    750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                    same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                    image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                    Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                    Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                    intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                    This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                    straight edges

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                    where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                    be written using the 4 nearest neighbors of point (x y)

                                                                                                                                    Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                    modest increase in computational effort

                                                                                                                                    Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                    nearest neighbors of the point 3 3

                                                                                                                                    0 0

                                                                                                                                    ( ) i ji j

                                                                                                                                    i jv x y c x y

                                                                                                                                    The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                    intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                    0 0

                                                                                                                                    ( )i ji j

                                                                                                                                    i jc x y x y

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                    bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                    programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                    Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                    the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                    then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                    neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                    Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                    interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                    from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Neighbors of a Pixel

                                                                                                                                    A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                    This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                    The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                    and are denoted ND(p)

                                                                                                                                    The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                    N8 (p)

                                                                                                                                    If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                    fall outside the image

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Adjacency Connectivity Regions Boundaries

                                                                                                                                    Denote by V the set of intensity levels used to define adjacency

                                                                                                                                    - in a binary image V 01 (V=0 V=1)

                                                                                                                                    - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                    We consider 3 types of adjacency

                                                                                                                                    (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                    (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                    (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                    m-adjacent if

                                                                                                                                    4( )q N p or

                                                                                                                                    ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                    Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                    ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    binary image

                                                                                                                                    0 1 1 0 1 1 0 1 1

                                                                                                                                    1 0 1 0 0 1 0 0 1 0

                                                                                                                                    0 0 1 0 0 1 0 0 1

                                                                                                                                    V

                                                                                                                                    The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                    8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                    m-adjacency

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                    is a sequence of distinct pixels with coordinates

                                                                                                                                    and are adjacent 0 0 1 1

                                                                                                                                    1 1

                                                                                                                                    ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                    n n

                                                                                                                                    i i i i

                                                                                                                                    x y x y x y x y s tx y x y i n

                                                                                                                                    The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                    Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                    Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                    in S if there exists a path between them consisting only of pixels from S

                                                                                                                                    S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                    Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                    Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                    that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                    8-adjacency are considered

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                    touches the image border

                                                                                                                                    the complement of 1

                                                                                                                                    ( )K

                                                                                                                                    cu k u u

                                                                                                                                    k

                                                                                                                                    R R R R

                                                                                                                                    We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                    background of the image

                                                                                                                                    The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                    points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                    region that have at least one background neighbor This definition is referred to as the

                                                                                                                                    inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                    border in the background

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Distance measures

                                                                                                                                    For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                    function or metric if

                                                                                                                                    (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                    (b) D(p q) = D(q p)

                                                                                                                                    (c) D(p z) le D(p q) + D(q z)

                                                                                                                                    The Euclidean distance between p and q is defined as 1

                                                                                                                                    2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                    The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                    centered at (x y)

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                    4( ) | | | |D p q x s y t

                                                                                                                                    The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                    4

                                                                                                                                    22 1 2

                                                                                                                                    2 2 1 0 1 22 1 2

                                                                                                                                    2

                                                                                                                                    D

                                                                                                                                    The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                    The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                    8( ) max| | | |D p q x s y t

                                                                                                                                    The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    8

                                                                                                                                    2 2 2 2 22 1 1 1 2

                                                                                                                                    2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                    D

                                                                                                                                    The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                    D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                    because these distances involve only the coordinates of the point

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Array versus Matrix Operations

                                                                                                                                    An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                    11 12 11 12

                                                                                                                                    21 22 21 22

                                                                                                                                    a a b ba a b b

                                                                                                                                    Array product

                                                                                                                                    11 12 11 12 11 11 12 12

                                                                                                                                    21 22 21 22 21 21 22 21

                                                                                                                                    a a b b a b a ba a b b a b a b

                                                                                                                                    Matrix product

                                                                                                                                    11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                    21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                    a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                    We assume array operations unless stated otherwise

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Linear versus Nonlinear Operations

                                                                                                                                    One of the most important classifications of image-processing methods is whether it is

                                                                                                                                    linear or nonlinear

                                                                                                                                    ( ) ( )H f x y g x y

                                                                                                                                    H is said to be a linear operator if

                                                                                                                                    images1 2 1 2

                                                                                                                                    1 2

                                                                                                                                    ( ) ( ) ( ) ( )

                                                                                                                                    H a f x y b f x y a H f x y b H f x y

                                                                                                                                    a b f f

                                                                                                                                    Example of nonlinear operator

                                                                                                                                    the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                    1 2

                                                                                                                                    0 2 6 5 1 1

                                                                                                                                    2 3 4 7f f a b

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    1 2

                                                                                                                                    0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                    2 3 4 7 2 4a f b f

                                                                                                                                    0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                    2 3 4 7

                                                                                                                                    Arithmetic Operations in Image Processing

                                                                                                                                    Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                    f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                    For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                    value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                    The two random variables are uncorrelated when their covariance is 0

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                    used in image enhancement)

                                                                                                                                    1

                                                                                                                                    1( ) ( )K

                                                                                                                                    ii

                                                                                                                                    g x y g x yK

                                                                                                                                    If the noise satisfies the properties stated above we have

                                                                                                                                    2 2( ) ( )

                                                                                                                                    1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                    ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                    and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                    the average image is

                                                                                                                                    ( ) ( )1

                                                                                                                                    g x y x yK

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                                                                                                                                    Week 1

                                                                                                                                    As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                    the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                    means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                    averaging process increases

                                                                                                                                    An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                    under very low light levels frequently causes sensor noise to render single images

                                                                                                                                    virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                    was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                    64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                    images respectively

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                                                                                                                                    Week 1

                                                                                                                                    Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                    100 noisy images

                                                                                                                                    a b c d e f

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                    images

                                                                                                                                    (a) (b) (c)

                                                                                                                                    Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                    significant bit of each pixel (c) the difference between the two images

                                                                                                                                    Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                    Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                    images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                    difference between images (a) and (b)

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                    h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                    intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                    source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                    bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                    anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                    images after injection of the contrast medium

                                                                                                                                    In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                    Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                    propagates through the various arteries in the area being observed

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    An important application of image multiplication (and division) is shading correction

                                                                                                                                    Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                    f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                    When the shading function is known

                                                                                                                                    ( )( )( )

                                                                                                                                    g x yf x yh x y

                                                                                                                                    h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                    approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                    sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    (a) (b) (c)

                                                                                                                                    Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                    operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                    1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                    image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                    (a) (b) (c)

                                                                                                                                    Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                    min( )mf f f

                                                                                                                                    0 ( 255)max( )

                                                                                                                                    ms

                                                                                                                                    m

                                                                                                                                    ff K K K

                                                                                                                                    f

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Spatial Operations

                                                                                                                                    - are performed directly on the pixels of a given image

                                                                                                                                    There are three categories of spatial operations

                                                                                                                                    single-pixel operations

                                                                                                                                    neighborhood operations

                                                                                                                                    geometric spatial transformations

                                                                                                                                    Single-pixel operations

                                                                                                                                    - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                    where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                    corresponding pixel in the processed image

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Neighborhood operations

                                                                                                                                    Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                    in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                    based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                    rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                    intensity by computing the average value of the pixels in Sxy

                                                                                                                                    ( )

                                                                                                                                    1( ) ( )xyr c S

                                                                                                                                    g x y f r cm n

                                                                                                                                    The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                    used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                    largest region of an image

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Geometric spatial transformations and image registration

                                                                                                                                    - modify the spatial relationship between pixels in an image

                                                                                                                                    - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                    printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                    predefined set of rules

                                                                                                                                    A geometric transformation consists of 2 basic operations

                                                                                                                                    1 a spatial transformation of coordinates

                                                                                                                                    2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                    pixels

                                                                                                                                    The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                    (v w) ndash pixel coordinates in the original image

                                                                                                                                    (x y) ndash pixel coordinates in the transformed image

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                    Affine transform

                                                                                                                                    11 1211 21 31

                                                                                                                                    21 2212 22 33

                                                                                                                                    31 32

                                                                                                                                    0[ 1] [ 1] [ 1] 0

                                                                                                                                    1

                                                                                                                                    t tx t v t w t

                                                                                                                                    x y v w T v w t ty t v t w t

                                                                                                                                    t t

                                                                                                                                    (AT)

                                                                                                                                    This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                    on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                    result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                    scaling rotation and translation matrices from Table 1

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Affine transformations

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                    the process we have to assign intensity values to those locations This task is done by

                                                                                                                                    using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                    In practice we can use equation (AT) in two basic ways

                                                                                                                                    forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                    location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                    Problems

                                                                                                                                    - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                    the same location in the output image

                                                                                                                                    - some output locations have no correspondent in the original image (no intensity

                                                                                                                                    assignment)

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                    computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                    It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                    pixel value

                                                                                                                                    Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                    numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Image registration ndash align two or more images of the same scene

                                                                                                                                    In image registration we have available the input and output images but the specific

                                                                                                                                    transformation that produced the output image from the input is generally unknown

                                                                                                                                    The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                    images

                                                                                                                                    - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                    same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                    - align images of a given location taken by the same instrument at different moments

                                                                                                                                    of time (satellite images)

                                                                                                                                    Solving the problem using tie points (also called control points) which are

                                                                                                                                    corresponding points whose locations are known precisely in the input and reference

                                                                                                                                    image

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    How to select tie points

                                                                                                                                    - interactively selecting them

                                                                                                                                    - use of algorithms that try to detect these points

                                                                                                                                    - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                    the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                    marks) directly on all images captured by the system which can be used as guides

                                                                                                                                    for establishing tie points

                                                                                                                                    The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                    of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                    a bilinear approximation is given by

                                                                                                                                    1 2 3 4

                                                                                                                                    5 6 7 8

                                                                                                                                    x c v c w c v w cy c v c w c v w c

                                                                                                                                    (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                    frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                    subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                    subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                    The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                    problem depend on the severity of the geometrical distortion

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Probabilistic Methods

                                                                                                                                    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                    p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                    ( ) kk

                                                                                                                                    np zM N

                                                                                                                                    nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                    pixels in the image) 1

                                                                                                                                    0( ) 1

                                                                                                                                    L

                                                                                                                                    kk

                                                                                                                                    p z

                                                                                                                                    The mean (average) intensity of an image is given by 1

                                                                                                                                    0( )

                                                                                                                                    L

                                                                                                                                    k kk

                                                                                                                                    m z p z

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    The variance of the intensities is 1

                                                                                                                                    2 2

                                                                                                                                    0( ) ( )

                                                                                                                                    L

                                                                                                                                    k kk

                                                                                                                                    z m p z

                                                                                                                                    The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                    measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                    ( ) is used

                                                                                                                                    The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                    0( ) ( ) ( )

                                                                                                                                    Ln

                                                                                                                                    n k kk

                                                                                                                                    z z m p z

                                                                                                                                    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                    3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                    ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                    mean

                                                                                                                                    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                    Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                    Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                    Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Intensity Transformations and Spatial Filtering

                                                                                                                                    ( ) ( )g x y T f x y

                                                                                                                                    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                    neighborhood of (x y)

                                                                                                                                    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                    and much smaller in size than the image

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                    called spatial filter (spatial mask kernel template or window)

                                                                                                                                    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                    ( )s T r

                                                                                                                                    s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                    Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                    darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                    is called contrast stretching

                                                                                                                                    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                    thresholding function

                                                                                                                                    Some Basic Intensity Transformation Functions

                                                                                                                                    Image Negatives

                                                                                                                                    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                    - equivalent of a photographic negative

                                                                                                                                    - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                    image

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Original Negative image

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                    Some basic intensity transformation functions

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                    range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                    while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                    transformation The log functions compress the dynamic range of images with large

                                                                                                                                    variations in pixel values

                                                                                                                                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Power-Law (Gamma) Transformations

                                                                                                                                    - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                    Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                    of output values with the opposite being true for higher values of input values The

                                                                                                                                    curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                    1c - identity transformation

                                                                                                                                    A variety of devices used for image capture printing and display respond according to a

                                                                                                                                    power law The process used to correct these power-law response phenomena is called

                                                                                                                                    gamma correction

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Piecewise-Linear Transformations Functions

                                                                                                                                    Contrast stretching

                                                                                                                                    - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                    intensity range of the recording tool or display device

                                                                                                                                    a b c d Fig5

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    11

                                                                                                                                    1

                                                                                                                                    2 1 1 21 2

                                                                                                                                    2 1 2 1

                                                                                                                                    22

                                                                                                                                    2

                                                                                                                                    [0 ]

                                                                                                                                    ( ) ( )( ) [ ]( ) ( )

                                                                                                                                    ( 1 ) [ 1]( 1 )

                                                                                                                                    s r r rrs r r s r rT r r r r

                                                                                                                                    r r r rs L r r r L

                                                                                                                                    L r

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    1 1 2 2r s r s identity transformation (no change)

                                                                                                                                    1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                    Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                    in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                    from their original range to the full range [0 L-1]

                                                                                                                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                    2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                    The original image on which these results are based is a scanning electron microscope

                                                                                                                                    image of pollen magnified approximately 700 times

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Intensity-level slicing

                                                                                                                                    - highlighting a specific range of intensities in an image

                                                                                                                                    There are two approaches for intensity-level slicing

                                                                                                                                    1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                    another (say black) all other intensities (Figure 311 (a))

                                                                                                                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                    intensities in the image (Figure 311 (b))

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                    the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                    Highlights range [A B] and preserves all other intensities

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                    blockageshellip)

                                                                                                                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                    image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                    Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Bit-plane slicing

                                                                                                                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                    This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    Digital Image Processing

                                                                                                                                    Week 1

                                                                                                                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                    • DIP 1 2017
                                                                                                                                    • DIP 02 (2017)

                                                                                                                                      Digital Image ProcessingDigital Image Processing

                                                                                                                                      Week 1Week 1

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Image Sampling and Quantization

                                                                                                                                      - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                                                      scene

                                                                                                                                      converting a continuous image f to digital form

                                                                                                                                      - digitizing (x y) is called sampling

                                                                                                                                      - digitizing f(x y) is called quantization

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                                      (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                                      ( )

                                                                                                                                      ( 10) ( 11) ( 1 1)

                                                                                                                                      f f f Nf f f N

                                                                                                                                      f x y

                                                                                                                                      f M f M f M N

                                                                                                                                      image element pixel

                                                                                                                                      00 01 0 1

                                                                                                                                      10 11 1 1

                                                                                                                                      10 11 1 1

                                                                                                                                      ( ) ( )

                                                                                                                                      N

                                                                                                                                      i jN M N

                                                                                                                                      i j

                                                                                                                                      M M M N

                                                                                                                                      a a aa f x i y j f i ja a a

                                                                                                                                      Aa

                                                                                                                                      a a a

                                                                                                                                      f(00) ndash the upper left corner of the image

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      M N ge 0 L=2k

                                                                                                                                      [0 1]i j i ja a L

                                                                                                                                      Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Number of bits required to store a digitized image

                                                                                                                                      for 2 b M N k M N b N k

                                                                                                                                      When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                                      Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                                      Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                                      (eg 100 line pairs per mm)

                                                                                                                                      Dots per unit distance are commonly used in printing and publishing

                                                                                                                                      In US the measure is expressed in dots per inch (dpi)

                                                                                                                                      (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                                      Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                                      The number of intensity levels (L) is determined by hardware considerations

                                                                                                                                      L=2k ndash most common k = 8

                                                                                                                                      Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                                      150 dpi (lower left) 72 dpi (lower right)

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Reducing the number of gray levels 256 128 64 32

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Reducing the number of gray levels 16 8 4 2

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                      Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                      Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                      Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                      750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                      same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                      image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                      Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                      Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                      intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                      This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                      straight edges

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                      where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                      be written using the 4 nearest neighbors of point (x y)

                                                                                                                                      Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                      modest increase in computational effort

                                                                                                                                      Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                      nearest neighbors of the point 3 3

                                                                                                                                      0 0

                                                                                                                                      ( ) i ji j

                                                                                                                                      i jv x y c x y

                                                                                                                                      The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                      intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                      0 0

                                                                                                                                      ( )i ji j

                                                                                                                                      i jc x y x y

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                      bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                      programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                      Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                      the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                      then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                      neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                      Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                      interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                      from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Neighbors of a Pixel

                                                                                                                                      A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                      This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                      The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                      and are denoted ND(p)

                                                                                                                                      The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                      N8 (p)

                                                                                                                                      If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                      fall outside the image

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Adjacency Connectivity Regions Boundaries

                                                                                                                                      Denote by V the set of intensity levels used to define adjacency

                                                                                                                                      - in a binary image V 01 (V=0 V=1)

                                                                                                                                      - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                      We consider 3 types of adjacency

                                                                                                                                      (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                      (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                      (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                      m-adjacent if

                                                                                                                                      4( )q N p or

                                                                                                                                      ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                      Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                      ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      binary image

                                                                                                                                      0 1 1 0 1 1 0 1 1

                                                                                                                                      1 0 1 0 0 1 0 0 1 0

                                                                                                                                      0 0 1 0 0 1 0 0 1

                                                                                                                                      V

                                                                                                                                      The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                      8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                      m-adjacency

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                      is a sequence of distinct pixels with coordinates

                                                                                                                                      and are adjacent 0 0 1 1

                                                                                                                                      1 1

                                                                                                                                      ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                      n n

                                                                                                                                      i i i i

                                                                                                                                      x y x y x y x y s tx y x y i n

                                                                                                                                      The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                      Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                      Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                      in S if there exists a path between them consisting only of pixels from S

                                                                                                                                      S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                      Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                      Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                      that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                      8-adjacency are considered

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                      touches the image border

                                                                                                                                      the complement of 1

                                                                                                                                      ( )K

                                                                                                                                      cu k u u

                                                                                                                                      k

                                                                                                                                      R R R R

                                                                                                                                      We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                      background of the image

                                                                                                                                      The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                      points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                      region that have at least one background neighbor This definition is referred to as the

                                                                                                                                      inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                      border in the background

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Distance measures

                                                                                                                                      For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                      function or metric if

                                                                                                                                      (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                      (b) D(p q) = D(q p)

                                                                                                                                      (c) D(p z) le D(p q) + D(q z)

                                                                                                                                      The Euclidean distance between p and q is defined as 1

                                                                                                                                      2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                      The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                      centered at (x y)

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                      4( ) | | | |D p q x s y t

                                                                                                                                      The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                      4

                                                                                                                                      22 1 2

                                                                                                                                      2 2 1 0 1 22 1 2

                                                                                                                                      2

                                                                                                                                      D

                                                                                                                                      The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                      The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                      8( ) max| | | |D p q x s y t

                                                                                                                                      The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      8

                                                                                                                                      2 2 2 2 22 1 1 1 2

                                                                                                                                      2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                      D

                                                                                                                                      The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                      D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                      because these distances involve only the coordinates of the point

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Array versus Matrix Operations

                                                                                                                                      An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                      11 12 11 12

                                                                                                                                      21 22 21 22

                                                                                                                                      a a b ba a b b

                                                                                                                                      Array product

                                                                                                                                      11 12 11 12 11 11 12 12

                                                                                                                                      21 22 21 22 21 21 22 21

                                                                                                                                      a a b b a b a ba a b b a b a b

                                                                                                                                      Matrix product

                                                                                                                                      11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                      21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                      a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                      We assume array operations unless stated otherwise

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Linear versus Nonlinear Operations

                                                                                                                                      One of the most important classifications of image-processing methods is whether it is

                                                                                                                                      linear or nonlinear

                                                                                                                                      ( ) ( )H f x y g x y

                                                                                                                                      H is said to be a linear operator if

                                                                                                                                      images1 2 1 2

                                                                                                                                      1 2

                                                                                                                                      ( ) ( ) ( ) ( )

                                                                                                                                      H a f x y b f x y a H f x y b H f x y

                                                                                                                                      a b f f

                                                                                                                                      Example of nonlinear operator

                                                                                                                                      the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                      1 2

                                                                                                                                      0 2 6 5 1 1

                                                                                                                                      2 3 4 7f f a b

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      1 2

                                                                                                                                      0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                      2 3 4 7 2 4a f b f

                                                                                                                                      0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                      2 3 4 7

                                                                                                                                      Arithmetic Operations in Image Processing

                                                                                                                                      Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                      f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                      For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                      value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                      The two random variables are uncorrelated when their covariance is 0

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                      used in image enhancement)

                                                                                                                                      1

                                                                                                                                      1( ) ( )K

                                                                                                                                      ii

                                                                                                                                      g x y g x yK

                                                                                                                                      If the noise satisfies the properties stated above we have

                                                                                                                                      2 2( ) ( )

                                                                                                                                      1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                      ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                      and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                      the average image is

                                                                                                                                      ( ) ( )1

                                                                                                                                      g x y x yK

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                      the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                      means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                      averaging process increases

                                                                                                                                      An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                      under very low light levels frequently causes sensor noise to render single images

                                                                                                                                      virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                      was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                      64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                      images respectively

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                      100 noisy images

                                                                                                                                      a b c d e f

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                      images

                                                                                                                                      (a) (b) (c)

                                                                                                                                      Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                      significant bit of each pixel (c) the difference between the two images

                                                                                                                                      Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                      Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                      images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                      difference between images (a) and (b)

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                      h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                      intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                      source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                      bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                      anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                      images after injection of the contrast medium

                                                                                                                                      In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                      Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                      propagates through the various arteries in the area being observed

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      An important application of image multiplication (and division) is shading correction

                                                                                                                                      Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                      f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                      When the shading function is known

                                                                                                                                      ( )( )( )

                                                                                                                                      g x yf x yh x y

                                                                                                                                      h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                      approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                      sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      (a) (b) (c)

                                                                                                                                      Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                      operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                      1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                      image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                      (a) (b) (c)

                                                                                                                                      Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                      min( )mf f f

                                                                                                                                      0 ( 255)max( )

                                                                                                                                      ms

                                                                                                                                      m

                                                                                                                                      ff K K K

                                                                                                                                      f

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Spatial Operations

                                                                                                                                      - are performed directly on the pixels of a given image

                                                                                                                                      There are three categories of spatial operations

                                                                                                                                      single-pixel operations

                                                                                                                                      neighborhood operations

                                                                                                                                      geometric spatial transformations

                                                                                                                                      Single-pixel operations

                                                                                                                                      - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                      where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                      corresponding pixel in the processed image

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Neighborhood operations

                                                                                                                                      Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                      in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                      based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                      rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                      intensity by computing the average value of the pixels in Sxy

                                                                                                                                      ( )

                                                                                                                                      1( ) ( )xyr c S

                                                                                                                                      g x y f r cm n

                                                                                                                                      The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                      used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                      largest region of an image

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Geometric spatial transformations and image registration

                                                                                                                                      - modify the spatial relationship between pixels in an image

                                                                                                                                      - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                      printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                      predefined set of rules

                                                                                                                                      A geometric transformation consists of 2 basic operations

                                                                                                                                      1 a spatial transformation of coordinates

                                                                                                                                      2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                      pixels

                                                                                                                                      The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                      (v w) ndash pixel coordinates in the original image

                                                                                                                                      (x y) ndash pixel coordinates in the transformed image

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                      Affine transform

                                                                                                                                      11 1211 21 31

                                                                                                                                      21 2212 22 33

                                                                                                                                      31 32

                                                                                                                                      0[ 1] [ 1] [ 1] 0

                                                                                                                                      1

                                                                                                                                      t tx t v t w t

                                                                                                                                      x y v w T v w t ty t v t w t

                                                                                                                                      t t

                                                                                                                                      (AT)

                                                                                                                                      This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                      on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                      result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                      scaling rotation and translation matrices from Table 1

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Affine transformations

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                      the process we have to assign intensity values to those locations This task is done by

                                                                                                                                      using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                      In practice we can use equation (AT) in two basic ways

                                                                                                                                      forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                      location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                      Problems

                                                                                                                                      - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                      the same location in the output image

                                                                                                                                      - some output locations have no correspondent in the original image (no intensity

                                                                                                                                      assignment)

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                      computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                      It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                      pixel value

                                                                                                                                      Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                      numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Image registration ndash align two or more images of the same scene

                                                                                                                                      In image registration we have available the input and output images but the specific

                                                                                                                                      transformation that produced the output image from the input is generally unknown

                                                                                                                                      The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                      images

                                                                                                                                      - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                      same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                      - align images of a given location taken by the same instrument at different moments

                                                                                                                                      of time (satellite images)

                                                                                                                                      Solving the problem using tie points (also called control points) which are

                                                                                                                                      corresponding points whose locations are known precisely in the input and reference

                                                                                                                                      image

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      How to select tie points

                                                                                                                                      - interactively selecting them

                                                                                                                                      - use of algorithms that try to detect these points

                                                                                                                                      - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                      the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                      marks) directly on all images captured by the system which can be used as guides

                                                                                                                                      for establishing tie points

                                                                                                                                      The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                      of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                      a bilinear approximation is given by

                                                                                                                                      1 2 3 4

                                                                                                                                      5 6 7 8

                                                                                                                                      x c v c w c v w cy c v c w c v w c

                                                                                                                                      (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                      frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                      subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                      subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                      The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                      problem depend on the severity of the geometrical distortion

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Probabilistic Methods

                                                                                                                                      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                      p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                      ( ) kk

                                                                                                                                      np zM N

                                                                                                                                      nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                      pixels in the image) 1

                                                                                                                                      0( ) 1

                                                                                                                                      L

                                                                                                                                      kk

                                                                                                                                      p z

                                                                                                                                      The mean (average) intensity of an image is given by 1

                                                                                                                                      0( )

                                                                                                                                      L

                                                                                                                                      k kk

                                                                                                                                      m z p z

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      The variance of the intensities is 1

                                                                                                                                      2 2

                                                                                                                                      0( ) ( )

                                                                                                                                      L

                                                                                                                                      k kk

                                                                                                                                      z m p z

                                                                                                                                      The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                      measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                      ( ) is used

                                                                                                                                      The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                      0( ) ( ) ( )

                                                                                                                                      Ln

                                                                                                                                      n k kk

                                                                                                                                      z z m p z

                                                                                                                                      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                      3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                      ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                      mean

                                                                                                                                      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                      Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                      Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                      Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Intensity Transformations and Spatial Filtering

                                                                                                                                      ( ) ( )g x y T f x y

                                                                                                                                      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                      neighborhood of (x y)

                                                                                                                                      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                      and much smaller in size than the image

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                      called spatial filter (spatial mask kernel template or window)

                                                                                                                                      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                      ( )s T r

                                                                                                                                      s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                      Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                      darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                      is called contrast stretching

                                                                                                                                      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                      thresholding function

                                                                                                                                      Some Basic Intensity Transformation Functions

                                                                                                                                      Image Negatives

                                                                                                                                      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                      - equivalent of a photographic negative

                                                                                                                                      - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                      image

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Original Negative image

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                      Some basic intensity transformation functions

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                      range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                      while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                      transformation The log functions compress the dynamic range of images with large

                                                                                                                                      variations in pixel values

                                                                                                                                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Power-Law (Gamma) Transformations

                                                                                                                                      - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                      Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                      of output values with the opposite being true for higher values of input values The

                                                                                                                                      curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                      1c - identity transformation

                                                                                                                                      A variety of devices used for image capture printing and display respond according to a

                                                                                                                                      power law The process used to correct these power-law response phenomena is called

                                                                                                                                      gamma correction

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Piecewise-Linear Transformations Functions

                                                                                                                                      Contrast stretching

                                                                                                                                      - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                      intensity range of the recording tool or display device

                                                                                                                                      a b c d Fig5

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      11

                                                                                                                                      1

                                                                                                                                      2 1 1 21 2

                                                                                                                                      2 1 2 1

                                                                                                                                      22

                                                                                                                                      2

                                                                                                                                      [0 ]

                                                                                                                                      ( ) ( )( ) [ ]( ) ( )

                                                                                                                                      ( 1 ) [ 1]( 1 )

                                                                                                                                      s r r rrs r r s r rT r r r r

                                                                                                                                      r r r rs L r r r L

                                                                                                                                      L r

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      1 1 2 2r s r s identity transformation (no change)

                                                                                                                                      1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                      Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                      in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                      from their original range to the full range [0 L-1]

                                                                                                                                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                      2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                      The original image on which these results are based is a scanning electron microscope

                                                                                                                                      image of pollen magnified approximately 700 times

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Intensity-level slicing

                                                                                                                                      - highlighting a specific range of intensities in an image

                                                                                                                                      There are two approaches for intensity-level slicing

                                                                                                                                      1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                      another (say black) all other intensities (Figure 311 (a))

                                                                                                                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                      intensities in the image (Figure 311 (b))

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                      the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                      Highlights range [A B] and preserves all other intensities

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                      blockageshellip)

                                                                                                                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                      image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                      Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Bit-plane slicing

                                                                                                                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                      This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      Digital Image Processing

                                                                                                                                      Week 1

                                                                                                                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                      • DIP 1 2017
                                                                                                                                      • DIP 02 (2017)

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Image Sampling and Quantization

                                                                                                                                        - the output of the sensors is a continuous voltage waveform related to the sensed

                                                                                                                                        scene

                                                                                                                                        converting a continuous image f to digital form

                                                                                                                                        - digitizing (x y) is called sampling

                                                                                                                                        - digitizing f(x y) is called quantization

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                                        (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                                        ( )

                                                                                                                                        ( 10) ( 11) ( 1 1)

                                                                                                                                        f f f Nf f f N

                                                                                                                                        f x y

                                                                                                                                        f M f M f M N

                                                                                                                                        image element pixel

                                                                                                                                        00 01 0 1

                                                                                                                                        10 11 1 1

                                                                                                                                        10 11 1 1

                                                                                                                                        ( ) ( )

                                                                                                                                        N

                                                                                                                                        i jN M N

                                                                                                                                        i j

                                                                                                                                        M M M N

                                                                                                                                        a a aa f x i y j f i ja a a

                                                                                                                                        Aa

                                                                                                                                        a a a

                                                                                                                                        f(00) ndash the upper left corner of the image

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        M N ge 0 L=2k

                                                                                                                                        [0 1]i j i ja a L

                                                                                                                                        Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Number of bits required to store a digitized image

                                                                                                                                        for 2 b M N k M N b N k

                                                                                                                                        When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                                        Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                                        Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                                        (eg 100 line pairs per mm)

                                                                                                                                        Dots per unit distance are commonly used in printing and publishing

                                                                                                                                        In US the measure is expressed in dots per inch (dpi)

                                                                                                                                        (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                                        Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                                        The number of intensity levels (L) is determined by hardware considerations

                                                                                                                                        L=2k ndash most common k = 8

                                                                                                                                        Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                                                        Week 1

                                                                                                                                        Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                                        150 dpi (lower left) 72 dpi (lower right)

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Reducing the number of gray levels 256 128 64 32

                                                                                                                                        Digital Image Processing

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                                                                                                                                        Reducing the number of gray levels 16 8 4 2

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                        Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                        Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                        Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                        750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                        same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                        image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                        Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                        Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                        intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                        This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                        straight edges

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                                                                                                                                        Week 1

                                                                                                                                        Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                        where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                        be written using the 4 nearest neighbors of point (x y)

                                                                                                                                        Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                        modest increase in computational effort

                                                                                                                                        Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                        nearest neighbors of the point 3 3

                                                                                                                                        0 0

                                                                                                                                        ( ) i ji j

                                                                                                                                        i jv x y c x y

                                                                                                                                        The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                        intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                        0 0

                                                                                                                                        ( )i ji j

                                                                                                                                        i jc x y x y

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                        bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                        programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                        Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                        the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                        then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                        neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                        Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                        interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                        from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Neighbors of a Pixel

                                                                                                                                        A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                        This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                        The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                        and are denoted ND(p)

                                                                                                                                        The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                        N8 (p)

                                                                                                                                        If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                        fall outside the image

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Adjacency Connectivity Regions Boundaries

                                                                                                                                        Denote by V the set of intensity levels used to define adjacency

                                                                                                                                        - in a binary image V 01 (V=0 V=1)

                                                                                                                                        - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                        We consider 3 types of adjacency

                                                                                                                                        (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                        (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                        (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                        m-adjacent if

                                                                                                                                        4( )q N p or

                                                                                                                                        ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                        Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                        ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        binary image

                                                                                                                                        0 1 1 0 1 1 0 1 1

                                                                                                                                        1 0 1 0 0 1 0 0 1 0

                                                                                                                                        0 0 1 0 0 1 0 0 1

                                                                                                                                        V

                                                                                                                                        The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                        8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                        m-adjacency

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                        is a sequence of distinct pixels with coordinates

                                                                                                                                        and are adjacent 0 0 1 1

                                                                                                                                        1 1

                                                                                                                                        ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                        n n

                                                                                                                                        i i i i

                                                                                                                                        x y x y x y x y s tx y x y i n

                                                                                                                                        The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                        Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                        Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                        in S if there exists a path between them consisting only of pixels from S

                                                                                                                                        S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                        Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                        Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                        that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                        8-adjacency are considered

                                                                                                                                        Digital Image Processing

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                                                                                                                                        Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                        touches the image border

                                                                                                                                        the complement of 1

                                                                                                                                        ( )K

                                                                                                                                        cu k u u

                                                                                                                                        k

                                                                                                                                        R R R R

                                                                                                                                        We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                        background of the image

                                                                                                                                        The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                        points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                        region that have at least one background neighbor This definition is referred to as the

                                                                                                                                        inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                        border in the background

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                                                                                                                                        Distance measures

                                                                                                                                        For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                        function or metric if

                                                                                                                                        (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                        (b) D(p q) = D(q p)

                                                                                                                                        (c) D(p z) le D(p q) + D(q z)

                                                                                                                                        The Euclidean distance between p and q is defined as 1

                                                                                                                                        2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                        The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                        centered at (x y)

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                        4( ) | | | |D p q x s y t

                                                                                                                                        The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                        4

                                                                                                                                        22 1 2

                                                                                                                                        2 2 1 0 1 22 1 2

                                                                                                                                        2

                                                                                                                                        D

                                                                                                                                        The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                        The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                        8( ) max| | | |D p q x s y t

                                                                                                                                        The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        8

                                                                                                                                        2 2 2 2 22 1 1 1 2

                                                                                                                                        2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                        D

                                                                                                                                        The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                        D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                        because these distances involve only the coordinates of the point

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Array versus Matrix Operations

                                                                                                                                        An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                        11 12 11 12

                                                                                                                                        21 22 21 22

                                                                                                                                        a a b ba a b b

                                                                                                                                        Array product

                                                                                                                                        11 12 11 12 11 11 12 12

                                                                                                                                        21 22 21 22 21 21 22 21

                                                                                                                                        a a b b a b a ba a b b a b a b

                                                                                                                                        Matrix product

                                                                                                                                        11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                        21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                        a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                        We assume array operations unless stated otherwise

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Linear versus Nonlinear Operations

                                                                                                                                        One of the most important classifications of image-processing methods is whether it is

                                                                                                                                        linear or nonlinear

                                                                                                                                        ( ) ( )H f x y g x y

                                                                                                                                        H is said to be a linear operator if

                                                                                                                                        images1 2 1 2

                                                                                                                                        1 2

                                                                                                                                        ( ) ( ) ( ) ( )

                                                                                                                                        H a f x y b f x y a H f x y b H f x y

                                                                                                                                        a b f f

                                                                                                                                        Example of nonlinear operator

                                                                                                                                        the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                        1 2

                                                                                                                                        0 2 6 5 1 1

                                                                                                                                        2 3 4 7f f a b

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        1 2

                                                                                                                                        0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                        2 3 4 7 2 4a f b f

                                                                                                                                        0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                        2 3 4 7

                                                                                                                                        Arithmetic Operations in Image Processing

                                                                                                                                        Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                        f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                        For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                        value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                        The two random variables are uncorrelated when their covariance is 0

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                                                                                                                                        Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                        used in image enhancement)

                                                                                                                                        1

                                                                                                                                        1( ) ( )K

                                                                                                                                        ii

                                                                                                                                        g x y g x yK

                                                                                                                                        If the noise satisfies the properties stated above we have

                                                                                                                                        2 2( ) ( )

                                                                                                                                        1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                        ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                        and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                        the average image is

                                                                                                                                        ( ) ( )1

                                                                                                                                        g x y x yK

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                                                                                                                                        Week 1

                                                                                                                                        As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                        the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                        means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                        averaging process increases

                                                                                                                                        An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                        under very low light levels frequently causes sensor noise to render single images

                                                                                                                                        virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                        was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                        64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                        images respectively

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                        100 noisy images

                                                                                                                                        a b c d e f

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                        images

                                                                                                                                        (a) (b) (c)

                                                                                                                                        Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                        significant bit of each pixel (c) the difference between the two images

                                                                                                                                        Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                        Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                        images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                        difference between images (a) and (b)

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                        h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                        intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                        source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                        bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                        anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                        images after injection of the contrast medium

                                                                                                                                        In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                        Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                        propagates through the various arteries in the area being observed

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        An important application of image multiplication (and division) is shading correction

                                                                                                                                        Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                        f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                        When the shading function is known

                                                                                                                                        ( )( )( )

                                                                                                                                        g x yf x yh x y

                                                                                                                                        h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                        approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                        sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        (a) (b) (c)

                                                                                                                                        Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                        operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                        1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                        image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                        (a) (b) (c)

                                                                                                                                        Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                        min( )mf f f

                                                                                                                                        0 ( 255)max( )

                                                                                                                                        ms

                                                                                                                                        m

                                                                                                                                        ff K K K

                                                                                                                                        f

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Spatial Operations

                                                                                                                                        - are performed directly on the pixels of a given image

                                                                                                                                        There are three categories of spatial operations

                                                                                                                                        single-pixel operations

                                                                                                                                        neighborhood operations

                                                                                                                                        geometric spatial transformations

                                                                                                                                        Single-pixel operations

                                                                                                                                        - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                        where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                        corresponding pixel in the processed image

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Neighborhood operations

                                                                                                                                        Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                        in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                        based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                        rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                        intensity by computing the average value of the pixels in Sxy

                                                                                                                                        ( )

                                                                                                                                        1( ) ( )xyr c S

                                                                                                                                        g x y f r cm n

                                                                                                                                        The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                        used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                        largest region of an image

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Geometric spatial transformations and image registration

                                                                                                                                        - modify the spatial relationship between pixels in an image

                                                                                                                                        - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                        printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                        predefined set of rules

                                                                                                                                        A geometric transformation consists of 2 basic operations

                                                                                                                                        1 a spatial transformation of coordinates

                                                                                                                                        2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                        pixels

                                                                                                                                        The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                        (v w) ndash pixel coordinates in the original image

                                                                                                                                        (x y) ndash pixel coordinates in the transformed image

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                        Affine transform

                                                                                                                                        11 1211 21 31

                                                                                                                                        21 2212 22 33

                                                                                                                                        31 32

                                                                                                                                        0[ 1] [ 1] [ 1] 0

                                                                                                                                        1

                                                                                                                                        t tx t v t w t

                                                                                                                                        x y v w T v w t ty t v t w t

                                                                                                                                        t t

                                                                                                                                        (AT)

                                                                                                                                        This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                        on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                        result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                        scaling rotation and translation matrices from Table 1

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Affine transformations

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                        the process we have to assign intensity values to those locations This task is done by

                                                                                                                                        using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                        In practice we can use equation (AT) in two basic ways

                                                                                                                                        forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                        location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                        Problems

                                                                                                                                        - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                        the same location in the output image

                                                                                                                                        - some output locations have no correspondent in the original image (no intensity

                                                                                                                                        assignment)

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                        computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                        It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                        pixel value

                                                                                                                                        Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                        numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Digital Image Processing

                                                                                                                                        Week 1

                                                                                                                                        Image registration ndash align two or more images of the same scene

                                                                                                                                        In image registration we have available the input and output images but the specific

                                                                                                                                        transformation that produced the output image from the input is generally unknown

                                                                                                                                        The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                        images

                                                                                                                                        - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                        same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                        - align images of a given location taken by the same instrument at different moments

                                                                                                                                        of time (satellite images)

                                                                                                                                        Solving the problem using tie points (also called control points) which are

                                                                                                                                        corresponding points whose locations are known precisely in the input and reference

                                                                                                                                        image

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                                                                                                                                        How to select tie points

                                                                                                                                        - interactively selecting them

                                                                                                                                        - use of algorithms that try to detect these points

                                                                                                                                        - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                        the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                        marks) directly on all images captured by the system which can be used as guides

                                                                                                                                        for establishing tie points

                                                                                                                                        The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                        of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                        a bilinear approximation is given by

                                                                                                                                        1 2 3 4

                                                                                                                                        5 6 7 8

                                                                                                                                        x c v c w c v w cy c v c w c v w c

                                                                                                                                        (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

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                                                                                                                                        When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                        frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                        subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                        subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                        The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                        problem depend on the severity of the geometrical distortion

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                                                                                                                                        Week 1

                                                                                                                                        a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

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                                                                                                                                        Week 1

                                                                                                                                        Probabilistic Methods

                                                                                                                                        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                        p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                        ( ) kk

                                                                                                                                        np zM N

                                                                                                                                        nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                        pixels in the image) 1

                                                                                                                                        0( ) 1

                                                                                                                                        L

                                                                                                                                        kk

                                                                                                                                        p z

                                                                                                                                        The mean (average) intensity of an image is given by 1

                                                                                                                                        0( )

                                                                                                                                        L

                                                                                                                                        k kk

                                                                                                                                        m z p z

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                                                                                                                                        The variance of the intensities is 1

                                                                                                                                        2 2

                                                                                                                                        0( ) ( )

                                                                                                                                        L

                                                                                                                                        k kk

                                                                                                                                        z m p z

                                                                                                                                        The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                        measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                        ( ) is used

                                                                                                                                        The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                        0( ) ( ) ( )

                                                                                                                                        Ln

                                                                                                                                        n k kk

                                                                                                                                        z z m p z

                                                                                                                                        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                        3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                        ( 3( ) 0z the intensities are biased to values lower than the mean

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                                                                                                                                        3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                        mean

                                                                                                                                        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                        Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                        Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                        Figure 1(c) ndash standard deviation 492 (variance = 24206)

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                                                                                                                                        Intensity Transformations and Spatial Filtering

                                                                                                                                        ( ) ( )g x y T f x y

                                                                                                                                        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                        neighborhood of (x y)

                                                                                                                                        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                        and much smaller in size than the image

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                                                                                                                                        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                        called spatial filter (spatial mask kernel template or window)

                                                                                                                                        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                        ( )s T r

                                                                                                                                        s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                        Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                        darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                        is called contrast stretching

                                                                                                                                        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

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                                                                                                                                        Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                        thresholding function

                                                                                                                                        Some Basic Intensity Transformation Functions

                                                                                                                                        Image Negatives

                                                                                                                                        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                        - equivalent of a photographic negative

                                                                                                                                        - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                        image

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                                                                                                                                        Original Negative image

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                                                                                                                                        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                        Some basic intensity transformation functions

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                                                                                                                                        This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                        range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                        while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                        transformation The log functions compress the dynamic range of images with large

                                                                                                                                        variations in pixel values

                                                                                                                                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

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                                                                                                                                        Power-Law (Gamma) Transformations

                                                                                                                                        - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                        Plots of gamma transformation for different values of γ (c=1)

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                                                                                                                                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                        of output values with the opposite being true for higher values of input values The

                                                                                                                                        curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                        1c - identity transformation

                                                                                                                                        A variety of devices used for image capture printing and display respond according to a

                                                                                                                                        power law The process used to correct these power-law response phenomena is called

                                                                                                                                        gamma correction

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                                                                                                                                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

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                                                                                                                                        Piecewise-Linear Transformations Functions

                                                                                                                                        Contrast stretching

                                                                                                                                        - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                        intensity range of the recording tool or display device

                                                                                                                                        a b c d Fig5

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                                                                                                                                        11

                                                                                                                                        1

                                                                                                                                        2 1 1 21 2

                                                                                                                                        2 1 2 1

                                                                                                                                        22

                                                                                                                                        2

                                                                                                                                        [0 ]

                                                                                                                                        ( ) ( )( ) [ ]( ) ( )

                                                                                                                                        ( 1 ) [ 1]( 1 )

                                                                                                                                        s r r rrs r r s r rT r r r r

                                                                                                                                        r r r rs L r r r L

                                                                                                                                        L r

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                                                                                                                                        1 1 2 2r s r s identity transformation (no change)

                                                                                                                                        1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                        Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                        in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                        from their original range to the full range [0 L-1]

                                                                                                                                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                        2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                        The original image on which these results are based is a scanning electron microscope

                                                                                                                                        image of pollen magnified approximately 700 times

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                                                                                                                                        Intensity-level slicing

                                                                                                                                        - highlighting a specific range of intensities in an image

                                                                                                                                        There are two approaches for intensity-level slicing

                                                                                                                                        1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                        another (say black) all other intensities (Figure 311 (a))

                                                                                                                                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                        intensities in the image (Figure 311 (b))

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                                                                                                                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                        the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                        Highlights range [A B] and preserves all other intensities

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                                                                                                                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                        blockageshellip)

                                                                                                                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                        image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                        Fig 6 - Aortic angiogram and intensity sliced versions

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                                                                                                                                        Bit-plane slicing

                                                                                                                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                        This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

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                                                                                                                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                        • DIP 1 2017
                                                                                                                                        • DIP 02 (2017)

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                                                                                                                                          Continuous image projected onto a sensor array Result of image sampling and quantization

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                                                                                                                                          Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                                          (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                                          ( )

                                                                                                                                          ( 10) ( 11) ( 1 1)

                                                                                                                                          f f f Nf f f N

                                                                                                                                          f x y

                                                                                                                                          f M f M f M N

                                                                                                                                          image element pixel

                                                                                                                                          00 01 0 1

                                                                                                                                          10 11 1 1

                                                                                                                                          10 11 1 1

                                                                                                                                          ( ) ( )

                                                                                                                                          N

                                                                                                                                          i jN M N

                                                                                                                                          i j

                                                                                                                                          M M M N

                                                                                                                                          a a aa f x i y j f i ja a a

                                                                                                                                          Aa

                                                                                                                                          a a a

                                                                                                                                          f(00) ndash the upper left corner of the image

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                                                                                                                                          M N ge 0 L=2k

                                                                                                                                          [0 1]i j i ja a L

                                                                                                                                          Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

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                                                                                                                                          Number of bits required to store a digitized image

                                                                                                                                          for 2 b M N k M N b N k

                                                                                                                                          When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

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                                                                                                                                          Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                                          Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                                          Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                                          (eg 100 line pairs per mm)

                                                                                                                                          Dots per unit distance are commonly used in printing and publishing

                                                                                                                                          In US the measure is expressed in dots per inch (dpi)

                                                                                                                                          (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                                          Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                                          The number of intensity levels (L) is determined by hardware considerations

                                                                                                                                          L=2k ndash most common k = 8

                                                                                                                                          Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                                                          Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                                          150 dpi (lower left) 72 dpi (lower right)

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                                                                                                                                          Reducing the number of gray levels 256 128 64 32

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                                                                                                                                          Reducing the number of gray levels 16 8 4 2

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                                                                                                                                          Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                          Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                          Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                          Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                          750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                          same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                          image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                          Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                          Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                          intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                          This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                          straight edges

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                                                                                                                                          Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                          where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                          be written using the 4 nearest neighbors of point (x y)

                                                                                                                                          Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                          modest increase in computational effort

                                                                                                                                          Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                          nearest neighbors of the point 3 3

                                                                                                                                          0 0

                                                                                                                                          ( ) i ji j

                                                                                                                                          i jv x y c x y

                                                                                                                                          The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                          intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                          0 0

                                                                                                                                          ( )i ji j

                                                                                                                                          i jc x y x y

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                                                                                                                                          Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                          bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                          programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                          Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                          the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                          then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                          neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                          Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                          interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                          from 1250 dpi to 150 dpi (instead of 72 dpi)

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                                                                                                                                          Week 1

                                                                                                                                          Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

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                                                                                                                                          Neighbors of a Pixel

                                                                                                                                          A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                          This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                          The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                          and are denoted ND(p)

                                                                                                                                          The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                          N8 (p)

                                                                                                                                          If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                          fall outside the image

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                                                                                                                                          Adjacency Connectivity Regions Boundaries

                                                                                                                                          Denote by V the set of intensity levels used to define adjacency

                                                                                                                                          - in a binary image V 01 (V=0 V=1)

                                                                                                                                          - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                          We consider 3 types of adjacency

                                                                                                                                          (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                          (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                          (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                          m-adjacent if

                                                                                                                                          4( )q N p or

                                                                                                                                          ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                          Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                          ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          binary image

                                                                                                                                          0 1 1 0 1 1 0 1 1

                                                                                                                                          1 0 1 0 0 1 0 0 1 0

                                                                                                                                          0 0 1 0 0 1 0 0 1

                                                                                                                                          V

                                                                                                                                          The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                          8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                          m-adjacency

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                                                                                                                                          A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                          is a sequence of distinct pixels with coordinates

                                                                                                                                          and are adjacent 0 0 1 1

                                                                                                                                          1 1

                                                                                                                                          ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                          n n

                                                                                                                                          i i i i

                                                                                                                                          x y x y x y x y s tx y x y i n

                                                                                                                                          The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                          Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                          Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                          in S if there exists a path between them consisting only of pixels from S

                                                                                                                                          S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                          Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                          Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                          that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                          8-adjacency are considered

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                                                                                                                                          Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                          touches the image border

                                                                                                                                          the complement of 1

                                                                                                                                          ( )K

                                                                                                                                          cu k u u

                                                                                                                                          k

                                                                                                                                          R R R R

                                                                                                                                          We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                          background of the image

                                                                                                                                          The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                          points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                          region that have at least one background neighbor This definition is referred to as the

                                                                                                                                          inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                          border in the background

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                                                                                                                                          Distance measures

                                                                                                                                          For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                          function or metric if

                                                                                                                                          (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                          (b) D(p q) = D(q p)

                                                                                                                                          (c) D(p z) le D(p q) + D(q z)

                                                                                                                                          The Euclidean distance between p and q is defined as 1

                                                                                                                                          2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                          The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                          centered at (x y)

                                                                                                                                          Digital Image Processing

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                                                                                                                                          The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                          4( ) | | | |D p q x s y t

                                                                                                                                          The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                          4

                                                                                                                                          22 1 2

                                                                                                                                          2 2 1 0 1 22 1 2

                                                                                                                                          2

                                                                                                                                          D

                                                                                                                                          The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                          The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                          8( ) max| | | |D p q x s y t

                                                                                                                                          The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                          Digital Image Processing

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                                                                                                                                          8

                                                                                                                                          2 2 2 2 22 1 1 1 2

                                                                                                                                          2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                          D

                                                                                                                                          The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                          D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                          because these distances involve only the coordinates of the point

                                                                                                                                          Digital Image Processing

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                                                                                                                                          Array versus Matrix Operations

                                                                                                                                          An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                          11 12 11 12

                                                                                                                                          21 22 21 22

                                                                                                                                          a a b ba a b b

                                                                                                                                          Array product

                                                                                                                                          11 12 11 12 11 11 12 12

                                                                                                                                          21 22 21 22 21 21 22 21

                                                                                                                                          a a b b a b a ba a b b a b a b

                                                                                                                                          Matrix product

                                                                                                                                          11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                          21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                          a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                          We assume array operations unless stated otherwise

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Linear versus Nonlinear Operations

                                                                                                                                          One of the most important classifications of image-processing methods is whether it is

                                                                                                                                          linear or nonlinear

                                                                                                                                          ( ) ( )H f x y g x y

                                                                                                                                          H is said to be a linear operator if

                                                                                                                                          images1 2 1 2

                                                                                                                                          1 2

                                                                                                                                          ( ) ( ) ( ) ( )

                                                                                                                                          H a f x y b f x y a H f x y b H f x y

                                                                                                                                          a b f f

                                                                                                                                          Example of nonlinear operator

                                                                                                                                          the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                          1 2

                                                                                                                                          0 2 6 5 1 1

                                                                                                                                          2 3 4 7f f a b

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          1 2

                                                                                                                                          0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                          2 3 4 7 2 4a f b f

                                                                                                                                          0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                          2 3 4 7

                                                                                                                                          Arithmetic Operations in Image Processing

                                                                                                                                          Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                          f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                          For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                          value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                          The two random variables are uncorrelated when their covariance is 0

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                          used in image enhancement)

                                                                                                                                          1

                                                                                                                                          1( ) ( )K

                                                                                                                                          ii

                                                                                                                                          g x y g x yK

                                                                                                                                          If the noise satisfies the properties stated above we have

                                                                                                                                          2 2( ) ( )

                                                                                                                                          1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                          ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                          and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                          the average image is

                                                                                                                                          ( ) ( )1

                                                                                                                                          g x y x yK

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                          the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                          means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                          averaging process increases

                                                                                                                                          An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                          under very low light levels frequently causes sensor noise to render single images

                                                                                                                                          virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                          was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                          64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                          images respectively

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                          100 noisy images

                                                                                                                                          a b c d e f

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                          images

                                                                                                                                          (a) (b) (c)

                                                                                                                                          Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                          significant bit of each pixel (c) the difference between the two images

                                                                                                                                          Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                          Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                          images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                          difference between images (a) and (b)

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                          h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                          intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                          source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                          bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                          anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                          images after injection of the contrast medium

                                                                                                                                          In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                          Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                          propagates through the various arteries in the area being observed

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          An important application of image multiplication (and division) is shading correction

                                                                                                                                          Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                          f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                          When the shading function is known

                                                                                                                                          ( )( )( )

                                                                                                                                          g x yf x yh x y

                                                                                                                                          h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                          approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                          sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          (a) (b) (c)

                                                                                                                                          Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                          operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                          1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                          image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                          (a) (b) (c)

                                                                                                                                          Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                          min( )mf f f

                                                                                                                                          0 ( 255)max( )

                                                                                                                                          ms

                                                                                                                                          m

                                                                                                                                          ff K K K

                                                                                                                                          f

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Spatial Operations

                                                                                                                                          - are performed directly on the pixels of a given image

                                                                                                                                          There are three categories of spatial operations

                                                                                                                                          single-pixel operations

                                                                                                                                          neighborhood operations

                                                                                                                                          geometric spatial transformations

                                                                                                                                          Single-pixel operations

                                                                                                                                          - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                          where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                          corresponding pixel in the processed image

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Neighborhood operations

                                                                                                                                          Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                          in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                          based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                          rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                          intensity by computing the average value of the pixels in Sxy

                                                                                                                                          ( )

                                                                                                                                          1( ) ( )xyr c S

                                                                                                                                          g x y f r cm n

                                                                                                                                          The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                          used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                          largest region of an image

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Geometric spatial transformations and image registration

                                                                                                                                          - modify the spatial relationship between pixels in an image

                                                                                                                                          - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                          printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                          predefined set of rules

                                                                                                                                          A geometric transformation consists of 2 basic operations

                                                                                                                                          1 a spatial transformation of coordinates

                                                                                                                                          2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                          pixels

                                                                                                                                          The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                          (v w) ndash pixel coordinates in the original image

                                                                                                                                          (x y) ndash pixel coordinates in the transformed image

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                          Affine transform

                                                                                                                                          11 1211 21 31

                                                                                                                                          21 2212 22 33

                                                                                                                                          31 32

                                                                                                                                          0[ 1] [ 1] [ 1] 0

                                                                                                                                          1

                                                                                                                                          t tx t v t w t

                                                                                                                                          x y v w T v w t ty t v t w t

                                                                                                                                          t t

                                                                                                                                          (AT)

                                                                                                                                          This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                          on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                          result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                          scaling rotation and translation matrices from Table 1

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Affine transformations

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                          the process we have to assign intensity values to those locations This task is done by

                                                                                                                                          using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                          In practice we can use equation (AT) in two basic ways

                                                                                                                                          forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                          location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                          Problems

                                                                                                                                          - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                          the same location in the output image

                                                                                                                                          - some output locations have no correspondent in the original image (no intensity

                                                                                                                                          assignment)

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                          computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                          It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                          pixel value

                                                                                                                                          Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                          numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Image registration ndash align two or more images of the same scene

                                                                                                                                          In image registration we have available the input and output images but the specific

                                                                                                                                          transformation that produced the output image from the input is generally unknown

                                                                                                                                          The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                          images

                                                                                                                                          - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                          same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                          - align images of a given location taken by the same instrument at different moments

                                                                                                                                          of time (satellite images)

                                                                                                                                          Solving the problem using tie points (also called control points) which are

                                                                                                                                          corresponding points whose locations are known precisely in the input and reference

                                                                                                                                          image

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          How to select tie points

                                                                                                                                          - interactively selecting them

                                                                                                                                          - use of algorithms that try to detect these points

                                                                                                                                          - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                          the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                          marks) directly on all images captured by the system which can be used as guides

                                                                                                                                          for establishing tie points

                                                                                                                                          The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                          of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                          a bilinear approximation is given by

                                                                                                                                          1 2 3 4

                                                                                                                                          5 6 7 8

                                                                                                                                          x c v c w c v w cy c v c w c v w c

                                                                                                                                          (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                          frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                          subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                          subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                          The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                          problem depend on the severity of the geometrical distortion

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Probabilistic Methods

                                                                                                                                          zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                          p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                          ( ) kk

                                                                                                                                          np zM N

                                                                                                                                          nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                          pixels in the image) 1

                                                                                                                                          0( ) 1

                                                                                                                                          L

                                                                                                                                          kk

                                                                                                                                          p z

                                                                                                                                          The mean (average) intensity of an image is given by 1

                                                                                                                                          0( )

                                                                                                                                          L

                                                                                                                                          k kk

                                                                                                                                          m z p z

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          The variance of the intensities is 1

                                                                                                                                          2 2

                                                                                                                                          0( ) ( )

                                                                                                                                          L

                                                                                                                                          k kk

                                                                                                                                          z m p z

                                                                                                                                          The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                          measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                          ( ) is used

                                                                                                                                          The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                          0( ) ( ) ( )

                                                                                                                                          Ln

                                                                                                                                          n k kk

                                                                                                                                          z z m p z

                                                                                                                                          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                          3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                          ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                          mean

                                                                                                                                          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                          Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                          Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                          Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Intensity Transformations and Spatial Filtering

                                                                                                                                          ( ) ( )g x y T f x y

                                                                                                                                          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                          neighborhood of (x y)

                                                                                                                                          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                          and much smaller in size than the image

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                          called spatial filter (spatial mask kernel template or window)

                                                                                                                                          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                          ( )s T r

                                                                                                                                          s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                          Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                          darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                          is called contrast stretching

                                                                                                                                          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                          thresholding function

                                                                                                                                          Some Basic Intensity Transformation Functions

                                                                                                                                          Image Negatives

                                                                                                                                          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                          - equivalent of a photographic negative

                                                                                                                                          - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                          image

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Original Negative image

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                          Some basic intensity transformation functions

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                          range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                          while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                          transformation The log functions compress the dynamic range of images with large

                                                                                                                                          variations in pixel values

                                                                                                                                          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Power-Law (Gamma) Transformations

                                                                                                                                          - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                          Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                          of output values with the opposite being true for higher values of input values The

                                                                                                                                          curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                          1c - identity transformation

                                                                                                                                          A variety of devices used for image capture printing and display respond according to a

                                                                                                                                          power law The process used to correct these power-law response phenomena is called

                                                                                                                                          gamma correction

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Piecewise-Linear Transformations Functions

                                                                                                                                          Contrast stretching

                                                                                                                                          - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                          intensity range of the recording tool or display device

                                                                                                                                          a b c d Fig5

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          11

                                                                                                                                          1

                                                                                                                                          2 1 1 21 2

                                                                                                                                          2 1 2 1

                                                                                                                                          22

                                                                                                                                          2

                                                                                                                                          [0 ]

                                                                                                                                          ( ) ( )( ) [ ]( ) ( )

                                                                                                                                          ( 1 ) [ 1]( 1 )

                                                                                                                                          s r r rrs r r s r rT r r r r

                                                                                                                                          r r r rs L r r r L

                                                                                                                                          L r

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          1 1 2 2r s r s identity transformation (no change)

                                                                                                                                          1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                          Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                          in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                          from their original range to the full range [0 L-1]

                                                                                                                                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                          2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                          The original image on which these results are based is a scanning electron microscope

                                                                                                                                          image of pollen magnified approximately 700 times

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Intensity-level slicing

                                                                                                                                          - highlighting a specific range of intensities in an image

                                                                                                                                          There are two approaches for intensity-level slicing

                                                                                                                                          1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                          another (say black) all other intensities (Figure 311 (a))

                                                                                                                                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                          intensities in the image (Figure 311 (b))

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                          the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                          Highlights range [A B] and preserves all other intensities

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                          blockageshellip)

                                                                                                                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                          image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                          Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Bit-plane slicing

                                                                                                                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                          This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          Digital Image Processing

                                                                                                                                          Week 1

                                                                                                                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                          • DIP 1 2017
                                                                                                                                          • DIP 02 (2017)

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Continuous image projected onto a sensor array Result of image sampling and quantization

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                                            (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                                            ( )

                                                                                                                                            ( 10) ( 11) ( 1 1)

                                                                                                                                            f f f Nf f f N

                                                                                                                                            f x y

                                                                                                                                            f M f M f M N

                                                                                                                                            image element pixel

                                                                                                                                            00 01 0 1

                                                                                                                                            10 11 1 1

                                                                                                                                            10 11 1 1

                                                                                                                                            ( ) ( )

                                                                                                                                            N

                                                                                                                                            i jN M N

                                                                                                                                            i j

                                                                                                                                            M M M N

                                                                                                                                            a a aa f x i y j f i ja a a

                                                                                                                                            Aa

                                                                                                                                            a a a

                                                                                                                                            f(00) ndash the upper left corner of the image

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            M N ge 0 L=2k

                                                                                                                                            [0 1]i j i ja a L

                                                                                                                                            Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Number of bits required to store a digitized image

                                                                                                                                            for 2 b M N k M N b N k

                                                                                                                                            When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                                            Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                                            Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                                            (eg 100 line pairs per mm)

                                                                                                                                            Dots per unit distance are commonly used in printing and publishing

                                                                                                                                            In US the measure is expressed in dots per inch (dpi)

                                                                                                                                            (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                                            Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                                            The number of intensity levels (L) is determined by hardware considerations

                                                                                                                                            L=2k ndash most common k = 8

                                                                                                                                            Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                                                            Week 1

                                                                                                                                            Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                                            150 dpi (lower left) 72 dpi (lower right)

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                                                                                                                                            Week 1

                                                                                                                                            Reducing the number of gray levels 256 128 64 32

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                                                                                                                                            Week 1

                                                                                                                                            Reducing the number of gray levels 16 8 4 2

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                            Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                            Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                            Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                            750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                            same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                            image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                            Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                            Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                            intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                            This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                            straight edges

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                            where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                            be written using the 4 nearest neighbors of point (x y)

                                                                                                                                            Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                            modest increase in computational effort

                                                                                                                                            Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                            nearest neighbors of the point 3 3

                                                                                                                                            0 0

                                                                                                                                            ( ) i ji j

                                                                                                                                            i jv x y c x y

                                                                                                                                            The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                            intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                            0 0

                                                                                                                                            ( )i ji j

                                                                                                                                            i jc x y x y

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                            bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                            programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                            Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                            the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                            then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                            neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                            Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                            interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                            from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Neighbors of a Pixel

                                                                                                                                            A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                            This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                            The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                            and are denoted ND(p)

                                                                                                                                            The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                            N8 (p)

                                                                                                                                            If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                            fall outside the image

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Adjacency Connectivity Regions Boundaries

                                                                                                                                            Denote by V the set of intensity levels used to define adjacency

                                                                                                                                            - in a binary image V 01 (V=0 V=1)

                                                                                                                                            - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                            We consider 3 types of adjacency

                                                                                                                                            (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                            (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                            (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                            m-adjacent if

                                                                                                                                            4( )q N p or

                                                                                                                                            ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                            Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                            ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            binary image

                                                                                                                                            0 1 1 0 1 1 0 1 1

                                                                                                                                            1 0 1 0 0 1 0 0 1 0

                                                                                                                                            0 0 1 0 0 1 0 0 1

                                                                                                                                            V

                                                                                                                                            The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                            8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                            m-adjacency

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                            is a sequence of distinct pixels with coordinates

                                                                                                                                            and are adjacent 0 0 1 1

                                                                                                                                            1 1

                                                                                                                                            ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                            n n

                                                                                                                                            i i i i

                                                                                                                                            x y x y x y x y s tx y x y i n

                                                                                                                                            The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                            Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                            Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                            in S if there exists a path between them consisting only of pixels from S

                                                                                                                                            S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                            Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                            Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                            that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                            8-adjacency are considered

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                            touches the image border

                                                                                                                                            the complement of 1

                                                                                                                                            ( )K

                                                                                                                                            cu k u u

                                                                                                                                            k

                                                                                                                                            R R R R

                                                                                                                                            We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                            background of the image

                                                                                                                                            The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                            points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                            region that have at least one background neighbor This definition is referred to as the

                                                                                                                                            inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                            border in the background

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Distance measures

                                                                                                                                            For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                            function or metric if

                                                                                                                                            (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                            (b) D(p q) = D(q p)

                                                                                                                                            (c) D(p z) le D(p q) + D(q z)

                                                                                                                                            The Euclidean distance between p and q is defined as 1

                                                                                                                                            2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                            The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                            centered at (x y)

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                            4( ) | | | |D p q x s y t

                                                                                                                                            The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                            4

                                                                                                                                            22 1 2

                                                                                                                                            2 2 1 0 1 22 1 2

                                                                                                                                            2

                                                                                                                                            D

                                                                                                                                            The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                            The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                            8( ) max| | | |D p q x s y t

                                                                                                                                            The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            8

                                                                                                                                            2 2 2 2 22 1 1 1 2

                                                                                                                                            2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                            D

                                                                                                                                            The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                            D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                            because these distances involve only the coordinates of the point

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Array versus Matrix Operations

                                                                                                                                            An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                            11 12 11 12

                                                                                                                                            21 22 21 22

                                                                                                                                            a a b ba a b b

                                                                                                                                            Array product

                                                                                                                                            11 12 11 12 11 11 12 12

                                                                                                                                            21 22 21 22 21 21 22 21

                                                                                                                                            a a b b a b a ba a b b a b a b

                                                                                                                                            Matrix product

                                                                                                                                            11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                            21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                            a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                            We assume array operations unless stated otherwise

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Linear versus Nonlinear Operations

                                                                                                                                            One of the most important classifications of image-processing methods is whether it is

                                                                                                                                            linear or nonlinear

                                                                                                                                            ( ) ( )H f x y g x y

                                                                                                                                            H is said to be a linear operator if

                                                                                                                                            images1 2 1 2

                                                                                                                                            1 2

                                                                                                                                            ( ) ( ) ( ) ( )

                                                                                                                                            H a f x y b f x y a H f x y b H f x y

                                                                                                                                            a b f f

                                                                                                                                            Example of nonlinear operator

                                                                                                                                            the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                            1 2

                                                                                                                                            0 2 6 5 1 1

                                                                                                                                            2 3 4 7f f a b

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            1 2

                                                                                                                                            0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                            2 3 4 7 2 4a f b f

                                                                                                                                            0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                            2 3 4 7

                                                                                                                                            Arithmetic Operations in Image Processing

                                                                                                                                            Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                            f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                            For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                            value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                            The two random variables are uncorrelated when their covariance is 0

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                                                                                                                                            Week 1

                                                                                                                                            Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                            used in image enhancement)

                                                                                                                                            1

                                                                                                                                            1( ) ( )K

                                                                                                                                            ii

                                                                                                                                            g x y g x yK

                                                                                                                                            If the noise satisfies the properties stated above we have

                                                                                                                                            2 2( ) ( )

                                                                                                                                            1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                            ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                            and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                            the average image is

                                                                                                                                            ( ) ( )1

                                                                                                                                            g x y x yK

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                                                                                                                                            Week 1

                                                                                                                                            As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                            the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                            means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                            averaging process increases

                                                                                                                                            An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                            under very low light levels frequently causes sensor noise to render single images

                                                                                                                                            virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                            was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                            64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                            images respectively

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                                                                                                                                            Week 1

                                                                                                                                            Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                            100 noisy images

                                                                                                                                            a b c d e f

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                            images

                                                                                                                                            (a) (b) (c)

                                                                                                                                            Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                            significant bit of each pixel (c) the difference between the two images

                                                                                                                                            Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                            Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                            images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                            difference between images (a) and (b)

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                            h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                            intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                            source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                            bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                            anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                            images after injection of the contrast medium

                                                                                                                                            In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                            Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                            propagates through the various arteries in the area being observed

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            An important application of image multiplication (and division) is shading correction

                                                                                                                                            Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                            f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                            When the shading function is known

                                                                                                                                            ( )( )( )

                                                                                                                                            g x yf x yh x y

                                                                                                                                            h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                            approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                            sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            (a) (b) (c)

                                                                                                                                            Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                            operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                            1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                            image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                            (a) (b) (c)

                                                                                                                                            Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                            min( )mf f f

                                                                                                                                            0 ( 255)max( )

                                                                                                                                            ms

                                                                                                                                            m

                                                                                                                                            ff K K K

                                                                                                                                            f

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Spatial Operations

                                                                                                                                            - are performed directly on the pixels of a given image

                                                                                                                                            There are three categories of spatial operations

                                                                                                                                            single-pixel operations

                                                                                                                                            neighborhood operations

                                                                                                                                            geometric spatial transformations

                                                                                                                                            Single-pixel operations

                                                                                                                                            - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                            where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                            corresponding pixel in the processed image

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Neighborhood operations

                                                                                                                                            Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                            in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                            based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                            rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                            intensity by computing the average value of the pixels in Sxy

                                                                                                                                            ( )

                                                                                                                                            1( ) ( )xyr c S

                                                                                                                                            g x y f r cm n

                                                                                                                                            The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                            used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                            largest region of an image

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Geometric spatial transformations and image registration

                                                                                                                                            - modify the spatial relationship between pixels in an image

                                                                                                                                            - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                            printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                            predefined set of rules

                                                                                                                                            A geometric transformation consists of 2 basic operations

                                                                                                                                            1 a spatial transformation of coordinates

                                                                                                                                            2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                            pixels

                                                                                                                                            The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                            (v w) ndash pixel coordinates in the original image

                                                                                                                                            (x y) ndash pixel coordinates in the transformed image

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                            Affine transform

                                                                                                                                            11 1211 21 31

                                                                                                                                            21 2212 22 33

                                                                                                                                            31 32

                                                                                                                                            0[ 1] [ 1] [ 1] 0

                                                                                                                                            1

                                                                                                                                            t tx t v t w t

                                                                                                                                            x y v w T v w t ty t v t w t

                                                                                                                                            t t

                                                                                                                                            (AT)

                                                                                                                                            This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                            on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                            result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                            scaling rotation and translation matrices from Table 1

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Affine transformations

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                            the process we have to assign intensity values to those locations This task is done by

                                                                                                                                            using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                            In practice we can use equation (AT) in two basic ways

                                                                                                                                            forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                            location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                            Problems

                                                                                                                                            - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                            the same location in the output image

                                                                                                                                            - some output locations have no correspondent in the original image (no intensity

                                                                                                                                            assignment)

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                            computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                            It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                            pixel value

                                                                                                                                            Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                            numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Image registration ndash align two or more images of the same scene

                                                                                                                                            In image registration we have available the input and output images but the specific

                                                                                                                                            transformation that produced the output image from the input is generally unknown

                                                                                                                                            The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                            images

                                                                                                                                            - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                            same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                            - align images of a given location taken by the same instrument at different moments

                                                                                                                                            of time (satellite images)

                                                                                                                                            Solving the problem using tie points (also called control points) which are

                                                                                                                                            corresponding points whose locations are known precisely in the input and reference

                                                                                                                                            image

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            How to select tie points

                                                                                                                                            - interactively selecting them

                                                                                                                                            - use of algorithms that try to detect these points

                                                                                                                                            - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                            the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                            marks) directly on all images captured by the system which can be used as guides

                                                                                                                                            for establishing tie points

                                                                                                                                            The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                            of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                            a bilinear approximation is given by

                                                                                                                                            1 2 3 4

                                                                                                                                            5 6 7 8

                                                                                                                                            x c v c w c v w cy c v c w c v w c

                                                                                                                                            (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                            frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                            subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                            subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                            The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                            problem depend on the severity of the geometrical distortion

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Probabilistic Methods

                                                                                                                                            zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                            p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                            ( ) kk

                                                                                                                                            np zM N

                                                                                                                                            nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                            pixels in the image) 1

                                                                                                                                            0( ) 1

                                                                                                                                            L

                                                                                                                                            kk

                                                                                                                                            p z

                                                                                                                                            The mean (average) intensity of an image is given by 1

                                                                                                                                            0( )

                                                                                                                                            L

                                                                                                                                            k kk

                                                                                                                                            m z p z

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            The variance of the intensities is 1

                                                                                                                                            2 2

                                                                                                                                            0( ) ( )

                                                                                                                                            L

                                                                                                                                            k kk

                                                                                                                                            z m p z

                                                                                                                                            The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                            measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                            ( ) is used

                                                                                                                                            The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                            0( ) ( ) ( )

                                                                                                                                            Ln

                                                                                                                                            n k kk

                                                                                                                                            z z m p z

                                                                                                                                            ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                            3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                            ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                            mean

                                                                                                                                            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                            Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                            Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                            Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Intensity Transformations and Spatial Filtering

                                                                                                                                            ( ) ( )g x y T f x y

                                                                                                                                            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                            neighborhood of (x y)

                                                                                                                                            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                            and much smaller in size than the image

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                            called spatial filter (spatial mask kernel template or window)

                                                                                                                                            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                            ( )s T r

                                                                                                                                            s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                            Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                            darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                            is called contrast stretching

                                                                                                                                            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                            thresholding function

                                                                                                                                            Some Basic Intensity Transformation Functions

                                                                                                                                            Image Negatives

                                                                                                                                            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                            - equivalent of a photographic negative

                                                                                                                                            - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                            image

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Original Negative image

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                            Some basic intensity transformation functions

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                            range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                            while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                            transformation The log functions compress the dynamic range of images with large

                                                                                                                                            variations in pixel values

                                                                                                                                            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Power-Law (Gamma) Transformations

                                                                                                                                            - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                            Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                            of output values with the opposite being true for higher values of input values The

                                                                                                                                            curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                            1c - identity transformation

                                                                                                                                            A variety of devices used for image capture printing and display respond according to a

                                                                                                                                            power law The process used to correct these power-law response phenomena is called

                                                                                                                                            gamma correction

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Piecewise-Linear Transformations Functions

                                                                                                                                            Contrast stretching

                                                                                                                                            - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                            intensity range of the recording tool or display device

                                                                                                                                            a b c d Fig5

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            11

                                                                                                                                            1

                                                                                                                                            2 1 1 21 2

                                                                                                                                            2 1 2 1

                                                                                                                                            22

                                                                                                                                            2

                                                                                                                                            [0 ]

                                                                                                                                            ( ) ( )( ) [ ]( ) ( )

                                                                                                                                            ( 1 ) [ 1]( 1 )

                                                                                                                                            s r r rrs r r s r rT r r r r

                                                                                                                                            r r r rs L r r r L

                                                                                                                                            L r

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            1 1 2 2r s r s identity transformation (no change)

                                                                                                                                            1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                            Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                            in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                            from their original range to the full range [0 L-1]

                                                                                                                                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                            2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                            The original image on which these results are based is a scanning electron microscope

                                                                                                                                            image of pollen magnified approximately 700 times

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Intensity-level slicing

                                                                                                                                            - highlighting a specific range of intensities in an image

                                                                                                                                            There are two approaches for intensity-level slicing

                                                                                                                                            1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                            another (say black) all other intensities (Figure 311 (a))

                                                                                                                                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                            intensities in the image (Figure 311 (b))

                                                                                                                                            Digital Image Processing

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                                                                                                                                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                            the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                            Highlights range [A B] and preserves all other intensities

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                            blockageshellip)

                                                                                                                                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                            image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                            Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                            Digital Image Processing

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                                                                                                                                            Bit-plane slicing

                                                                                                                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                            This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            Digital Image Processing

                                                                                                                                            Week 1

                                                                                                                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                            • DIP 1 2017
                                                                                                                                            • DIP 02 (2017)

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Representing Digital Images (xy) x = 01hellipM-1 y = 01hellipN-1 ndash spatial variables or spatial coordinates

                                                                                                                                              (00) (01) (0 1)(10) (11) (1 1)

                                                                                                                                              ( )

                                                                                                                                              ( 10) ( 11) ( 1 1)

                                                                                                                                              f f f Nf f f N

                                                                                                                                              f x y

                                                                                                                                              f M f M f M N

                                                                                                                                              image element pixel

                                                                                                                                              00 01 0 1

                                                                                                                                              10 11 1 1

                                                                                                                                              10 11 1 1

                                                                                                                                              ( ) ( )

                                                                                                                                              N

                                                                                                                                              i jN M N

                                                                                                                                              i j

                                                                                                                                              M M M N

                                                                                                                                              a a aa f x i y j f i ja a a

                                                                                                                                              Aa

                                                                                                                                              a a a

                                                                                                                                              f(00) ndash the upper left corner of the image

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              M N ge 0 L=2k

                                                                                                                                              [0 1]i j i ja a L

                                                                                                                                              Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                                                              Digital Image Processing

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                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Number of bits required to store a digitized image

                                                                                                                                              for 2 b M N k M N b N k

                                                                                                                                              When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                                              Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                                              Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                                              (eg 100 line pairs per mm)

                                                                                                                                              Dots per unit distance are commonly used in printing and publishing

                                                                                                                                              In US the measure is expressed in dots per inch (dpi)

                                                                                                                                              (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                                              Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                                              The number of intensity levels (L) is determined by hardware considerations

                                                                                                                                              L=2k ndash most common k = 8

                                                                                                                                              Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                                              150 dpi (lower left) 72 dpi (lower right)

                                                                                                                                              Digital Image Processing

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                                                                                                                                              Reducing the number of gray levels 256 128 64 32

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Reducing the number of gray levels 16 8 4 2

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                              Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                              Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                              Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                              750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                              same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                              image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                              Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                              Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                              intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                              This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                              straight edges

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                              where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                              be written using the 4 nearest neighbors of point (x y)

                                                                                                                                              Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                              modest increase in computational effort

                                                                                                                                              Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                              nearest neighbors of the point 3 3

                                                                                                                                              0 0

                                                                                                                                              ( ) i ji j

                                                                                                                                              i jv x y c x y

                                                                                                                                              The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                              intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                              0 0

                                                                                                                                              ( )i ji j

                                                                                                                                              i jc x y x y

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                              bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                              programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                              Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                              the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                              then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                              neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                              Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                              interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                              from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Neighbors of a Pixel

                                                                                                                                              A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                              This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                              The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                              and are denoted ND(p)

                                                                                                                                              The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                              N8 (p)

                                                                                                                                              If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                              fall outside the image

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Adjacency Connectivity Regions Boundaries

                                                                                                                                              Denote by V the set of intensity levels used to define adjacency

                                                                                                                                              - in a binary image V 01 (V=0 V=1)

                                                                                                                                              - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                              We consider 3 types of adjacency

                                                                                                                                              (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                              (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                              (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                              m-adjacent if

                                                                                                                                              4( )q N p or

                                                                                                                                              ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                              Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                              ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              binary image

                                                                                                                                              0 1 1 0 1 1 0 1 1

                                                                                                                                              1 0 1 0 0 1 0 0 1 0

                                                                                                                                              0 0 1 0 0 1 0 0 1

                                                                                                                                              V

                                                                                                                                              The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                              8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                              m-adjacency

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                              is a sequence of distinct pixels with coordinates

                                                                                                                                              and are adjacent 0 0 1 1

                                                                                                                                              1 1

                                                                                                                                              ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                              n n

                                                                                                                                              i i i i

                                                                                                                                              x y x y x y x y s tx y x y i n

                                                                                                                                              The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                              Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                              Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                              in S if there exists a path between them consisting only of pixels from S

                                                                                                                                              S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                              Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                              Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                              that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                              8-adjacency are considered

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                              touches the image border

                                                                                                                                              the complement of 1

                                                                                                                                              ( )K

                                                                                                                                              cu k u u

                                                                                                                                              k

                                                                                                                                              R R R R

                                                                                                                                              We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                              background of the image

                                                                                                                                              The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                              points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                              region that have at least one background neighbor This definition is referred to as the

                                                                                                                                              inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                              border in the background

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Distance measures

                                                                                                                                              For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                              function or metric if

                                                                                                                                              (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                              (b) D(p q) = D(q p)

                                                                                                                                              (c) D(p z) le D(p q) + D(q z)

                                                                                                                                              The Euclidean distance between p and q is defined as 1

                                                                                                                                              2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                              The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                              centered at (x y)

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                              4( ) | | | |D p q x s y t

                                                                                                                                              The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                              4

                                                                                                                                              22 1 2

                                                                                                                                              2 2 1 0 1 22 1 2

                                                                                                                                              2

                                                                                                                                              D

                                                                                                                                              The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                              The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                              8( ) max| | | |D p q x s y t

                                                                                                                                              The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              8

                                                                                                                                              2 2 2 2 22 1 1 1 2

                                                                                                                                              2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                              D

                                                                                                                                              The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                              D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                              because these distances involve only the coordinates of the point

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Array versus Matrix Operations

                                                                                                                                              An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                              11 12 11 12

                                                                                                                                              21 22 21 22

                                                                                                                                              a a b ba a b b

                                                                                                                                              Array product

                                                                                                                                              11 12 11 12 11 11 12 12

                                                                                                                                              21 22 21 22 21 21 22 21

                                                                                                                                              a a b b a b a ba a b b a b a b

                                                                                                                                              Matrix product

                                                                                                                                              11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                              21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                              a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                              We assume array operations unless stated otherwise

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Linear versus Nonlinear Operations

                                                                                                                                              One of the most important classifications of image-processing methods is whether it is

                                                                                                                                              linear or nonlinear

                                                                                                                                              ( ) ( )H f x y g x y

                                                                                                                                              H is said to be a linear operator if

                                                                                                                                              images1 2 1 2

                                                                                                                                              1 2

                                                                                                                                              ( ) ( ) ( ) ( )

                                                                                                                                              H a f x y b f x y a H f x y b H f x y

                                                                                                                                              a b f f

                                                                                                                                              Example of nonlinear operator

                                                                                                                                              the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                              1 2

                                                                                                                                              0 2 6 5 1 1

                                                                                                                                              2 3 4 7f f a b

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              1 2

                                                                                                                                              0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                              2 3 4 7 2 4a f b f

                                                                                                                                              0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                              2 3 4 7

                                                                                                                                              Arithmetic Operations in Image Processing

                                                                                                                                              Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                              f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                              For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                              value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                              The two random variables are uncorrelated when their covariance is 0

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                              used in image enhancement)

                                                                                                                                              1

                                                                                                                                              1( ) ( )K

                                                                                                                                              ii

                                                                                                                                              g x y g x yK

                                                                                                                                              If the noise satisfies the properties stated above we have

                                                                                                                                              2 2( ) ( )

                                                                                                                                              1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                              ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                              and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                              the average image is

                                                                                                                                              ( ) ( )1

                                                                                                                                              g x y x yK

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                              the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                              means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                              averaging process increases

                                                                                                                                              An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                              under very low light levels frequently causes sensor noise to render single images

                                                                                                                                              virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                              was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                              64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                              images respectively

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                              100 noisy images

                                                                                                                                              a b c d e f

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                              images

                                                                                                                                              (a) (b) (c)

                                                                                                                                              Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                              significant bit of each pixel (c) the difference between the two images

                                                                                                                                              Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                              Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                              images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                              difference between images (a) and (b)

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                              h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                              intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                              source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                              bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                              anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                              images after injection of the contrast medium

                                                                                                                                              In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                              Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                              propagates through the various arteries in the area being observed

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              An important application of image multiplication (and division) is shading correction

                                                                                                                                              Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                              f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                              When the shading function is known

                                                                                                                                              ( )( )( )

                                                                                                                                              g x yf x yh x y

                                                                                                                                              h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                              approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                              sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              (a) (b) (c)

                                                                                                                                              Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                              operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                              1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                              image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                              (a) (b) (c)

                                                                                                                                              Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                              min( )mf f f

                                                                                                                                              0 ( 255)max( )

                                                                                                                                              ms

                                                                                                                                              m

                                                                                                                                              ff K K K

                                                                                                                                              f

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Spatial Operations

                                                                                                                                              - are performed directly on the pixels of a given image

                                                                                                                                              There are three categories of spatial operations

                                                                                                                                              single-pixel operations

                                                                                                                                              neighborhood operations

                                                                                                                                              geometric spatial transformations

                                                                                                                                              Single-pixel operations

                                                                                                                                              - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                              where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                              corresponding pixel in the processed image

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Neighborhood operations

                                                                                                                                              Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                              in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                              based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                              rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                              intensity by computing the average value of the pixels in Sxy

                                                                                                                                              ( )

                                                                                                                                              1( ) ( )xyr c S

                                                                                                                                              g x y f r cm n

                                                                                                                                              The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                              used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                              largest region of an image

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Geometric spatial transformations and image registration

                                                                                                                                              - modify the spatial relationship between pixels in an image

                                                                                                                                              - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                              printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                              predefined set of rules

                                                                                                                                              A geometric transformation consists of 2 basic operations

                                                                                                                                              1 a spatial transformation of coordinates

                                                                                                                                              2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                              pixels

                                                                                                                                              The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                              (v w) ndash pixel coordinates in the original image

                                                                                                                                              (x y) ndash pixel coordinates in the transformed image

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                              Affine transform

                                                                                                                                              11 1211 21 31

                                                                                                                                              21 2212 22 33

                                                                                                                                              31 32

                                                                                                                                              0[ 1] [ 1] [ 1] 0

                                                                                                                                              1

                                                                                                                                              t tx t v t w t

                                                                                                                                              x y v w T v w t ty t v t w t

                                                                                                                                              t t

                                                                                                                                              (AT)

                                                                                                                                              This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                              on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                              result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                              scaling rotation and translation matrices from Table 1

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Affine transformations

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                              the process we have to assign intensity values to those locations This task is done by

                                                                                                                                              using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                              In practice we can use equation (AT) in two basic ways

                                                                                                                                              forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                              location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                              Problems

                                                                                                                                              - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                              the same location in the output image

                                                                                                                                              - some output locations have no correspondent in the original image (no intensity

                                                                                                                                              assignment)

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                              computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                              It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                              pixel value

                                                                                                                                              Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                              numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Image registration ndash align two or more images of the same scene

                                                                                                                                              In image registration we have available the input and output images but the specific

                                                                                                                                              transformation that produced the output image from the input is generally unknown

                                                                                                                                              The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                              images

                                                                                                                                              - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                              same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                              - align images of a given location taken by the same instrument at different moments

                                                                                                                                              of time (satellite images)

                                                                                                                                              Solving the problem using tie points (also called control points) which are

                                                                                                                                              corresponding points whose locations are known precisely in the input and reference

                                                                                                                                              image

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              How to select tie points

                                                                                                                                              - interactively selecting them

                                                                                                                                              - use of algorithms that try to detect these points

                                                                                                                                              - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                              the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                              marks) directly on all images captured by the system which can be used as guides

                                                                                                                                              for establishing tie points

                                                                                                                                              The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                              of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                              a bilinear approximation is given by

                                                                                                                                              1 2 3 4

                                                                                                                                              5 6 7 8

                                                                                                                                              x c v c w c v w cy c v c w c v w c

                                                                                                                                              (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                              frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                              subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                              subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                              The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                              problem depend on the severity of the geometrical distortion

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Probabilistic Methods

                                                                                                                                              zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                              p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                              ( ) kk

                                                                                                                                              np zM N

                                                                                                                                              nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                              pixels in the image) 1

                                                                                                                                              0( ) 1

                                                                                                                                              L

                                                                                                                                              kk

                                                                                                                                              p z

                                                                                                                                              The mean (average) intensity of an image is given by 1

                                                                                                                                              0( )

                                                                                                                                              L

                                                                                                                                              k kk

                                                                                                                                              m z p z

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              The variance of the intensities is 1

                                                                                                                                              2 2

                                                                                                                                              0( ) ( )

                                                                                                                                              L

                                                                                                                                              k kk

                                                                                                                                              z m p z

                                                                                                                                              The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                              measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                              ( ) is used

                                                                                                                                              The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                              0( ) ( ) ( )

                                                                                                                                              Ln

                                                                                                                                              n k kk

                                                                                                                                              z z m p z

                                                                                                                                              ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                              3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                              ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                              mean

                                                                                                                                              Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                              Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                              Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                              Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Intensity Transformations and Spatial Filtering

                                                                                                                                              ( ) ( )g x y T f x y

                                                                                                                                              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                              neighborhood of (x y)

                                                                                                                                              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                              and much smaller in size than the image

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                              called spatial filter (spatial mask kernel template or window)

                                                                                                                                              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                              ( )s T r

                                                                                                                                              s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                              Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                              darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                              is called contrast stretching

                                                                                                                                              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                              thresholding function

                                                                                                                                              Some Basic Intensity Transformation Functions

                                                                                                                                              Image Negatives

                                                                                                                                              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                              - equivalent of a photographic negative

                                                                                                                                              - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                              image

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Original Negative image

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                              Some basic intensity transformation functions

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                              range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                              while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                              transformation The log functions compress the dynamic range of images with large

                                                                                                                                              variations in pixel values

                                                                                                                                              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Power-Law (Gamma) Transformations

                                                                                                                                              - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                              Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                              of output values with the opposite being true for higher values of input values The

                                                                                                                                              curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                              1c - identity transformation

                                                                                                                                              A variety of devices used for image capture printing and display respond according to a

                                                                                                                                              power law The process used to correct these power-law response phenomena is called

                                                                                                                                              gamma correction

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Piecewise-Linear Transformations Functions

                                                                                                                                              Contrast stretching

                                                                                                                                              - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                              intensity range of the recording tool or display device

                                                                                                                                              a b c d Fig5

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              11

                                                                                                                                              1

                                                                                                                                              2 1 1 21 2

                                                                                                                                              2 1 2 1

                                                                                                                                              22

                                                                                                                                              2

                                                                                                                                              [0 ]

                                                                                                                                              ( ) ( )( ) [ ]( ) ( )

                                                                                                                                              ( 1 ) [ 1]( 1 )

                                                                                                                                              s r r rrs r r s r rT r r r r

                                                                                                                                              r r r rs L r r r L

                                                                                                                                              L r

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              1 1 2 2r s r s identity transformation (no change)

                                                                                                                                              1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                              Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                              in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                              from their original range to the full range [0 L-1]

                                                                                                                                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                              2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                              The original image on which these results are based is a scanning electron microscope

                                                                                                                                              image of pollen magnified approximately 700 times

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Intensity-level slicing

                                                                                                                                              - highlighting a specific range of intensities in an image

                                                                                                                                              There are two approaches for intensity-level slicing

                                                                                                                                              1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                              another (say black) all other intensities (Figure 311 (a))

                                                                                                                                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                              intensities in the image (Figure 311 (b))

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                              the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                              Highlights range [A B] and preserves all other intensities

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                              blockageshellip)

                                                                                                                                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                              image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                              Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Bit-plane slicing

                                                                                                                                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                              This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              Digital Image Processing

                                                                                                                                              Week 1

                                                                                                                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                              • DIP 1 2017
                                                                                                                                              • DIP 02 (2017)

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                M N ge 0 L=2k

                                                                                                                                                [0 1]i j i ja a L

                                                                                                                                                Dynamic range of an image = the ratio of the maximum measurable intensity to the minimum detectable intensity level in the system Upper limit ndash determined by saturation lower limit - noise

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Number of bits required to store a digitized image

                                                                                                                                                for 2 b M N k M N b N k

                                                                                                                                                When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                                                Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                                                Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                                                (eg 100 line pairs per mm)

                                                                                                                                                Dots per unit distance are commonly used in printing and publishing

                                                                                                                                                In US the measure is expressed in dots per inch (dpi)

                                                                                                                                                (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                                                Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                                                The number of intensity levels (L) is determined by hardware considerations

                                                                                                                                                L=2k ndash most common k = 8

                                                                                                                                                Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                                                150 dpi (lower left) 72 dpi (lower right)

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Reducing the number of gray levels 256 128 64 32

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Reducing the number of gray levels 16 8 4 2

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                                Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                                Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                                Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                                750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                                same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                                image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                                Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                                Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                                intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                                This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                                straight edges

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                                where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                                be written using the 4 nearest neighbors of point (x y)

                                                                                                                                                Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                                modest increase in computational effort

                                                                                                                                                Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                                nearest neighbors of the point 3 3

                                                                                                                                                0 0

                                                                                                                                                ( ) i ji j

                                                                                                                                                i jv x y c x y

                                                                                                                                                The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                                intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                                0 0

                                                                                                                                                ( )i ji j

                                                                                                                                                i jc x y x y

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                                bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                                programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                                Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                                the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                                then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                                neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                                Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                                interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                                from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Neighbors of a Pixel

                                                                                                                                                A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                                This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                                The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                                and are denoted ND(p)

                                                                                                                                                The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                                N8 (p)

                                                                                                                                                If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                                fall outside the image

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Adjacency Connectivity Regions Boundaries

                                                                                                                                                Denote by V the set of intensity levels used to define adjacency

                                                                                                                                                - in a binary image V 01 (V=0 V=1)

                                                                                                                                                - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                                We consider 3 types of adjacency

                                                                                                                                                (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                                (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                                (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                                m-adjacent if

                                                                                                                                                4( )q N p or

                                                                                                                                                ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                                Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                                ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                binary image

                                                                                                                                                0 1 1 0 1 1 0 1 1

                                                                                                                                                1 0 1 0 0 1 0 0 1 0

                                                                                                                                                0 0 1 0 0 1 0 0 1

                                                                                                                                                V

                                                                                                                                                The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                                8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                                m-adjacency

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                is a sequence of distinct pixels with coordinates

                                                                                                                                                and are adjacent 0 0 1 1

                                                                                                                                                1 1

                                                                                                                                                ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                n n

                                                                                                                                                i i i i

                                                                                                                                                x y x y x y x y s tx y x y i n

                                                                                                                                                The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                8-adjacency are considered

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                touches the image border

                                                                                                                                                the complement of 1

                                                                                                                                                ( )K

                                                                                                                                                cu k u u

                                                                                                                                                k

                                                                                                                                                R R R R

                                                                                                                                                We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                background of the image

                                                                                                                                                The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                border in the background

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Distance measures

                                                                                                                                                For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                function or metric if

                                                                                                                                                (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                (b) D(p q) = D(q p)

                                                                                                                                                (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                The Euclidean distance between p and q is defined as 1

                                                                                                                                                2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                centered at (x y)

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                4( ) | | | |D p q x s y t

                                                                                                                                                The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                4

                                                                                                                                                22 1 2

                                                                                                                                                2 2 1 0 1 22 1 2

                                                                                                                                                2

                                                                                                                                                D

                                                                                                                                                The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                8( ) max| | | |D p q x s y t

                                                                                                                                                The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                8

                                                                                                                                                2 2 2 2 22 1 1 1 2

                                                                                                                                                2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                D

                                                                                                                                                The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                because these distances involve only the coordinates of the point

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Array versus Matrix Operations

                                                                                                                                                An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                11 12 11 12

                                                                                                                                                21 22 21 22

                                                                                                                                                a a b ba a b b

                                                                                                                                                Array product

                                                                                                                                                11 12 11 12 11 11 12 12

                                                                                                                                                21 22 21 22 21 21 22 21

                                                                                                                                                a a b b a b a ba a b b a b a b

                                                                                                                                                Matrix product

                                                                                                                                                11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                We assume array operations unless stated otherwise

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Linear versus Nonlinear Operations

                                                                                                                                                One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                linear or nonlinear

                                                                                                                                                ( ) ( )H f x y g x y

                                                                                                                                                H is said to be a linear operator if

                                                                                                                                                images1 2 1 2

                                                                                                                                                1 2

                                                                                                                                                ( ) ( ) ( ) ( )

                                                                                                                                                H a f x y b f x y a H f x y b H f x y

                                                                                                                                                a b f f

                                                                                                                                                Example of nonlinear operator

                                                                                                                                                the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                1 2

                                                                                                                                                0 2 6 5 1 1

                                                                                                                                                2 3 4 7f f a b

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                1 2

                                                                                                                                                0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                2 3 4 7 2 4a f b f

                                                                                                                                                0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                2 3 4 7

                                                                                                                                                Arithmetic Operations in Image Processing

                                                                                                                                                Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                used in image enhancement)

                                                                                                                                                1

                                                                                                                                                1( ) ( )K

                                                                                                                                                ii

                                                                                                                                                g x y g x yK

                                                                                                                                                If the noise satisfies the properties stated above we have

                                                                                                                                                2 2( ) ( )

                                                                                                                                                1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                the average image is

                                                                                                                                                ( ) ( )1

                                                                                                                                                g x y x yK

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                                                                                                                                                Week 1

                                                                                                                                                As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                averaging process increases

                                                                                                                                                An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                images respectively

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                100 noisy images

                                                                                                                                                a b c d e f

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                images

                                                                                                                                                (a) (b) (c)

                                                                                                                                                Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                significant bit of each pixel (c) the difference between the two images

                                                                                                                                                Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                difference between images (a) and (b)

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                images after injection of the contrast medium

                                                                                                                                                In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                propagates through the various arteries in the area being observed

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                An important application of image multiplication (and division) is shading correction

                                                                                                                                                Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                When the shading function is known

                                                                                                                                                ( )( )( )

                                                                                                                                                g x yf x yh x y

                                                                                                                                                h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                (a) (b) (c)

                                                                                                                                                Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                (a) (b) (c)

                                                                                                                                                Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                min( )mf f f

                                                                                                                                                0 ( 255)max( )

                                                                                                                                                ms

                                                                                                                                                m

                                                                                                                                                ff K K K

                                                                                                                                                f

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Spatial Operations

                                                                                                                                                - are performed directly on the pixels of a given image

                                                                                                                                                There are three categories of spatial operations

                                                                                                                                                single-pixel operations

                                                                                                                                                neighborhood operations

                                                                                                                                                geometric spatial transformations

                                                                                                                                                Single-pixel operations

                                                                                                                                                - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                corresponding pixel in the processed image

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Neighborhood operations

                                                                                                                                                Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                intensity by computing the average value of the pixels in Sxy

                                                                                                                                                ( )

                                                                                                                                                1( ) ( )xyr c S

                                                                                                                                                g x y f r cm n

                                                                                                                                                The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                largest region of an image

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Geometric spatial transformations and image registration

                                                                                                                                                - modify the spatial relationship between pixels in an image

                                                                                                                                                - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                predefined set of rules

                                                                                                                                                A geometric transformation consists of 2 basic operations

                                                                                                                                                1 a spatial transformation of coordinates

                                                                                                                                                2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                pixels

                                                                                                                                                The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                (v w) ndash pixel coordinates in the original image

                                                                                                                                                (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                Affine transform

                                                                                                                                                11 1211 21 31

                                                                                                                                                21 2212 22 33

                                                                                                                                                31 32

                                                                                                                                                0[ 1] [ 1] [ 1] 0

                                                                                                                                                1

                                                                                                                                                t tx t v t w t

                                                                                                                                                x y v w T v w t ty t v t w t

                                                                                                                                                t t

                                                                                                                                                (AT)

                                                                                                                                                This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                scaling rotation and translation matrices from Table 1

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Affine transformations

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                In practice we can use equation (AT) in two basic ways

                                                                                                                                                forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                Problems

                                                                                                                                                - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                the same location in the output image

                                                                                                                                                - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                assignment)

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                pixel value

                                                                                                                                                Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Image registration ndash align two or more images of the same scene

                                                                                                                                                In image registration we have available the input and output images but the specific

                                                                                                                                                transformation that produced the output image from the input is generally unknown

                                                                                                                                                The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                images

                                                                                                                                                - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                - align images of a given location taken by the same instrument at different moments

                                                                                                                                                of time (satellite images)

                                                                                                                                                Solving the problem using tie points (also called control points) which are

                                                                                                                                                corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                image

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                How to select tie points

                                                                                                                                                - interactively selecting them

                                                                                                                                                - use of algorithms that try to detect these points

                                                                                                                                                - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                for establishing tie points

                                                                                                                                                The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                a bilinear approximation is given by

                                                                                                                                                1 2 3 4

                                                                                                                                                5 6 7 8

                                                                                                                                                x c v c w c v w cy c v c w c v w c

                                                                                                                                                (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                problem depend on the severity of the geometrical distortion

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                Digital Image Processing

                                                                                                                                                Week 1

                                                                                                                                                Probabilistic Methods

                                                                                                                                                zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                ( ) kk

                                                                                                                                                np zM N

                                                                                                                                                nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                pixels in the image) 1

                                                                                                                                                0( ) 1

                                                                                                                                                L

                                                                                                                                                kk

                                                                                                                                                p z

                                                                                                                                                The mean (average) intensity of an image is given by 1

                                                                                                                                                0( )

                                                                                                                                                L

                                                                                                                                                k kk

                                                                                                                                                m z p z

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                                                                                                                                                The variance of the intensities is 1

                                                                                                                                                2 2

                                                                                                                                                0( ) ( )

                                                                                                                                                L

                                                                                                                                                k kk

                                                                                                                                                z m p z

                                                                                                                                                The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                ( ) is used

                                                                                                                                                The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                0( ) ( ) ( )

                                                                                                                                                Ln

                                                                                                                                                n k kk

                                                                                                                                                z z m p z

                                                                                                                                                ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                ( 3( ) 0z the intensities are biased to values lower than the mean

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                                                                                                                                                3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                mean

                                                                                                                                                Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                Figure 1(c) ndash standard deviation 492 (variance = 24206)

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                                                                                                                                                Intensity Transformations and Spatial Filtering

                                                                                                                                                ( ) ( )g x y T f x y

                                                                                                                                                f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                neighborhood of (x y)

                                                                                                                                                - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                and much smaller in size than the image

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                                                                                                                                                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                called spatial filter (spatial mask kernel template or window)

                                                                                                                                                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                ( )s T r

                                                                                                                                                s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                is called contrast stretching

                                                                                                                                                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

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                                                                                                                                                Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                thresholding function

                                                                                                                                                Some Basic Intensity Transformation Functions

                                                                                                                                                Image Negatives

                                                                                                                                                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                - equivalent of a photographic negative

                                                                                                                                                - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                image

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                                                                                                                                                Original Negative image

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                                                                                                                                                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                Some basic intensity transformation functions

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                                                                                                                                                This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                transformation The log functions compress the dynamic range of images with large

                                                                                                                                                variations in pixel values

                                                                                                                                                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

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                                                                                                                                                Power-Law (Gamma) Transformations

                                                                                                                                                - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                Plots of gamma transformation for different values of γ (c=1)

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                                                                                                                                                Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                of output values with the opposite being true for higher values of input values The

                                                                                                                                                curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                1c - identity transformation

                                                                                                                                                A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                power law The process used to correct these power-law response phenomena is called

                                                                                                                                                gamma correction

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                                                                                                                                                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

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                                                                                                                                                Piecewise-Linear Transformations Functions

                                                                                                                                                Contrast stretching

                                                                                                                                                - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                intensity range of the recording tool or display device

                                                                                                                                                a b c d Fig5

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                                                                                                                                                11

                                                                                                                                                1

                                                                                                                                                2 1 1 21 2

                                                                                                                                                2 1 2 1

                                                                                                                                                22

                                                                                                                                                2

                                                                                                                                                [0 ]

                                                                                                                                                ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                ( 1 ) [ 1]( 1 )

                                                                                                                                                s r r rrs r r s r rT r r r r

                                                                                                                                                r r r rs L r r r L

                                                                                                                                                L r

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                                                                                                                                                1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                from their original range to the full range [0 L-1]

                                                                                                                                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                The original image on which these results are based is a scanning electron microscope

                                                                                                                                                image of pollen magnified approximately 700 times

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                                                                                                                                                Intensity-level slicing

                                                                                                                                                - highlighting a specific range of intensities in an image

                                                                                                                                                There are two approaches for intensity-level slicing

                                                                                                                                                1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                intensities in the image (Figure 311 (b))

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                                                                                                                                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                Highlights range [A B] and preserves all other intensities

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                                                                                                                                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                blockageshellip)

                                                                                                                                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                Fig 6 - Aortic angiogram and intensity sliced versions

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                                                                                                                                                Bit-plane slicing

                                                                                                                                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

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                                                                                                                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                • DIP 1 2017
                                                                                                                                                • DIP 02 (2017)

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                                                                                                                                                  Number of bits required to store a digitized image

                                                                                                                                                  for 2 b M N k M N b N k

                                                                                                                                                  When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

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                                                                                                                                                  Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                                                  Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                                                  Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                                                  (eg 100 line pairs per mm)

                                                                                                                                                  Dots per unit distance are commonly used in printing and publishing

                                                                                                                                                  In US the measure is expressed in dots per inch (dpi)

                                                                                                                                                  (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                                                  Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                                                  The number of intensity levels (L) is determined by hardware considerations

                                                                                                                                                  L=2k ndash most common k = 8

                                                                                                                                                  Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                                                                  Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                                                  150 dpi (lower left) 72 dpi (lower right)

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                                                                                                                                                  Reducing the number of gray levels 256 128 64 32

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                                                                                                                                                  Reducing the number of gray levels 16 8 4 2

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                                                                                                                                                  Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                                  Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                                  Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                                  Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                                  750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                                  same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                                  image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                                  Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                                  Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                                  intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                                  This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                                  straight edges

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                                                                                                                                                  Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                                  where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                                  be written using the 4 nearest neighbors of point (x y)

                                                                                                                                                  Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                                  modest increase in computational effort

                                                                                                                                                  Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                                  nearest neighbors of the point 3 3

                                                                                                                                                  0 0

                                                                                                                                                  ( ) i ji j

                                                                                                                                                  i jv x y c x y

                                                                                                                                                  The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                                  intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                                  0 0

                                                                                                                                                  ( )i ji j

                                                                                                                                                  i jc x y x y

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                                                                                                                                                  Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                                  bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                                  programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                                  Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                                  the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                                  then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                                  neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                                  Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                                  interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                                  from 1250 dpi to 150 dpi (instead of 72 dpi)

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                                                                                                                                                  Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

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                                                                                                                                                  Neighbors of a Pixel

                                                                                                                                                  A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                                  This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                                  The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                                  and are denoted ND(p)

                                                                                                                                                  The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                                  N8 (p)

                                                                                                                                                  If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                                  fall outside the image

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                                                                                                                                                  Adjacency Connectivity Regions Boundaries

                                                                                                                                                  Denote by V the set of intensity levels used to define adjacency

                                                                                                                                                  - in a binary image V 01 (V=0 V=1)

                                                                                                                                                  - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                                  We consider 3 types of adjacency

                                                                                                                                                  (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                                  (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                                  (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                                  m-adjacent if

                                                                                                                                                  4( )q N p or

                                                                                                                                                  ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                                  Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                                  ambiguities that often arise when 8-adjacency is used Consider the example

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                                                                                                                                                  binary image

                                                                                                                                                  0 1 1 0 1 1 0 1 1

                                                                                                                                                  1 0 1 0 0 1 0 0 1 0

                                                                                                                                                  0 0 1 0 0 1 0 0 1

                                                                                                                                                  V

                                                                                                                                                  The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                                  8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                                  m-adjacency

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                                                                                                                                                  A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                  is a sequence of distinct pixels with coordinates

                                                                                                                                                  and are adjacent 0 0 1 1

                                                                                                                                                  1 1

                                                                                                                                                  ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                  n n

                                                                                                                                                  i i i i

                                                                                                                                                  x y x y x y x y s tx y x y i n

                                                                                                                                                  The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                  Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                  Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                  in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                  S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                  Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                  Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                  that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                  8-adjacency are considered

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                                                                                                                                                  Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                  touches the image border

                                                                                                                                                  the complement of 1

                                                                                                                                                  ( )K

                                                                                                                                                  cu k u u

                                                                                                                                                  k

                                                                                                                                                  R R R R

                                                                                                                                                  We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                  background of the image

                                                                                                                                                  The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                  points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                  region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                  inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                  border in the background

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                                                                                                                                                  Distance measures

                                                                                                                                                  For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                  function or metric if

                                                                                                                                                  (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                  (b) D(p q) = D(q p)

                                                                                                                                                  (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                  The Euclidean distance between p and q is defined as 1

                                                                                                                                                  2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                  The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                  centered at (x y)

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                                                                                                                                                  The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                  4( ) | | | |D p q x s y t

                                                                                                                                                  The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                  4

                                                                                                                                                  22 1 2

                                                                                                                                                  2 2 1 0 1 22 1 2

                                                                                                                                                  2

                                                                                                                                                  D

                                                                                                                                                  The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                  The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                  8( ) max| | | |D p q x s y t

                                                                                                                                                  The pixels q for which 8( )D p q r form a square centered at (x y)

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                                                                                                                                                  8

                                                                                                                                                  2 2 2 2 22 1 1 1 2

                                                                                                                                                  2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                  D

                                                                                                                                                  The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                  D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                  because these distances involve only the coordinates of the point

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                                                                                                                                                  Array versus Matrix Operations

                                                                                                                                                  An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                  11 12 11 12

                                                                                                                                                  21 22 21 22

                                                                                                                                                  a a b ba a b b

                                                                                                                                                  Array product

                                                                                                                                                  11 12 11 12 11 11 12 12

                                                                                                                                                  21 22 21 22 21 21 22 21

                                                                                                                                                  a a b b a b a ba a b b a b a b

                                                                                                                                                  Matrix product

                                                                                                                                                  11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                  21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                  a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                  We assume array operations unless stated otherwise

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                                                                                                                                                  Linear versus Nonlinear Operations

                                                                                                                                                  One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                  linear or nonlinear

                                                                                                                                                  ( ) ( )H f x y g x y

                                                                                                                                                  H is said to be a linear operator if

                                                                                                                                                  images1 2 1 2

                                                                                                                                                  1 2

                                                                                                                                                  ( ) ( ) ( ) ( )

                                                                                                                                                  H a f x y b f x y a H f x y b H f x y

                                                                                                                                                  a b f f

                                                                                                                                                  Example of nonlinear operator

                                                                                                                                                  the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                  1 2

                                                                                                                                                  0 2 6 5 1 1

                                                                                                                                                  2 3 4 7f f a b

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                                                                                                                                                  1 2

                                                                                                                                                  0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                  2 3 4 7 2 4a f b f

                                                                                                                                                  0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                  2 3 4 7

                                                                                                                                                  Arithmetic Operations in Image Processing

                                                                                                                                                  Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                  f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                  For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                  value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                  The two random variables are uncorrelated when their covariance is 0

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                                                                                                                                                  Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                  used in image enhancement)

                                                                                                                                                  1

                                                                                                                                                  1( ) ( )K

                                                                                                                                                  ii

                                                                                                                                                  g x y g x yK

                                                                                                                                                  If the noise satisfies the properties stated above we have

                                                                                                                                                  2 2( ) ( )

                                                                                                                                                  1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                  ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                  and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                  the average image is

                                                                                                                                                  ( ) ( )1

                                                                                                                                                  g x y x yK

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                                                                                                                                                  Week 1

                                                                                                                                                  As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                  the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                  means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                  averaging process increases

                                                                                                                                                  An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                  under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                  virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                  was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                  64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                  images respectively

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                  100 noisy images

                                                                                                                                                  a b c d e f

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                  images

                                                                                                                                                  (a) (b) (c)

                                                                                                                                                  Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                  significant bit of each pixel (c) the difference between the two images

                                                                                                                                                  Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                  Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                  images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                  difference between images (a) and (b)

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                  h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                  intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                  source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                  bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                  anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                  images after injection of the contrast medium

                                                                                                                                                  In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                  Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                  propagates through the various arteries in the area being observed

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  An important application of image multiplication (and division) is shading correction

                                                                                                                                                  Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                  f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                  When the shading function is known

                                                                                                                                                  ( )( )( )

                                                                                                                                                  g x yf x yh x y

                                                                                                                                                  h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                  approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                  sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  (a) (b) (c)

                                                                                                                                                  Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                  operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                  1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                  image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                  (a) (b) (c)

                                                                                                                                                  Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                  min( )mf f f

                                                                                                                                                  0 ( 255)max( )

                                                                                                                                                  ms

                                                                                                                                                  m

                                                                                                                                                  ff K K K

                                                                                                                                                  f

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Spatial Operations

                                                                                                                                                  - are performed directly on the pixels of a given image

                                                                                                                                                  There are three categories of spatial operations

                                                                                                                                                  single-pixel operations

                                                                                                                                                  neighborhood operations

                                                                                                                                                  geometric spatial transformations

                                                                                                                                                  Single-pixel operations

                                                                                                                                                  - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                  where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                  corresponding pixel in the processed image

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Neighborhood operations

                                                                                                                                                  Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                  in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                  based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                  rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                  intensity by computing the average value of the pixels in Sxy

                                                                                                                                                  ( )

                                                                                                                                                  1( ) ( )xyr c S

                                                                                                                                                  g x y f r cm n

                                                                                                                                                  The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                  used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                  largest region of an image

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Geometric spatial transformations and image registration

                                                                                                                                                  - modify the spatial relationship between pixels in an image

                                                                                                                                                  - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                  printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                  predefined set of rules

                                                                                                                                                  A geometric transformation consists of 2 basic operations

                                                                                                                                                  1 a spatial transformation of coordinates

                                                                                                                                                  2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                  pixels

                                                                                                                                                  The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                  (v w) ndash pixel coordinates in the original image

                                                                                                                                                  (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                  Affine transform

                                                                                                                                                  11 1211 21 31

                                                                                                                                                  21 2212 22 33

                                                                                                                                                  31 32

                                                                                                                                                  0[ 1] [ 1] [ 1] 0

                                                                                                                                                  1

                                                                                                                                                  t tx t v t w t

                                                                                                                                                  x y v w T v w t ty t v t w t

                                                                                                                                                  t t

                                                                                                                                                  (AT)

                                                                                                                                                  This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                  on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                  result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                  scaling rotation and translation matrices from Table 1

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Affine transformations

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                  the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                  using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                  In practice we can use equation (AT) in two basic ways

                                                                                                                                                  forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                  location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                  Problems

                                                                                                                                                  - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                  the same location in the output image

                                                                                                                                                  - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                  assignment)

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                  computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                  It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                  pixel value

                                                                                                                                                  Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                  numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Image registration ndash align two or more images of the same scene

                                                                                                                                                  In image registration we have available the input and output images but the specific

                                                                                                                                                  transformation that produced the output image from the input is generally unknown

                                                                                                                                                  The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                  images

                                                                                                                                                  - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                  same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                  - align images of a given location taken by the same instrument at different moments

                                                                                                                                                  of time (satellite images)

                                                                                                                                                  Solving the problem using tie points (also called control points) which are

                                                                                                                                                  corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                  image

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  How to select tie points

                                                                                                                                                  - interactively selecting them

                                                                                                                                                  - use of algorithms that try to detect these points

                                                                                                                                                  - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                  the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                  marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                  for establishing tie points

                                                                                                                                                  The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                  of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                  a bilinear approximation is given by

                                                                                                                                                  1 2 3 4

                                                                                                                                                  5 6 7 8

                                                                                                                                                  x c v c w c v w cy c v c w c v w c

                                                                                                                                                  (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                  frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                  subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                  subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                  The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                  problem depend on the severity of the geometrical distortion

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Probabilistic Methods

                                                                                                                                                  zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                  p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                  ( ) kk

                                                                                                                                                  np zM N

                                                                                                                                                  nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                  pixels in the image) 1

                                                                                                                                                  0( ) 1

                                                                                                                                                  L

                                                                                                                                                  kk

                                                                                                                                                  p z

                                                                                                                                                  The mean (average) intensity of an image is given by 1

                                                                                                                                                  0( )

                                                                                                                                                  L

                                                                                                                                                  k kk

                                                                                                                                                  m z p z

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  The variance of the intensities is 1

                                                                                                                                                  2 2

                                                                                                                                                  0( ) ( )

                                                                                                                                                  L

                                                                                                                                                  k kk

                                                                                                                                                  z m p z

                                                                                                                                                  The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                  measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                  ( ) is used

                                                                                                                                                  The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                  0( ) ( ) ( )

                                                                                                                                                  Ln

                                                                                                                                                  n k kk

                                                                                                                                                  z z m p z

                                                                                                                                                  ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                  3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                  ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                  mean

                                                                                                                                                  Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                  Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                  Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                  Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Intensity Transformations and Spatial Filtering

                                                                                                                                                  ( ) ( )g x y T f x y

                                                                                                                                                  f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                  neighborhood of (x y)

                                                                                                                                                  - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                  and much smaller in size than the image

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                  called spatial filter (spatial mask kernel template or window)

                                                                                                                                                  ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                  ( )s T r

                                                                                                                                                  s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                  Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                  darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                  is called contrast stretching

                                                                                                                                                  Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                  thresholding function

                                                                                                                                                  Some Basic Intensity Transformation Functions

                                                                                                                                                  Image Negatives

                                                                                                                                                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                  - equivalent of a photographic negative

                                                                                                                                                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                  image

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Original Negative image

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                  Some basic intensity transformation functions

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                  range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                  while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                  transformation The log functions compress the dynamic range of images with large

                                                                                                                                                  variations in pixel values

                                                                                                                                                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Power-Law (Gamma) Transformations

                                                                                                                                                  - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                  Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                  of output values with the opposite being true for higher values of input values The

                                                                                                                                                  curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                  1c - identity transformation

                                                                                                                                                  A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                  power law The process used to correct these power-law response phenomena is called

                                                                                                                                                  gamma correction

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Piecewise-Linear Transformations Functions

                                                                                                                                                  Contrast stretching

                                                                                                                                                  - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                  intensity range of the recording tool or display device

                                                                                                                                                  a b c d Fig5

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  11

                                                                                                                                                  1

                                                                                                                                                  2 1 1 21 2

                                                                                                                                                  2 1 2 1

                                                                                                                                                  22

                                                                                                                                                  2

                                                                                                                                                  [0 ]

                                                                                                                                                  ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                  ( 1 ) [ 1]( 1 )

                                                                                                                                                  s r r rrs r r s r rT r r r r

                                                                                                                                                  r r r rs L r r r L

                                                                                                                                                  L r

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                  1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                  Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                  in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                  from their original range to the full range [0 L-1]

                                                                                                                                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                  2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                  The original image on which these results are based is a scanning electron microscope

                                                                                                                                                  image of pollen magnified approximately 700 times

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Intensity-level slicing

                                                                                                                                                  - highlighting a specific range of intensities in an image

                                                                                                                                                  There are two approaches for intensity-level slicing

                                                                                                                                                  1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                  another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                  intensities in the image (Figure 311 (b))

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                  the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                  Highlights range [A B] and preserves all other intensities

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                  blockageshellip)

                                                                                                                                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                  image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                  Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Bit-plane slicing

                                                                                                                                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                  This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  Digital Image Processing

                                                                                                                                                  Week 1

                                                                                                                                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                  • DIP 1 2017
                                                                                                                                                  • DIP 02 (2017)

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Number of bits required to store a digitized image

                                                                                                                                                    for 2 b M N k M N b N k

                                                                                                                                                    When an image can have 2k intensity levels the image is referred as a k-bit image 256 discrete intensity values ndash 8-bit image

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                                                    Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                                                    Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                                                    (eg 100 line pairs per mm)

                                                                                                                                                    Dots per unit distance are commonly used in printing and publishing

                                                                                                                                                    In US the measure is expressed in dots per inch (dpi)

                                                                                                                                                    (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                                                    Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                                                    The number of intensity levels (L) is determined by hardware considerations

                                                                                                                                                    L=2k ndash most common k = 8

                                                                                                                                                    Intensity resolution in practice is given by k (number of bits used to quantize intensity)

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                                                    150 dpi (lower left) 72 dpi (lower right)

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Reducing the number of gray levels 256 128 64 32

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Reducing the number of gray levels 16 8 4 2

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                                    Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                                    Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                                    Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                                    750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                                    same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                                    image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                                    Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                                    Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                                    intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                                    This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                                    straight edges

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                                    where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                                    be written using the 4 nearest neighbors of point (x y)

                                                                                                                                                    Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                                    modest increase in computational effort

                                                                                                                                                    Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                                    nearest neighbors of the point 3 3

                                                                                                                                                    0 0

                                                                                                                                                    ( ) i ji j

                                                                                                                                                    i jv x y c x y

                                                                                                                                                    The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                                    intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                                    0 0

                                                                                                                                                    ( )i ji j

                                                                                                                                                    i jc x y x y

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                                    bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                                    programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                                    Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                                    the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                                    then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                                    neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                                    Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                                    interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                                    from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Neighbors of a Pixel

                                                                                                                                                    A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                                    This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                                    The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                                    and are denoted ND(p)

                                                                                                                                                    The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                                    N8 (p)

                                                                                                                                                    If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                                    fall outside the image

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Adjacency Connectivity Regions Boundaries

                                                                                                                                                    Denote by V the set of intensity levels used to define adjacency

                                                                                                                                                    - in a binary image V 01 (V=0 V=1)

                                                                                                                                                    - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                                    We consider 3 types of adjacency

                                                                                                                                                    (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                                    (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                                    (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                                    m-adjacent if

                                                                                                                                                    4( )q N p or

                                                                                                                                                    ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                                    Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                                    ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    binary image

                                                                                                                                                    0 1 1 0 1 1 0 1 1

                                                                                                                                                    1 0 1 0 0 1 0 0 1 0

                                                                                                                                                    0 0 1 0 0 1 0 0 1

                                                                                                                                                    V

                                                                                                                                                    The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                                    8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                                    m-adjacency

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                    is a sequence of distinct pixels with coordinates

                                                                                                                                                    and are adjacent 0 0 1 1

                                                                                                                                                    1 1

                                                                                                                                                    ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                    n n

                                                                                                                                                    i i i i

                                                                                                                                                    x y x y x y x y s tx y x y i n

                                                                                                                                                    The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                    Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                    Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                    in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                    S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                    Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                    Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                    that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                    8-adjacency are considered

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                    touches the image border

                                                                                                                                                    the complement of 1

                                                                                                                                                    ( )K

                                                                                                                                                    cu k u u

                                                                                                                                                    k

                                                                                                                                                    R R R R

                                                                                                                                                    We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                    background of the image

                                                                                                                                                    The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                    points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                    region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                    inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                    border in the background

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Distance measures

                                                                                                                                                    For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                    function or metric if

                                                                                                                                                    (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                    (b) D(p q) = D(q p)

                                                                                                                                                    (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                    The Euclidean distance between p and q is defined as 1

                                                                                                                                                    2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                    The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                    centered at (x y)

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                    4( ) | | | |D p q x s y t

                                                                                                                                                    The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                    4

                                                                                                                                                    22 1 2

                                                                                                                                                    2 2 1 0 1 22 1 2

                                                                                                                                                    2

                                                                                                                                                    D

                                                                                                                                                    The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                    The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                    8( ) max| | | |D p q x s y t

                                                                                                                                                    The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    8

                                                                                                                                                    2 2 2 2 22 1 1 1 2

                                                                                                                                                    2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                    D

                                                                                                                                                    The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                    D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                    because these distances involve only the coordinates of the point

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Array versus Matrix Operations

                                                                                                                                                    An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                    11 12 11 12

                                                                                                                                                    21 22 21 22

                                                                                                                                                    a a b ba a b b

                                                                                                                                                    Array product

                                                                                                                                                    11 12 11 12 11 11 12 12

                                                                                                                                                    21 22 21 22 21 21 22 21

                                                                                                                                                    a a b b a b a ba a b b a b a b

                                                                                                                                                    Matrix product

                                                                                                                                                    11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                    21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                    a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                    We assume array operations unless stated otherwise

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Linear versus Nonlinear Operations

                                                                                                                                                    One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                    linear or nonlinear

                                                                                                                                                    ( ) ( )H f x y g x y

                                                                                                                                                    H is said to be a linear operator if

                                                                                                                                                    images1 2 1 2

                                                                                                                                                    1 2

                                                                                                                                                    ( ) ( ) ( ) ( )

                                                                                                                                                    H a f x y b f x y a H f x y b H f x y

                                                                                                                                                    a b f f

                                                                                                                                                    Example of nonlinear operator

                                                                                                                                                    the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                    1 2

                                                                                                                                                    0 2 6 5 1 1

                                                                                                                                                    2 3 4 7f f a b

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    1 2

                                                                                                                                                    0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                    2 3 4 7 2 4a f b f

                                                                                                                                                    0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                    2 3 4 7

                                                                                                                                                    Arithmetic Operations in Image Processing

                                                                                                                                                    Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                    f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                    For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                    value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                    The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                    used in image enhancement)

                                                                                                                                                    1

                                                                                                                                                    1( ) ( )K

                                                                                                                                                    ii

                                                                                                                                                    g x y g x yK

                                                                                                                                                    If the noise satisfies the properties stated above we have

                                                                                                                                                    2 2( ) ( )

                                                                                                                                                    1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                    ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                    and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                    the average image is

                                                                                                                                                    ( ) ( )1

                                                                                                                                                    g x y x yK

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                    the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                    means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                    averaging process increases

                                                                                                                                                    An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                    under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                    virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                    was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                    64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                    images respectively

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                    100 noisy images

                                                                                                                                                    a b c d e f

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                    images

                                                                                                                                                    (a) (b) (c)

                                                                                                                                                    Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                    significant bit of each pixel (c) the difference between the two images

                                                                                                                                                    Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                    Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                    images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                    difference between images (a) and (b)

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                    h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                    intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                    source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                    bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                    anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                    images after injection of the contrast medium

                                                                                                                                                    In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                    Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                    propagates through the various arteries in the area being observed

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    An important application of image multiplication (and division) is shading correction

                                                                                                                                                    Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                    f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                    When the shading function is known

                                                                                                                                                    ( )( )( )

                                                                                                                                                    g x yf x yh x y

                                                                                                                                                    h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                    approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                    sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    (a) (b) (c)

                                                                                                                                                    Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                    operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                    1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                    image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                    (a) (b) (c)

                                                                                                                                                    Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                    min( )mf f f

                                                                                                                                                    0 ( 255)max( )

                                                                                                                                                    ms

                                                                                                                                                    m

                                                                                                                                                    ff K K K

                                                                                                                                                    f

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Spatial Operations

                                                                                                                                                    - are performed directly on the pixels of a given image

                                                                                                                                                    There are three categories of spatial operations

                                                                                                                                                    single-pixel operations

                                                                                                                                                    neighborhood operations

                                                                                                                                                    geometric spatial transformations

                                                                                                                                                    Single-pixel operations

                                                                                                                                                    - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                    where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                    corresponding pixel in the processed image

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Neighborhood operations

                                                                                                                                                    Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                    in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                    based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                    rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                    intensity by computing the average value of the pixels in Sxy

                                                                                                                                                    ( )

                                                                                                                                                    1( ) ( )xyr c S

                                                                                                                                                    g x y f r cm n

                                                                                                                                                    The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                    used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                    largest region of an image

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Geometric spatial transformations and image registration

                                                                                                                                                    - modify the spatial relationship between pixels in an image

                                                                                                                                                    - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                    printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                    predefined set of rules

                                                                                                                                                    A geometric transformation consists of 2 basic operations

                                                                                                                                                    1 a spatial transformation of coordinates

                                                                                                                                                    2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                    pixels

                                                                                                                                                    The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                    (v w) ndash pixel coordinates in the original image

                                                                                                                                                    (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                    Affine transform

                                                                                                                                                    11 1211 21 31

                                                                                                                                                    21 2212 22 33

                                                                                                                                                    31 32

                                                                                                                                                    0[ 1] [ 1] [ 1] 0

                                                                                                                                                    1

                                                                                                                                                    t tx t v t w t

                                                                                                                                                    x y v w T v w t ty t v t w t

                                                                                                                                                    t t

                                                                                                                                                    (AT)

                                                                                                                                                    This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                    on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                    result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                    scaling rotation and translation matrices from Table 1

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Affine transformations

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                    the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                    using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                    In practice we can use equation (AT) in two basic ways

                                                                                                                                                    forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                    location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                    Problems

                                                                                                                                                    - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                    the same location in the output image

                                                                                                                                                    - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                    assignment)

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                    computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                    It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                    pixel value

                                                                                                                                                    Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                    numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Image registration ndash align two or more images of the same scene

                                                                                                                                                    In image registration we have available the input and output images but the specific

                                                                                                                                                    transformation that produced the output image from the input is generally unknown

                                                                                                                                                    The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                    images

                                                                                                                                                    - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                    same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                    - align images of a given location taken by the same instrument at different moments

                                                                                                                                                    of time (satellite images)

                                                                                                                                                    Solving the problem using tie points (also called control points) which are

                                                                                                                                                    corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                    image

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    How to select tie points

                                                                                                                                                    - interactively selecting them

                                                                                                                                                    - use of algorithms that try to detect these points

                                                                                                                                                    - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                    the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                    marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                    for establishing tie points

                                                                                                                                                    The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                    of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                    a bilinear approximation is given by

                                                                                                                                                    1 2 3 4

                                                                                                                                                    5 6 7 8

                                                                                                                                                    x c v c w c v w cy c v c w c v w c

                                                                                                                                                    (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                    frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                    subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                    subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                    The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                    problem depend on the severity of the geometrical distortion

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                    Digital Image Processing

                                                                                                                                                    Week 1

                                                                                                                                                    Probabilistic Methods

                                                                                                                                                    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                    p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                    ( ) kk

                                                                                                                                                    np zM N

                                                                                                                                                    nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                    pixels in the image) 1

                                                                                                                                                    0( ) 1

                                                                                                                                                    L

                                                                                                                                                    kk

                                                                                                                                                    p z

                                                                                                                                                    The mean (average) intensity of an image is given by 1

                                                                                                                                                    0( )

                                                                                                                                                    L

                                                                                                                                                    k kk

                                                                                                                                                    m z p z

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                                                                                                                                                    Week 1

                                                                                                                                                    The variance of the intensities is 1

                                                                                                                                                    2 2

                                                                                                                                                    0( ) ( )

                                                                                                                                                    L

                                                                                                                                                    k kk

                                                                                                                                                    z m p z

                                                                                                                                                    The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                    measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                    ( ) is used

                                                                                                                                                    The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                    0( ) ( ) ( )

                                                                                                                                                    Ln

                                                                                                                                                    n k kk

                                                                                                                                                    z z m p z

                                                                                                                                                    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                    3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                    ( 3( ) 0z the intensities are biased to values lower than the mean

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                                                                                                                                                    Week 1

                                                                                                                                                    3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                    mean

                                                                                                                                                    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                    Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                    Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                    Figure 1(c) ndash standard deviation 492 (variance = 24206)

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                                                                                                                                                    Week 1

                                                                                                                                                    Intensity Transformations and Spatial Filtering

                                                                                                                                                    ( ) ( )g x y T f x y

                                                                                                                                                    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                    neighborhood of (x y)

                                                                                                                                                    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                    and much smaller in size than the image

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                                                                                                                                                    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                    called spatial filter (spatial mask kernel template or window)

                                                                                                                                                    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                    ( )s T r

                                                                                                                                                    s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                    Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                    darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                    is called contrast stretching

                                                                                                                                                    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

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                                                                                                                                                    Week 1

                                                                                                                                                    Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                    thresholding function

                                                                                                                                                    Some Basic Intensity Transformation Functions

                                                                                                                                                    Image Negatives

                                                                                                                                                    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                    - equivalent of a photographic negative

                                                                                                                                                    - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                    image

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                                                                                                                                                    Week 1

                                                                                                                                                    Original Negative image

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                                                                                                                                                    Week 1

                                                                                                                                                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                    Some basic intensity transformation functions

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                                                                                                                                                    This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                    range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                    while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                    transformation The log functions compress the dynamic range of images with large

                                                                                                                                                    variations in pixel values

                                                                                                                                                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

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                                                                                                                                                    Power-Law (Gamma) Transformations

                                                                                                                                                    - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                    Plots of gamma transformation for different values of γ (c=1)

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                                                                                                                                                    Week 1

                                                                                                                                                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                    of output values with the opposite being true for higher values of input values The

                                                                                                                                                    curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                    1c - identity transformation

                                                                                                                                                    A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                    power law The process used to correct these power-law response phenomena is called

                                                                                                                                                    gamma correction

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                                                                                                                                                    Week 1

                                                                                                                                                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

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                                                                                                                                                    Piecewise-Linear Transformations Functions

                                                                                                                                                    Contrast stretching

                                                                                                                                                    - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                    intensity range of the recording tool or display device

                                                                                                                                                    a b c d Fig5

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                                                                                                                                                    Week 1

                                                                                                                                                    11

                                                                                                                                                    1

                                                                                                                                                    2 1 1 21 2

                                                                                                                                                    2 1 2 1

                                                                                                                                                    22

                                                                                                                                                    2

                                                                                                                                                    [0 ]

                                                                                                                                                    ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                    ( 1 ) [ 1]( 1 )

                                                                                                                                                    s r r rrs r r s r rT r r r r

                                                                                                                                                    r r r rs L r r r L

                                                                                                                                                    L r

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                                                                                                                                                    1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                    1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                    Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                    in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                    from their original range to the full range [0 L-1]

                                                                                                                                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                    2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                    The original image on which these results are based is a scanning electron microscope

                                                                                                                                                    image of pollen magnified approximately 700 times

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                                                                                                                                                    Intensity-level slicing

                                                                                                                                                    - highlighting a specific range of intensities in an image

                                                                                                                                                    There are two approaches for intensity-level slicing

                                                                                                                                                    1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                    another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                    intensities in the image (Figure 311 (b))

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                                                                                                                                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                    the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                    Highlights range [A B] and preserves all other intensities

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                                                                                                                                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                    blockageshellip)

                                                                                                                                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                    image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                    Fig 6 - Aortic angiogram and intensity sliced versions

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                                                                                                                                                    Bit-plane slicing

                                                                                                                                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                    This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

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                                                                                                                                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                    • DIP 1 2017
                                                                                                                                                    • DIP 02 (2017)

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                                                                                                                                                      Spatial and Intensity Resolution Spatial resolution ndash the smallest discernible detail in an image

                                                                                                                                                      Measures line pairs per unit distance dots (pixels) per unit distance

                                                                                                                                                      Image resolution = the largest number of discernible line pairs per unit distance

                                                                                                                                                      (eg 100 line pairs per mm)

                                                                                                                                                      Dots per unit distance are commonly used in printing and publishing

                                                                                                                                                      In US the measure is expressed in dots per inch (dpi)

                                                                                                                                                      (newspapers are printed with 75 dpi glossy brochures at 175 dpi)

                                                                                                                                                      Intensity resolution ndash the smallest discernible change in intensity level

                                                                                                                                                      The number of intensity levels (L) is determined by hardware considerations

                                                                                                                                                      L=2k ndash most common k = 8

                                                                                                                                                      Intensity resolution in practice is given by k (number of bits used to quantize intensity)

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                                                                                                                                                      Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                                                      150 dpi (lower left) 72 dpi (lower right)

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                                                                                                                                                      Reducing the number of gray levels 256 128 64 32

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                                                                                                                                                      Reducing the number of gray levels 16 8 4 2

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                                                                                                                                                      Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                                      Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                                      Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                                      Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                                      750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                                      same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                                      image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                                      Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                                      Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                                      intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                                      This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                                      straight edges

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                                                                                                                                                      Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                                      where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                                      be written using the 4 nearest neighbors of point (x y)

                                                                                                                                                      Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                                      modest increase in computational effort

                                                                                                                                                      Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                                      nearest neighbors of the point 3 3

                                                                                                                                                      0 0

                                                                                                                                                      ( ) i ji j

                                                                                                                                                      i jv x y c x y

                                                                                                                                                      The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                                      intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                                      0 0

                                                                                                                                                      ( )i ji j

                                                                                                                                                      i jc x y x y

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                                                                                                                                                      Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                                      bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                                      programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                                      Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                                      the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                                      then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                                      neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                                      Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                                      interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                                      from 1250 dpi to 150 dpi (instead of 72 dpi)

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                                                                                                                                                      Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

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                                                                                                                                                      Neighbors of a Pixel

                                                                                                                                                      A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                                      This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                                      The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                                      and are denoted ND(p)

                                                                                                                                                      The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                                      N8 (p)

                                                                                                                                                      If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                                      fall outside the image

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                                                                                                                                                      Adjacency Connectivity Regions Boundaries

                                                                                                                                                      Denote by V the set of intensity levels used to define adjacency

                                                                                                                                                      - in a binary image V 01 (V=0 V=1)

                                                                                                                                                      - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                                      We consider 3 types of adjacency

                                                                                                                                                      (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                                      (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                                      (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                                      m-adjacent if

                                                                                                                                                      4( )q N p or

                                                                                                                                                      ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                                      Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                                      ambiguities that often arise when 8-adjacency is used Consider the example

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                                                                                                                                                      Week 1

                                                                                                                                                      binary image

                                                                                                                                                      0 1 1 0 1 1 0 1 1

                                                                                                                                                      1 0 1 0 0 1 0 0 1 0

                                                                                                                                                      0 0 1 0 0 1 0 0 1

                                                                                                                                                      V

                                                                                                                                                      The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                                      8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                                      m-adjacency

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                                                                                                                                                      A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                      is a sequence of distinct pixels with coordinates

                                                                                                                                                      and are adjacent 0 0 1 1

                                                                                                                                                      1 1

                                                                                                                                                      ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                      n n

                                                                                                                                                      i i i i

                                                                                                                                                      x y x y x y x y s tx y x y i n

                                                                                                                                                      The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                      Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                      Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                      in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                      S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                      Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                      Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                      that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                      8-adjacency are considered

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                                                                                                                                                      Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                      touches the image border

                                                                                                                                                      the complement of 1

                                                                                                                                                      ( )K

                                                                                                                                                      cu k u u

                                                                                                                                                      k

                                                                                                                                                      R R R R

                                                                                                                                                      We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                      background of the image

                                                                                                                                                      The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                      points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                      region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                      inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                      border in the background

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                                                                                                                                                      Distance measures

                                                                                                                                                      For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                      function or metric if

                                                                                                                                                      (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                      (b) D(p q) = D(q p)

                                                                                                                                                      (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                      The Euclidean distance between p and q is defined as 1

                                                                                                                                                      2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                      The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                      centered at (x y)

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                                                                                                                                                      The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                      4( ) | | | |D p q x s y t

                                                                                                                                                      The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                      4

                                                                                                                                                      22 1 2

                                                                                                                                                      2 2 1 0 1 22 1 2

                                                                                                                                                      2

                                                                                                                                                      D

                                                                                                                                                      The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                      The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                      8( ) max| | | |D p q x s y t

                                                                                                                                                      The pixels q for which 8( )D p q r form a square centered at (x y)

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                                                                                                                                                      8

                                                                                                                                                      2 2 2 2 22 1 1 1 2

                                                                                                                                                      2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                      D

                                                                                                                                                      The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                      D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                      because these distances involve only the coordinates of the point

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                                                                                                                                                      Array versus Matrix Operations

                                                                                                                                                      An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                      11 12 11 12

                                                                                                                                                      21 22 21 22

                                                                                                                                                      a a b ba a b b

                                                                                                                                                      Array product

                                                                                                                                                      11 12 11 12 11 11 12 12

                                                                                                                                                      21 22 21 22 21 21 22 21

                                                                                                                                                      a a b b a b a ba a b b a b a b

                                                                                                                                                      Matrix product

                                                                                                                                                      11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                      21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                      a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                      We assume array operations unless stated otherwise

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                                                                                                                                                      Linear versus Nonlinear Operations

                                                                                                                                                      One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                      linear or nonlinear

                                                                                                                                                      ( ) ( )H f x y g x y

                                                                                                                                                      H is said to be a linear operator if

                                                                                                                                                      images1 2 1 2

                                                                                                                                                      1 2

                                                                                                                                                      ( ) ( ) ( ) ( )

                                                                                                                                                      H a f x y b f x y a H f x y b H f x y

                                                                                                                                                      a b f f

                                                                                                                                                      Example of nonlinear operator

                                                                                                                                                      the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                      1 2

                                                                                                                                                      0 2 6 5 1 1

                                                                                                                                                      2 3 4 7f f a b

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      1 2

                                                                                                                                                      0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                      2 3 4 7 2 4a f b f

                                                                                                                                                      0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                      2 3 4 7

                                                                                                                                                      Arithmetic Operations in Image Processing

                                                                                                                                                      Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                      f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                      For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                      value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                      The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                      used in image enhancement)

                                                                                                                                                      1

                                                                                                                                                      1( ) ( )K

                                                                                                                                                      ii

                                                                                                                                                      g x y g x yK

                                                                                                                                                      If the noise satisfies the properties stated above we have

                                                                                                                                                      2 2( ) ( )

                                                                                                                                                      1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                      ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                      and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                      the average image is

                                                                                                                                                      ( ) ( )1

                                                                                                                                                      g x y x yK

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                                                                                                                                                      Week 1

                                                                                                                                                      As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                      the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                      means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                      averaging process increases

                                                                                                                                                      An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                      under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                      virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                      was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                      64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                      images respectively

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                      100 noisy images

                                                                                                                                                      a b c d e f

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                      images

                                                                                                                                                      (a) (b) (c)

                                                                                                                                                      Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                      significant bit of each pixel (c) the difference between the two images

                                                                                                                                                      Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                      Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                      images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                      difference between images (a) and (b)

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                      h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                      intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                      source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                      bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                      anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                      images after injection of the contrast medium

                                                                                                                                                      In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                      Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                      propagates through the various arteries in the area being observed

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      An important application of image multiplication (and division) is shading correction

                                                                                                                                                      Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                      f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                      When the shading function is known

                                                                                                                                                      ( )( )( )

                                                                                                                                                      g x yf x yh x y

                                                                                                                                                      h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                      approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                      sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      (a) (b) (c)

                                                                                                                                                      Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                      operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                      1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                      image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                      (a) (b) (c)

                                                                                                                                                      Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                      min( )mf f f

                                                                                                                                                      0 ( 255)max( )

                                                                                                                                                      ms

                                                                                                                                                      m

                                                                                                                                                      ff K K K

                                                                                                                                                      f

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Spatial Operations

                                                                                                                                                      - are performed directly on the pixels of a given image

                                                                                                                                                      There are three categories of spatial operations

                                                                                                                                                      single-pixel operations

                                                                                                                                                      neighborhood operations

                                                                                                                                                      geometric spatial transformations

                                                                                                                                                      Single-pixel operations

                                                                                                                                                      - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                      where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                      corresponding pixel in the processed image

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Neighborhood operations

                                                                                                                                                      Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                      in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                      based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                      rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                      intensity by computing the average value of the pixels in Sxy

                                                                                                                                                      ( )

                                                                                                                                                      1( ) ( )xyr c S

                                                                                                                                                      g x y f r cm n

                                                                                                                                                      The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                      used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                      largest region of an image

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Geometric spatial transformations and image registration

                                                                                                                                                      - modify the spatial relationship between pixels in an image

                                                                                                                                                      - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                      printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                      predefined set of rules

                                                                                                                                                      A geometric transformation consists of 2 basic operations

                                                                                                                                                      1 a spatial transformation of coordinates

                                                                                                                                                      2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                      pixels

                                                                                                                                                      The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                      (v w) ndash pixel coordinates in the original image

                                                                                                                                                      (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                      Affine transform

                                                                                                                                                      11 1211 21 31

                                                                                                                                                      21 2212 22 33

                                                                                                                                                      31 32

                                                                                                                                                      0[ 1] [ 1] [ 1] 0

                                                                                                                                                      1

                                                                                                                                                      t tx t v t w t

                                                                                                                                                      x y v w T v w t ty t v t w t

                                                                                                                                                      t t

                                                                                                                                                      (AT)

                                                                                                                                                      This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                      on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                      result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                      scaling rotation and translation matrices from Table 1

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Affine transformations

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                      the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                      using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                      In practice we can use equation (AT) in two basic ways

                                                                                                                                                      forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                      location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                      Problems

                                                                                                                                                      - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                      the same location in the output image

                                                                                                                                                      - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                      assignment)

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                      computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                      It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                      pixel value

                                                                                                                                                      Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                      numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Image registration ndash align two or more images of the same scene

                                                                                                                                                      In image registration we have available the input and output images but the specific

                                                                                                                                                      transformation that produced the output image from the input is generally unknown

                                                                                                                                                      The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                      images

                                                                                                                                                      - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                      same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                      - align images of a given location taken by the same instrument at different moments

                                                                                                                                                      of time (satellite images)

                                                                                                                                                      Solving the problem using tie points (also called control points) which are

                                                                                                                                                      corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                      image

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      How to select tie points

                                                                                                                                                      - interactively selecting them

                                                                                                                                                      - use of algorithms that try to detect these points

                                                                                                                                                      - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                      the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                      marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                      for establishing tie points

                                                                                                                                                      The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                      of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                      a bilinear approximation is given by

                                                                                                                                                      1 2 3 4

                                                                                                                                                      5 6 7 8

                                                                                                                                                      x c v c w c v w cy c v c w c v w c

                                                                                                                                                      (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                      frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                      subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                      subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                      The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                      problem depend on the severity of the geometrical distortion

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Probabilistic Methods

                                                                                                                                                      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                      p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                      ( ) kk

                                                                                                                                                      np zM N

                                                                                                                                                      nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                      pixels in the image) 1

                                                                                                                                                      0( ) 1

                                                                                                                                                      L

                                                                                                                                                      kk

                                                                                                                                                      p z

                                                                                                                                                      The mean (average) intensity of an image is given by 1

                                                                                                                                                      0( )

                                                                                                                                                      L

                                                                                                                                                      k kk

                                                                                                                                                      m z p z

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      The variance of the intensities is 1

                                                                                                                                                      2 2

                                                                                                                                                      0( ) ( )

                                                                                                                                                      L

                                                                                                                                                      k kk

                                                                                                                                                      z m p z

                                                                                                                                                      The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                      measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                      ( ) is used

                                                                                                                                                      The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                      0( ) ( ) ( )

                                                                                                                                                      Ln

                                                                                                                                                      n k kk

                                                                                                                                                      z z m p z

                                                                                                                                                      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                      3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                      ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                      mean

                                                                                                                                                      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                      Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                      Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                      Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Intensity Transformations and Spatial Filtering

                                                                                                                                                      ( ) ( )g x y T f x y

                                                                                                                                                      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                      neighborhood of (x y)

                                                                                                                                                      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                      and much smaller in size than the image

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                      called spatial filter (spatial mask kernel template or window)

                                                                                                                                                      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                      ( )s T r

                                                                                                                                                      s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                      Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                      darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                      is called contrast stretching

                                                                                                                                                      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                      thresholding function

                                                                                                                                                      Some Basic Intensity Transformation Functions

                                                                                                                                                      Image Negatives

                                                                                                                                                      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                      - equivalent of a photographic negative

                                                                                                                                                      - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                      image

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Original Negative image

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                      Some basic intensity transformation functions

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                      range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                      while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                      transformation The log functions compress the dynamic range of images with large

                                                                                                                                                      variations in pixel values

                                                                                                                                                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Power-Law (Gamma) Transformations

                                                                                                                                                      - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                      Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                      of output values with the opposite being true for higher values of input values The

                                                                                                                                                      curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                      1c - identity transformation

                                                                                                                                                      A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                      power law The process used to correct these power-law response phenomena is called

                                                                                                                                                      gamma correction

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Piecewise-Linear Transformations Functions

                                                                                                                                                      Contrast stretching

                                                                                                                                                      - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                      intensity range of the recording tool or display device

                                                                                                                                                      a b c d Fig5

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      11

                                                                                                                                                      1

                                                                                                                                                      2 1 1 21 2

                                                                                                                                                      2 1 2 1

                                                                                                                                                      22

                                                                                                                                                      2

                                                                                                                                                      [0 ]

                                                                                                                                                      ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                      ( 1 ) [ 1]( 1 )

                                                                                                                                                      s r r rrs r r s r rT r r r r

                                                                                                                                                      r r r rs L r r r L

                                                                                                                                                      L r

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                      1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                      Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                      in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                      from their original range to the full range [0 L-1]

                                                                                                                                                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                      2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                      The original image on which these results are based is a scanning electron microscope

                                                                                                                                                      image of pollen magnified approximately 700 times

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Intensity-level slicing

                                                                                                                                                      - highlighting a specific range of intensities in an image

                                                                                                                                                      There are two approaches for intensity-level slicing

                                                                                                                                                      1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                      another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                      intensities in the image (Figure 311 (b))

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                      the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                      Highlights range [A B] and preserves all other intensities

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                      blockageshellip)

                                                                                                                                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                      image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                      Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Bit-plane slicing

                                                                                                                                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                      This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      Digital Image Processing

                                                                                                                                                      Week 1

                                                                                                                                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                      • DIP 1 2017
                                                                                                                                                      • DIP 02 (2017)

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Fig1 Reducing spatial resolution 1250 dpi(upper left) 300 dpi (upper right)

                                                                                                                                                        150 dpi (lower left) 72 dpi (lower right)

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Reducing the number of gray levels 256 128 64 32

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Reducing the number of gray levels 16 8 4 2

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                                        Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                                        Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                                        Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                                        750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                                        same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                                        image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                                        Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                                        Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                                        intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                                        This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                                        straight edges

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                                        where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                                        be written using the 4 nearest neighbors of point (x y)

                                                                                                                                                        Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                                        modest increase in computational effort

                                                                                                                                                        Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                                        nearest neighbors of the point 3 3

                                                                                                                                                        0 0

                                                                                                                                                        ( ) i ji j

                                                                                                                                                        i jv x y c x y

                                                                                                                                                        The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                                        intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                                        0 0

                                                                                                                                                        ( )i ji j

                                                                                                                                                        i jc x y x y

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                                        bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                                        programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                                        Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                                        the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                                        then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                                        neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                                        Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                                        interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                                        from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Neighbors of a Pixel

                                                                                                                                                        A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                                        This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                                        The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                                        and are denoted ND(p)

                                                                                                                                                        The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                                        N8 (p)

                                                                                                                                                        If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                                        fall outside the image

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Adjacency Connectivity Regions Boundaries

                                                                                                                                                        Denote by V the set of intensity levels used to define adjacency

                                                                                                                                                        - in a binary image V 01 (V=0 V=1)

                                                                                                                                                        - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                                        We consider 3 types of adjacency

                                                                                                                                                        (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                                        (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                                        (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                                        m-adjacent if

                                                                                                                                                        4( )q N p or

                                                                                                                                                        ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                                        Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                                        ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        binary image

                                                                                                                                                        0 1 1 0 1 1 0 1 1

                                                                                                                                                        1 0 1 0 0 1 0 0 1 0

                                                                                                                                                        0 0 1 0 0 1 0 0 1

                                                                                                                                                        V

                                                                                                                                                        The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                                        8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                                        m-adjacency

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                        is a sequence of distinct pixels with coordinates

                                                                                                                                                        and are adjacent 0 0 1 1

                                                                                                                                                        1 1

                                                                                                                                                        ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                        n n

                                                                                                                                                        i i i i

                                                                                                                                                        x y x y x y x y s tx y x y i n

                                                                                                                                                        The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                        Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                        Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                        in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                        S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                        Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                        Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                        that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                        8-adjacency are considered

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                        touches the image border

                                                                                                                                                        the complement of 1

                                                                                                                                                        ( )K

                                                                                                                                                        cu k u u

                                                                                                                                                        k

                                                                                                                                                        R R R R

                                                                                                                                                        We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                        background of the image

                                                                                                                                                        The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                        points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                        region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                        inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                        border in the background

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Distance measures

                                                                                                                                                        For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                        function or metric if

                                                                                                                                                        (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                        (b) D(p q) = D(q p)

                                                                                                                                                        (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                        The Euclidean distance between p and q is defined as 1

                                                                                                                                                        2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                        The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                        centered at (x y)

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                        4( ) | | | |D p q x s y t

                                                                                                                                                        The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                        4

                                                                                                                                                        22 1 2

                                                                                                                                                        2 2 1 0 1 22 1 2

                                                                                                                                                        2

                                                                                                                                                        D

                                                                                                                                                        The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                        The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                        8( ) max| | | |D p q x s y t

                                                                                                                                                        The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        8

                                                                                                                                                        2 2 2 2 22 1 1 1 2

                                                                                                                                                        2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                        D

                                                                                                                                                        The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                        D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                        because these distances involve only the coordinates of the point

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Array versus Matrix Operations

                                                                                                                                                        An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                        11 12 11 12

                                                                                                                                                        21 22 21 22

                                                                                                                                                        a a b ba a b b

                                                                                                                                                        Array product

                                                                                                                                                        11 12 11 12 11 11 12 12

                                                                                                                                                        21 22 21 22 21 21 22 21

                                                                                                                                                        a a b b a b a ba a b b a b a b

                                                                                                                                                        Matrix product

                                                                                                                                                        11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                        21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                        a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                        We assume array operations unless stated otherwise

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Linear versus Nonlinear Operations

                                                                                                                                                        One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                        linear or nonlinear

                                                                                                                                                        ( ) ( )H f x y g x y

                                                                                                                                                        H is said to be a linear operator if

                                                                                                                                                        images1 2 1 2

                                                                                                                                                        1 2

                                                                                                                                                        ( ) ( ) ( ) ( )

                                                                                                                                                        H a f x y b f x y a H f x y b H f x y

                                                                                                                                                        a b f f

                                                                                                                                                        Example of nonlinear operator

                                                                                                                                                        the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                        1 2

                                                                                                                                                        0 2 6 5 1 1

                                                                                                                                                        2 3 4 7f f a b

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        1 2

                                                                                                                                                        0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                        2 3 4 7 2 4a f b f

                                                                                                                                                        0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                        2 3 4 7

                                                                                                                                                        Arithmetic Operations in Image Processing

                                                                                                                                                        Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                        f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                        For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                        value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                        The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                        used in image enhancement)

                                                                                                                                                        1

                                                                                                                                                        1( ) ( )K

                                                                                                                                                        ii

                                                                                                                                                        g x y g x yK

                                                                                                                                                        If the noise satisfies the properties stated above we have

                                                                                                                                                        2 2( ) ( )

                                                                                                                                                        1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                        ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                        and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                        the average image is

                                                                                                                                                        ( ) ( )1

                                                                                                                                                        g x y x yK

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                                                                                                                                                        Week 1

                                                                                                                                                        As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                        the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                        means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                        averaging process increases

                                                                                                                                                        An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                        under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                        virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                        was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                        64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                        images respectively

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                                                                                                                                                        Week 1

                                                                                                                                                        Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                        100 noisy images

                                                                                                                                                        a b c d e f

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                        images

                                                                                                                                                        (a) (b) (c)

                                                                                                                                                        Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                        significant bit of each pixel (c) the difference between the two images

                                                                                                                                                        Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                        Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                        images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                        difference between images (a) and (b)

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                        h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                        intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                        source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                        bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                        anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                        images after injection of the contrast medium

                                                                                                                                                        In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                        Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                        propagates through the various arteries in the area being observed

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        An important application of image multiplication (and division) is shading correction

                                                                                                                                                        Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                        f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                        When the shading function is known

                                                                                                                                                        ( )( )( )

                                                                                                                                                        g x yf x yh x y

                                                                                                                                                        h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                        approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                        sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        (a) (b) (c)

                                                                                                                                                        Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                        operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                        1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                        image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                        (a) (b) (c)

                                                                                                                                                        Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                        min( )mf f f

                                                                                                                                                        0 ( 255)max( )

                                                                                                                                                        ms

                                                                                                                                                        m

                                                                                                                                                        ff K K K

                                                                                                                                                        f

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Spatial Operations

                                                                                                                                                        - are performed directly on the pixels of a given image

                                                                                                                                                        There are three categories of spatial operations

                                                                                                                                                        single-pixel operations

                                                                                                                                                        neighborhood operations

                                                                                                                                                        geometric spatial transformations

                                                                                                                                                        Single-pixel operations

                                                                                                                                                        - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                        where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                        corresponding pixel in the processed image

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Neighborhood operations

                                                                                                                                                        Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                        in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                        based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                        rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                        intensity by computing the average value of the pixels in Sxy

                                                                                                                                                        ( )

                                                                                                                                                        1( ) ( )xyr c S

                                                                                                                                                        g x y f r cm n

                                                                                                                                                        The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                        used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                        largest region of an image

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Geometric spatial transformations and image registration

                                                                                                                                                        - modify the spatial relationship between pixels in an image

                                                                                                                                                        - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                        printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                        predefined set of rules

                                                                                                                                                        A geometric transformation consists of 2 basic operations

                                                                                                                                                        1 a spatial transformation of coordinates

                                                                                                                                                        2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                        pixels

                                                                                                                                                        The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                        (v w) ndash pixel coordinates in the original image

                                                                                                                                                        (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                        Affine transform

                                                                                                                                                        11 1211 21 31

                                                                                                                                                        21 2212 22 33

                                                                                                                                                        31 32

                                                                                                                                                        0[ 1] [ 1] [ 1] 0

                                                                                                                                                        1

                                                                                                                                                        t tx t v t w t

                                                                                                                                                        x y v w T v w t ty t v t w t

                                                                                                                                                        t t

                                                                                                                                                        (AT)

                                                                                                                                                        This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                        on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                        result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                        scaling rotation and translation matrices from Table 1

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Affine transformations

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                        the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                        using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                        In practice we can use equation (AT) in two basic ways

                                                                                                                                                        forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                        location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                        Problems

                                                                                                                                                        - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                        the same location in the output image

                                                                                                                                                        - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                        assignment)

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                        computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                        It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                        pixel value

                                                                                                                                                        Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                        numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Image registration ndash align two or more images of the same scene

                                                                                                                                                        In image registration we have available the input and output images but the specific

                                                                                                                                                        transformation that produced the output image from the input is generally unknown

                                                                                                                                                        The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                        images

                                                                                                                                                        - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                        same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                        - align images of a given location taken by the same instrument at different moments

                                                                                                                                                        of time (satellite images)

                                                                                                                                                        Solving the problem using tie points (also called control points) which are

                                                                                                                                                        corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                        image

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        How to select tie points

                                                                                                                                                        - interactively selecting them

                                                                                                                                                        - use of algorithms that try to detect these points

                                                                                                                                                        - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                        the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                        marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                        for establishing tie points

                                                                                                                                                        The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                        of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                        a bilinear approximation is given by

                                                                                                                                                        1 2 3 4

                                                                                                                                                        5 6 7 8

                                                                                                                                                        x c v c w c v w cy c v c w c v w c

                                                                                                                                                        (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                        frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                        subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                        subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                        The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                        problem depend on the severity of the geometrical distortion

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Probabilistic Methods

                                                                                                                                                        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                        p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                        ( ) kk

                                                                                                                                                        np zM N

                                                                                                                                                        nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                        pixels in the image) 1

                                                                                                                                                        0( ) 1

                                                                                                                                                        L

                                                                                                                                                        kk

                                                                                                                                                        p z

                                                                                                                                                        The mean (average) intensity of an image is given by 1

                                                                                                                                                        0( )

                                                                                                                                                        L

                                                                                                                                                        k kk

                                                                                                                                                        m z p z

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        The variance of the intensities is 1

                                                                                                                                                        2 2

                                                                                                                                                        0( ) ( )

                                                                                                                                                        L

                                                                                                                                                        k kk

                                                                                                                                                        z m p z

                                                                                                                                                        The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                        measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                        ( ) is used

                                                                                                                                                        The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                        0( ) ( ) ( )

                                                                                                                                                        Ln

                                                                                                                                                        n k kk

                                                                                                                                                        z z m p z

                                                                                                                                                        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                        3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                        ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                        mean

                                                                                                                                                        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                        Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                        Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                        Figure 1(c) ndash standard deviation 492 (variance = 24206)

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                                                                                                                                                        Week 1

                                                                                                                                                        Intensity Transformations and Spatial Filtering

                                                                                                                                                        ( ) ( )g x y T f x y

                                                                                                                                                        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                        neighborhood of (x y)

                                                                                                                                                        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                        and much smaller in size than the image

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                                                                                                                                                        Week 1

                                                                                                                                                        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                        called spatial filter (spatial mask kernel template or window)

                                                                                                                                                        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                        ( )s T r

                                                                                                                                                        s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                        Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                        darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                        is called contrast stretching

                                                                                                                                                        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

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                                                                                                                                                        Week 1

                                                                                                                                                        Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                        thresholding function

                                                                                                                                                        Some Basic Intensity Transformation Functions

                                                                                                                                                        Image Negatives

                                                                                                                                                        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                        - equivalent of a photographic negative

                                                                                                                                                        - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                        image

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                                                                                                                                                        Week 1

                                                                                                                                                        Original Negative image

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                                                                                                                                                        Week 1

                                                                                                                                                        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                        Some basic intensity transformation functions

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                                                                                                                                                        Week 1

                                                                                                                                                        This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                        range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                        while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                        transformation The log functions compress the dynamic range of images with large

                                                                                                                                                        variations in pixel values

                                                                                                                                                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

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                                                                                                                                                        Week 1

                                                                                                                                                        Power-Law (Gamma) Transformations

                                                                                                                                                        - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                        Plots of gamma transformation for different values of γ (c=1)

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                                                                                                                                                        Week 1

                                                                                                                                                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                        of output values with the opposite being true for higher values of input values The

                                                                                                                                                        curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                        1c - identity transformation

                                                                                                                                                        A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                        power law The process used to correct these power-law response phenomena is called

                                                                                                                                                        gamma correction

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                                                                                                                                                        Week 1

                                                                                                                                                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

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                                                                                                                                                        Week 1

                                                                                                                                                        Piecewise-Linear Transformations Functions

                                                                                                                                                        Contrast stretching

                                                                                                                                                        - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                        intensity range of the recording tool or display device

                                                                                                                                                        a b c d Fig5

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        11

                                                                                                                                                        1

                                                                                                                                                        2 1 1 21 2

                                                                                                                                                        2 1 2 1

                                                                                                                                                        22

                                                                                                                                                        2

                                                                                                                                                        [0 ]

                                                                                                                                                        ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                        ( 1 ) [ 1]( 1 )

                                                                                                                                                        s r r rrs r r s r rT r r r r

                                                                                                                                                        r r r rs L r r r L

                                                                                                                                                        L r

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                                                                                                                                                        Week 1

                                                                                                                                                        1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                        1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                        Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                        in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                        from their original range to the full range [0 L-1]

                                                                                                                                                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                        2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                        The original image on which these results are based is a scanning electron microscope

                                                                                                                                                        image of pollen magnified approximately 700 times

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Intensity-level slicing

                                                                                                                                                        - highlighting a specific range of intensities in an image

                                                                                                                                                        There are two approaches for intensity-level slicing

                                                                                                                                                        1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                        another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                        intensities in the image (Figure 311 (b))

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                                                                                                                                                        Week 1

                                                                                                                                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                        the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                        Highlights range [A B] and preserves all other intensities

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                        blockageshellip)

                                                                                                                                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                        image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                        Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Bit-plane slicing

                                                                                                                                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                        This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        Digital Image Processing

                                                                                                                                                        Week 1

                                                                                                                                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                        • DIP 1 2017
                                                                                                                                                        • DIP 02 (2017)

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                                                                                                                                                          Week 1

                                                                                                                                                          Reducing the number of gray levels 256 128 64 32

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Reducing the number of gray levels 16 8 4 2

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                                                                                                                                                          Week 1

                                                                                                                                                          Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                                          Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                                          Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                                          Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                                          750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                                          same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                                          image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                                          Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                                          Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                                          intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                                          This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                                          straight edges

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                                          where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                                          be written using the 4 nearest neighbors of point (x y)

                                                                                                                                                          Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                                          modest increase in computational effort

                                                                                                                                                          Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                                          nearest neighbors of the point 3 3

                                                                                                                                                          0 0

                                                                                                                                                          ( ) i ji j

                                                                                                                                                          i jv x y c x y

                                                                                                                                                          The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                                          intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                                          0 0

                                                                                                                                                          ( )i ji j

                                                                                                                                                          i jc x y x y

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                                          bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                                          programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                                          Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                                          the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                                          then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                                          neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                                          Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                                          interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                                          from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Neighbors of a Pixel

                                                                                                                                                          A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                                          This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                                          The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                                          and are denoted ND(p)

                                                                                                                                                          The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                                          N8 (p)

                                                                                                                                                          If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                                          fall outside the image

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Adjacency Connectivity Regions Boundaries

                                                                                                                                                          Denote by V the set of intensity levels used to define adjacency

                                                                                                                                                          - in a binary image V 01 (V=0 V=1)

                                                                                                                                                          - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                                          We consider 3 types of adjacency

                                                                                                                                                          (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                                          (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                                          (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                                          m-adjacent if

                                                                                                                                                          4( )q N p or

                                                                                                                                                          ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                                          Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                                          ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          binary image

                                                                                                                                                          0 1 1 0 1 1 0 1 1

                                                                                                                                                          1 0 1 0 0 1 0 0 1 0

                                                                                                                                                          0 0 1 0 0 1 0 0 1

                                                                                                                                                          V

                                                                                                                                                          The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                                          8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                                          m-adjacency

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                          is a sequence of distinct pixels with coordinates

                                                                                                                                                          and are adjacent 0 0 1 1

                                                                                                                                                          1 1

                                                                                                                                                          ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                          n n

                                                                                                                                                          i i i i

                                                                                                                                                          x y x y x y x y s tx y x y i n

                                                                                                                                                          The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                          Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                          Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                          in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                          S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                          Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                          Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                          that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                          8-adjacency are considered

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                          touches the image border

                                                                                                                                                          the complement of 1

                                                                                                                                                          ( )K

                                                                                                                                                          cu k u u

                                                                                                                                                          k

                                                                                                                                                          R R R R

                                                                                                                                                          We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                          background of the image

                                                                                                                                                          The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                          points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                          region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                          inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                          border in the background

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Distance measures

                                                                                                                                                          For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                          function or metric if

                                                                                                                                                          (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                          (b) D(p q) = D(q p)

                                                                                                                                                          (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                          The Euclidean distance between p and q is defined as 1

                                                                                                                                                          2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                          The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                          centered at (x y)

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                          4( ) | | | |D p q x s y t

                                                                                                                                                          The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                          4

                                                                                                                                                          22 1 2

                                                                                                                                                          2 2 1 0 1 22 1 2

                                                                                                                                                          2

                                                                                                                                                          D

                                                                                                                                                          The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                          The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                          8( ) max| | | |D p q x s y t

                                                                                                                                                          The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          8

                                                                                                                                                          2 2 2 2 22 1 1 1 2

                                                                                                                                                          2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                          D

                                                                                                                                                          The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                          D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                          because these distances involve only the coordinates of the point

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Array versus Matrix Operations

                                                                                                                                                          An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                          11 12 11 12

                                                                                                                                                          21 22 21 22

                                                                                                                                                          a a b ba a b b

                                                                                                                                                          Array product

                                                                                                                                                          11 12 11 12 11 11 12 12

                                                                                                                                                          21 22 21 22 21 21 22 21

                                                                                                                                                          a a b b a b a ba a b b a b a b

                                                                                                                                                          Matrix product

                                                                                                                                                          11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                          21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                          a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                          We assume array operations unless stated otherwise

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Linear versus Nonlinear Operations

                                                                                                                                                          One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                          linear or nonlinear

                                                                                                                                                          ( ) ( )H f x y g x y

                                                                                                                                                          H is said to be a linear operator if

                                                                                                                                                          images1 2 1 2

                                                                                                                                                          1 2

                                                                                                                                                          ( ) ( ) ( ) ( )

                                                                                                                                                          H a f x y b f x y a H f x y b H f x y

                                                                                                                                                          a b f f

                                                                                                                                                          Example of nonlinear operator

                                                                                                                                                          the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                          1 2

                                                                                                                                                          0 2 6 5 1 1

                                                                                                                                                          2 3 4 7f f a b

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          1 2

                                                                                                                                                          0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                          2 3 4 7 2 4a f b f

                                                                                                                                                          0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                          2 3 4 7

                                                                                                                                                          Arithmetic Operations in Image Processing

                                                                                                                                                          Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                          f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                          For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                          value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                          The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                          used in image enhancement)

                                                                                                                                                          1

                                                                                                                                                          1( ) ( )K

                                                                                                                                                          ii

                                                                                                                                                          g x y g x yK

                                                                                                                                                          If the noise satisfies the properties stated above we have

                                                                                                                                                          2 2( ) ( )

                                                                                                                                                          1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                          ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                          and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                          the average image is

                                                                                                                                                          ( ) ( )1

                                                                                                                                                          g x y x yK

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                          the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                          means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                          averaging process increases

                                                                                                                                                          An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                          under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                          virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                          was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                          64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                          images respectively

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                          100 noisy images

                                                                                                                                                          a b c d e f

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                          images

                                                                                                                                                          (a) (b) (c)

                                                                                                                                                          Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                          significant bit of each pixel (c) the difference between the two images

                                                                                                                                                          Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                          Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                          images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                          difference between images (a) and (b)

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                          h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                          intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                          source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                          bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                          anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                          images after injection of the contrast medium

                                                                                                                                                          In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                          Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                          propagates through the various arteries in the area being observed

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          An important application of image multiplication (and division) is shading correction

                                                                                                                                                          Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                          f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                          When the shading function is known

                                                                                                                                                          ( )( )( )

                                                                                                                                                          g x yf x yh x y

                                                                                                                                                          h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                          approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                          sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          (a) (b) (c)

                                                                                                                                                          Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                          operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                          1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                          image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                          (a) (b) (c)

                                                                                                                                                          Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                          min( )mf f f

                                                                                                                                                          0 ( 255)max( )

                                                                                                                                                          ms

                                                                                                                                                          m

                                                                                                                                                          ff K K K

                                                                                                                                                          f

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Spatial Operations

                                                                                                                                                          - are performed directly on the pixels of a given image

                                                                                                                                                          There are three categories of spatial operations

                                                                                                                                                          single-pixel operations

                                                                                                                                                          neighborhood operations

                                                                                                                                                          geometric spatial transformations

                                                                                                                                                          Single-pixel operations

                                                                                                                                                          - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                          where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                          corresponding pixel in the processed image

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Neighborhood operations

                                                                                                                                                          Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                          in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                          based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                          rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                          intensity by computing the average value of the pixels in Sxy

                                                                                                                                                          ( )

                                                                                                                                                          1( ) ( )xyr c S

                                                                                                                                                          g x y f r cm n

                                                                                                                                                          The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                          used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                          largest region of an image

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Geometric spatial transformations and image registration

                                                                                                                                                          - modify the spatial relationship between pixels in an image

                                                                                                                                                          - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                          printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                          predefined set of rules

                                                                                                                                                          A geometric transformation consists of 2 basic operations

                                                                                                                                                          1 a spatial transformation of coordinates

                                                                                                                                                          2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                          pixels

                                                                                                                                                          The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                          (v w) ndash pixel coordinates in the original image

                                                                                                                                                          (x y) ndash pixel coordinates in the transformed image

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                                                                                                                                                          Week 1

                                                                                                                                                          [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                          Affine transform

                                                                                                                                                          11 1211 21 31

                                                                                                                                                          21 2212 22 33

                                                                                                                                                          31 32

                                                                                                                                                          0[ 1] [ 1] [ 1] 0

                                                                                                                                                          1

                                                                                                                                                          t tx t v t w t

                                                                                                                                                          x y v w T v w t ty t v t w t

                                                                                                                                                          t t

                                                                                                                                                          (AT)

                                                                                                                                                          This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                          on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                          result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                          scaling rotation and translation matrices from Table 1

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Affine transformations

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                          the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                          using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                          In practice we can use equation (AT) in two basic ways

                                                                                                                                                          forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                          location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                          Problems

                                                                                                                                                          - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                          the same location in the output image

                                                                                                                                                          - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                          assignment)

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                          computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                          It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                          pixel value

                                                                                                                                                          Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                          numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Image registration ndash align two or more images of the same scene

                                                                                                                                                          In image registration we have available the input and output images but the specific

                                                                                                                                                          transformation that produced the output image from the input is generally unknown

                                                                                                                                                          The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                          images

                                                                                                                                                          - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                          same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                          - align images of a given location taken by the same instrument at different moments

                                                                                                                                                          of time (satellite images)

                                                                                                                                                          Solving the problem using tie points (also called control points) which are

                                                                                                                                                          corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                          image

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          How to select tie points

                                                                                                                                                          - interactively selecting them

                                                                                                                                                          - use of algorithms that try to detect these points

                                                                                                                                                          - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                          the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                          marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                          for establishing tie points

                                                                                                                                                          The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                          of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                          a bilinear approximation is given by

                                                                                                                                                          1 2 3 4

                                                                                                                                                          5 6 7 8

                                                                                                                                                          x c v c w c v w cy c v c w c v w c

                                                                                                                                                          (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                          frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                          subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                          subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                          The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                          problem depend on the severity of the geometrical distortion

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Probabilistic Methods

                                                                                                                                                          zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                          p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                          ( ) kk

                                                                                                                                                          np zM N

                                                                                                                                                          nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                          pixels in the image) 1

                                                                                                                                                          0( ) 1

                                                                                                                                                          L

                                                                                                                                                          kk

                                                                                                                                                          p z

                                                                                                                                                          The mean (average) intensity of an image is given by 1

                                                                                                                                                          0( )

                                                                                                                                                          L

                                                                                                                                                          k kk

                                                                                                                                                          m z p z

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          The variance of the intensities is 1

                                                                                                                                                          2 2

                                                                                                                                                          0( ) ( )

                                                                                                                                                          L

                                                                                                                                                          k kk

                                                                                                                                                          z m p z

                                                                                                                                                          The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                          measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                          ( ) is used

                                                                                                                                                          The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                          0( ) ( ) ( )

                                                                                                                                                          Ln

                                                                                                                                                          n k kk

                                                                                                                                                          z z m p z

                                                                                                                                                          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                          3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                          ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                          mean

                                                                                                                                                          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                          Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                          Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                          Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Intensity Transformations and Spatial Filtering

                                                                                                                                                          ( ) ( )g x y T f x y

                                                                                                                                                          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                          neighborhood of (x y)

                                                                                                                                                          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                          and much smaller in size than the image

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                          called spatial filter (spatial mask kernel template or window)

                                                                                                                                                          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                          ( )s T r

                                                                                                                                                          s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                          Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                          darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                          is called contrast stretching

                                                                                                                                                          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                          thresholding function

                                                                                                                                                          Some Basic Intensity Transformation Functions

                                                                                                                                                          Image Negatives

                                                                                                                                                          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                          - equivalent of a photographic negative

                                                                                                                                                          - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                          image

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Original Negative image

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                          Some basic intensity transformation functions

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                          range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                          while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                          transformation The log functions compress the dynamic range of images with large

                                                                                                                                                          variations in pixel values

                                                                                                                                                          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Power-Law (Gamma) Transformations

                                                                                                                                                          - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                          Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                          of output values with the opposite being true for higher values of input values The

                                                                                                                                                          curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                          1c - identity transformation

                                                                                                                                                          A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                          power law The process used to correct these power-law response phenomena is called

                                                                                                                                                          gamma correction

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Piecewise-Linear Transformations Functions

                                                                                                                                                          Contrast stretching

                                                                                                                                                          - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                          intensity range of the recording tool or display device

                                                                                                                                                          a b c d Fig5

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          11

                                                                                                                                                          1

                                                                                                                                                          2 1 1 21 2

                                                                                                                                                          2 1 2 1

                                                                                                                                                          22

                                                                                                                                                          2

                                                                                                                                                          [0 ]

                                                                                                                                                          ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                          ( 1 ) [ 1]( 1 )

                                                                                                                                                          s r r rrs r r s r rT r r r r

                                                                                                                                                          r r r rs L r r r L

                                                                                                                                                          L r

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                          1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                          Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                          in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                          from their original range to the full range [0 L-1]

                                                                                                                                                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                          2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                          The original image on which these results are based is a scanning electron microscope

                                                                                                                                                          image of pollen magnified approximately 700 times

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Intensity-level slicing

                                                                                                                                                          - highlighting a specific range of intensities in an image

                                                                                                                                                          There are two approaches for intensity-level slicing

                                                                                                                                                          1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                          another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                          intensities in the image (Figure 311 (b))

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                          the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                          Highlights range [A B] and preserves all other intensities

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                          blockageshellip)

                                                                                                                                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                          image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                          Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Bit-plane slicing

                                                                                                                                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                          This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          Digital Image Processing

                                                                                                                                                          Week 1

                                                                                                                                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                          • DIP 1 2017
                                                                                                                                                          • DIP 02 (2017)

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Reducing the number of gray levels 16 8 4 2

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                                            Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                                            Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                                            Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                                            750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                                            same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                                            image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                                            Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                                            Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                                            intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                                            This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                                            straight edges

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                                            where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                                            be written using the 4 nearest neighbors of point (x y)

                                                                                                                                                            Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                                            modest increase in computational effort

                                                                                                                                                            Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                                            nearest neighbors of the point 3 3

                                                                                                                                                            0 0

                                                                                                                                                            ( ) i ji j

                                                                                                                                                            i jv x y c x y

                                                                                                                                                            The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                                            intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                                            0 0

                                                                                                                                                            ( )i ji j

                                                                                                                                                            i jc x y x y

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                                            bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                                            programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                                            Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                                            the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                                            then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                                            neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                                            Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                                            interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                                            from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Neighbors of a Pixel

                                                                                                                                                            A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                                            This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                                            The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                                            and are denoted ND(p)

                                                                                                                                                            The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                                            N8 (p)

                                                                                                                                                            If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                                            fall outside the image

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Adjacency Connectivity Regions Boundaries

                                                                                                                                                            Denote by V the set of intensity levels used to define adjacency

                                                                                                                                                            - in a binary image V 01 (V=0 V=1)

                                                                                                                                                            - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                                            We consider 3 types of adjacency

                                                                                                                                                            (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                                            (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                                            (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                                            m-adjacent if

                                                                                                                                                            4( )q N p or

                                                                                                                                                            ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                                            Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                                            ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            binary image

                                                                                                                                                            0 1 1 0 1 1 0 1 1

                                                                                                                                                            1 0 1 0 0 1 0 0 1 0

                                                                                                                                                            0 0 1 0 0 1 0 0 1

                                                                                                                                                            V

                                                                                                                                                            The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                                            8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                                            m-adjacency

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                            is a sequence of distinct pixels with coordinates

                                                                                                                                                            and are adjacent 0 0 1 1

                                                                                                                                                            1 1

                                                                                                                                                            ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                            n n

                                                                                                                                                            i i i i

                                                                                                                                                            x y x y x y x y s tx y x y i n

                                                                                                                                                            The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                            Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                            Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                            in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                            S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                            Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                            Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                            that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                            8-adjacency are considered

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                            touches the image border

                                                                                                                                                            the complement of 1

                                                                                                                                                            ( )K

                                                                                                                                                            cu k u u

                                                                                                                                                            k

                                                                                                                                                            R R R R

                                                                                                                                                            We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                            background of the image

                                                                                                                                                            The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                            points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                            region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                            inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                            border in the background

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Distance measures

                                                                                                                                                            For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                            function or metric if

                                                                                                                                                            (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                            (b) D(p q) = D(q p)

                                                                                                                                                            (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                            The Euclidean distance between p and q is defined as 1

                                                                                                                                                            2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                            The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                            centered at (x y)

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                            4( ) | | | |D p q x s y t

                                                                                                                                                            The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                            4

                                                                                                                                                            22 1 2

                                                                                                                                                            2 2 1 0 1 22 1 2

                                                                                                                                                            2

                                                                                                                                                            D

                                                                                                                                                            The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                            The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                            8( ) max| | | |D p q x s y t

                                                                                                                                                            The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            8

                                                                                                                                                            2 2 2 2 22 1 1 1 2

                                                                                                                                                            2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                            D

                                                                                                                                                            The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                            D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                            because these distances involve only the coordinates of the point

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Array versus Matrix Operations

                                                                                                                                                            An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                            11 12 11 12

                                                                                                                                                            21 22 21 22

                                                                                                                                                            a a b ba a b b

                                                                                                                                                            Array product

                                                                                                                                                            11 12 11 12 11 11 12 12

                                                                                                                                                            21 22 21 22 21 21 22 21

                                                                                                                                                            a a b b a b a ba a b b a b a b

                                                                                                                                                            Matrix product

                                                                                                                                                            11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                            21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                            a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                            We assume array operations unless stated otherwise

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Linear versus Nonlinear Operations

                                                                                                                                                            One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                            linear or nonlinear

                                                                                                                                                            ( ) ( )H f x y g x y

                                                                                                                                                            H is said to be a linear operator if

                                                                                                                                                            images1 2 1 2

                                                                                                                                                            1 2

                                                                                                                                                            ( ) ( ) ( ) ( )

                                                                                                                                                            H a f x y b f x y a H f x y b H f x y

                                                                                                                                                            a b f f

                                                                                                                                                            Example of nonlinear operator

                                                                                                                                                            the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                            1 2

                                                                                                                                                            0 2 6 5 1 1

                                                                                                                                                            2 3 4 7f f a b

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            1 2

                                                                                                                                                            0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                            2 3 4 7 2 4a f b f

                                                                                                                                                            0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                            2 3 4 7

                                                                                                                                                            Arithmetic Operations in Image Processing

                                                                                                                                                            Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                            f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                            For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                            value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                            The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                            used in image enhancement)

                                                                                                                                                            1

                                                                                                                                                            1( ) ( )K

                                                                                                                                                            ii

                                                                                                                                                            g x y g x yK

                                                                                                                                                            If the noise satisfies the properties stated above we have

                                                                                                                                                            2 2( ) ( )

                                                                                                                                                            1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                            ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                            and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                            the average image is

                                                                                                                                                            ( ) ( )1

                                                                                                                                                            g x y x yK

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                            the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                            means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                            averaging process increases

                                                                                                                                                            An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                            under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                            virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                            was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                            64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                            images respectively

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                            100 noisy images

                                                                                                                                                            a b c d e f

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                            images

                                                                                                                                                            (a) (b) (c)

                                                                                                                                                            Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                            significant bit of each pixel (c) the difference between the two images

                                                                                                                                                            Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                            Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                            images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                            difference between images (a) and (b)

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                            h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                            intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                            source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                            bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                            anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                            images after injection of the contrast medium

                                                                                                                                                            In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                            Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                            propagates through the various arteries in the area being observed

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            An important application of image multiplication (and division) is shading correction

                                                                                                                                                            Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                            f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                            When the shading function is known

                                                                                                                                                            ( )( )( )

                                                                                                                                                            g x yf x yh x y

                                                                                                                                                            h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                            approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                            sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            (a) (b) (c)

                                                                                                                                                            Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                            operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                            1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                            image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                            (a) (b) (c)

                                                                                                                                                            Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                            min( )mf f f

                                                                                                                                                            0 ( 255)max( )

                                                                                                                                                            ms

                                                                                                                                                            m

                                                                                                                                                            ff K K K

                                                                                                                                                            f

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Spatial Operations

                                                                                                                                                            - are performed directly on the pixels of a given image

                                                                                                                                                            There are three categories of spatial operations

                                                                                                                                                            single-pixel operations

                                                                                                                                                            neighborhood operations

                                                                                                                                                            geometric spatial transformations

                                                                                                                                                            Single-pixel operations

                                                                                                                                                            - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                            where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                            corresponding pixel in the processed image

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Neighborhood operations

                                                                                                                                                            Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                            in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                            based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                            rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                            intensity by computing the average value of the pixels in Sxy

                                                                                                                                                            ( )

                                                                                                                                                            1( ) ( )xyr c S

                                                                                                                                                            g x y f r cm n

                                                                                                                                                            The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                            used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                            largest region of an image

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Geometric spatial transformations and image registration

                                                                                                                                                            - modify the spatial relationship between pixels in an image

                                                                                                                                                            - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                            printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                            predefined set of rules

                                                                                                                                                            A geometric transformation consists of 2 basic operations

                                                                                                                                                            1 a spatial transformation of coordinates

                                                                                                                                                            2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                            pixels

                                                                                                                                                            The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                            (v w) ndash pixel coordinates in the original image

                                                                                                                                                            (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                            Affine transform

                                                                                                                                                            11 1211 21 31

                                                                                                                                                            21 2212 22 33

                                                                                                                                                            31 32

                                                                                                                                                            0[ 1] [ 1] [ 1] 0

                                                                                                                                                            1

                                                                                                                                                            t tx t v t w t

                                                                                                                                                            x y v w T v w t ty t v t w t

                                                                                                                                                            t t

                                                                                                                                                            (AT)

                                                                                                                                                            This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                            on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                            result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                            scaling rotation and translation matrices from Table 1

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Affine transformations

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                            the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                            using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                            In practice we can use equation (AT) in two basic ways

                                                                                                                                                            forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                            location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                            Problems

                                                                                                                                                            - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                            the same location in the output image

                                                                                                                                                            - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                            assignment)

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                            computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                            It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                            pixel value

                                                                                                                                                            Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                            numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Image registration ndash align two or more images of the same scene

                                                                                                                                                            In image registration we have available the input and output images but the specific

                                                                                                                                                            transformation that produced the output image from the input is generally unknown

                                                                                                                                                            The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                            images

                                                                                                                                                            - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                            same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                            - align images of a given location taken by the same instrument at different moments

                                                                                                                                                            of time (satellite images)

                                                                                                                                                            Solving the problem using tie points (also called control points) which are

                                                                                                                                                            corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                            image

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            How to select tie points

                                                                                                                                                            - interactively selecting them

                                                                                                                                                            - use of algorithms that try to detect these points

                                                                                                                                                            - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                            the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                            marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                            for establishing tie points

                                                                                                                                                            The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                            of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                            a bilinear approximation is given by

                                                                                                                                                            1 2 3 4

                                                                                                                                                            5 6 7 8

                                                                                                                                                            x c v c w c v w cy c v c w c v w c

                                                                                                                                                            (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                            frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                            subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                            subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                            The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                            problem depend on the severity of the geometrical distortion

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Probabilistic Methods

                                                                                                                                                            zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                            p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                            ( ) kk

                                                                                                                                                            np zM N

                                                                                                                                                            nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                            pixels in the image) 1

                                                                                                                                                            0( ) 1

                                                                                                                                                            L

                                                                                                                                                            kk

                                                                                                                                                            p z

                                                                                                                                                            The mean (average) intensity of an image is given by 1

                                                                                                                                                            0( )

                                                                                                                                                            L

                                                                                                                                                            k kk

                                                                                                                                                            m z p z

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            The variance of the intensities is 1

                                                                                                                                                            2 2

                                                                                                                                                            0( ) ( )

                                                                                                                                                            L

                                                                                                                                                            k kk

                                                                                                                                                            z m p z

                                                                                                                                                            The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                            measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                            ( ) is used

                                                                                                                                                            The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                            0( ) ( ) ( )

                                                                                                                                                            Ln

                                                                                                                                                            n k kk

                                                                                                                                                            z z m p z

                                                                                                                                                            ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                            3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                            ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                            mean

                                                                                                                                                            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                            Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                            Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                            Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Intensity Transformations and Spatial Filtering

                                                                                                                                                            ( ) ( )g x y T f x y

                                                                                                                                                            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                            neighborhood of (x y)

                                                                                                                                                            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                            and much smaller in size than the image

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                            called spatial filter (spatial mask kernel template or window)

                                                                                                                                                            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                            ( )s T r

                                                                                                                                                            s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                            Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                            darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                            is called contrast stretching

                                                                                                                                                            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                            thresholding function

                                                                                                                                                            Some Basic Intensity Transformation Functions

                                                                                                                                                            Image Negatives

                                                                                                                                                            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                            - equivalent of a photographic negative

                                                                                                                                                            - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                            image

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Original Negative image

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                            Some basic intensity transformation functions

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                            range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                            while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                            transformation The log functions compress the dynamic range of images with large

                                                                                                                                                            variations in pixel values

                                                                                                                                                            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Power-Law (Gamma) Transformations

                                                                                                                                                            - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                            Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                            of output values with the opposite being true for higher values of input values The

                                                                                                                                                            curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                            1c - identity transformation

                                                                                                                                                            A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                            power law The process used to correct these power-law response phenomena is called

                                                                                                                                                            gamma correction

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Piecewise-Linear Transformations Functions

                                                                                                                                                            Contrast stretching

                                                                                                                                                            - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                            intensity range of the recording tool or display device

                                                                                                                                                            a b c d Fig5

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            11

                                                                                                                                                            1

                                                                                                                                                            2 1 1 21 2

                                                                                                                                                            2 1 2 1

                                                                                                                                                            22

                                                                                                                                                            2

                                                                                                                                                            [0 ]

                                                                                                                                                            ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                            ( 1 ) [ 1]( 1 )

                                                                                                                                                            s r r rrs r r s r rT r r r r

                                                                                                                                                            r r r rs L r r r L

                                                                                                                                                            L r

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                            1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                            Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                            in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                            from their original range to the full range [0 L-1]

                                                                                                                                                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                            2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                            The original image on which these results are based is a scanning electron microscope

                                                                                                                                                            image of pollen magnified approximately 700 times

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Intensity-level slicing

                                                                                                                                                            - highlighting a specific range of intensities in an image

                                                                                                                                                            There are two approaches for intensity-level slicing

                                                                                                                                                            1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                            another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                            intensities in the image (Figure 311 (b))

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                                                                                                                                                            Week 1

                                                                                                                                                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                            the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                            Highlights range [A B] and preserves all other intensities

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                            blockageshellip)

                                                                                                                                                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                            image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                            Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Bit-plane slicing

                                                                                                                                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                            This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            Digital Image Processing

                                                                                                                                                            Week 1

                                                                                                                                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                            • DIP 1 2017
                                                                                                                                                            • DIP 02 (2017)

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Image Interpolation - used in zooming shrinking rotating and geometric corrections

                                                                                                                                                              Shrinking zooming ndash image resizing ndash image resampling methods

                                                                                                                                                              Interpolation is the process of using known data to estimate values at unknown locations

                                                                                                                                                              Suppose we have an image of size 500 times 500 pixels that has to be enlarged 15 times to

                                                                                                                                                              750times750 pixels One way to do this is to create an imaginary 750 times 750 grid with the

                                                                                                                                                              same spacing as the original and then shrink it so that it fits exactly over the original

                                                                                                                                                              image The pixel spacing in the 750 times 750 grid will be less than in the original image

                                                                                                                                                              Problem assignment of intensity-level in the new 750 times 750 grid

                                                                                                                                                              Nearest neighbor interpolation assign for every point in the new grid (750 times 750) the

                                                                                                                                                              intensity of the closest pixel (nearest neighbor) from the oldoriginal grid (500 times 500)

                                                                                                                                                              This technique has the tendency to produce undesirable effects like severe distortion of

                                                                                                                                                              straight edges

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                                              where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                                              be written using the 4 nearest neighbors of point (x y)

                                                                                                                                                              Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                                              modest increase in computational effort

                                                                                                                                                              Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                                              nearest neighbors of the point 3 3

                                                                                                                                                              0 0

                                                                                                                                                              ( ) i ji j

                                                                                                                                                              i jv x y c x y

                                                                                                                                                              The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                                              intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                                              0 0

                                                                                                                                                              ( )i ji j

                                                                                                                                                              i jc x y x y

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                                              bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                                              programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                                              Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                                              the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                                              then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                                              neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                                              Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                                              interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                                              from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Neighbors of a Pixel

                                                                                                                                                              A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                                              This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                                              The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                                              and are denoted ND(p)

                                                                                                                                                              The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                                              N8 (p)

                                                                                                                                                              If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                                              fall outside the image

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Adjacency Connectivity Regions Boundaries

                                                                                                                                                              Denote by V the set of intensity levels used to define adjacency

                                                                                                                                                              - in a binary image V 01 (V=0 V=1)

                                                                                                                                                              - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                                              We consider 3 types of adjacency

                                                                                                                                                              (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                                              (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                                              (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                                              m-adjacent if

                                                                                                                                                              4( )q N p or

                                                                                                                                                              ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                                              Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                                              ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              binary image

                                                                                                                                                              0 1 1 0 1 1 0 1 1

                                                                                                                                                              1 0 1 0 0 1 0 0 1 0

                                                                                                                                                              0 0 1 0 0 1 0 0 1

                                                                                                                                                              V

                                                                                                                                                              The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                                              8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                                              m-adjacency

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                              is a sequence of distinct pixels with coordinates

                                                                                                                                                              and are adjacent 0 0 1 1

                                                                                                                                                              1 1

                                                                                                                                                              ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                              n n

                                                                                                                                                              i i i i

                                                                                                                                                              x y x y x y x y s tx y x y i n

                                                                                                                                                              The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                              Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                              Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                              in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                              S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                              Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                              Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                              that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                              8-adjacency are considered

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                              touches the image border

                                                                                                                                                              the complement of 1

                                                                                                                                                              ( )K

                                                                                                                                                              cu k u u

                                                                                                                                                              k

                                                                                                                                                              R R R R

                                                                                                                                                              We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                              background of the image

                                                                                                                                                              The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                              points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                              region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                              inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                              border in the background

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Distance measures

                                                                                                                                                              For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                              function or metric if

                                                                                                                                                              (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                              (b) D(p q) = D(q p)

                                                                                                                                                              (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                              The Euclidean distance between p and q is defined as 1

                                                                                                                                                              2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                              The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                              centered at (x y)

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                              4( ) | | | |D p q x s y t

                                                                                                                                                              The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                              4

                                                                                                                                                              22 1 2

                                                                                                                                                              2 2 1 0 1 22 1 2

                                                                                                                                                              2

                                                                                                                                                              D

                                                                                                                                                              The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                              The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                              8( ) max| | | |D p q x s y t

                                                                                                                                                              The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              8

                                                                                                                                                              2 2 2 2 22 1 1 1 2

                                                                                                                                                              2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                              D

                                                                                                                                                              The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                              D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                              because these distances involve only the coordinates of the point

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Array versus Matrix Operations

                                                                                                                                                              An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                              11 12 11 12

                                                                                                                                                              21 22 21 22

                                                                                                                                                              a a b ba a b b

                                                                                                                                                              Array product

                                                                                                                                                              11 12 11 12 11 11 12 12

                                                                                                                                                              21 22 21 22 21 21 22 21

                                                                                                                                                              a a b b a b a ba a b b a b a b

                                                                                                                                                              Matrix product

                                                                                                                                                              11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                              21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                              a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                              We assume array operations unless stated otherwise

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Linear versus Nonlinear Operations

                                                                                                                                                              One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                              linear or nonlinear

                                                                                                                                                              ( ) ( )H f x y g x y

                                                                                                                                                              H is said to be a linear operator if

                                                                                                                                                              images1 2 1 2

                                                                                                                                                              1 2

                                                                                                                                                              ( ) ( ) ( ) ( )

                                                                                                                                                              H a f x y b f x y a H f x y b H f x y

                                                                                                                                                              a b f f

                                                                                                                                                              Example of nonlinear operator

                                                                                                                                                              the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                              1 2

                                                                                                                                                              0 2 6 5 1 1

                                                                                                                                                              2 3 4 7f f a b

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              1 2

                                                                                                                                                              0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                              2 3 4 7 2 4a f b f

                                                                                                                                                              0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                              2 3 4 7

                                                                                                                                                              Arithmetic Operations in Image Processing

                                                                                                                                                              Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                              f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                              For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                              value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                              The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                              used in image enhancement)

                                                                                                                                                              1

                                                                                                                                                              1( ) ( )K

                                                                                                                                                              ii

                                                                                                                                                              g x y g x yK

                                                                                                                                                              If the noise satisfies the properties stated above we have

                                                                                                                                                              2 2( ) ( )

                                                                                                                                                              1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                              ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                              and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                              the average image is

                                                                                                                                                              ( ) ( )1

                                                                                                                                                              g x y x yK

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                              the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                              means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                              averaging process increases

                                                                                                                                                              An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                              under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                              virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                              was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                              64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                              images respectively

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                              100 noisy images

                                                                                                                                                              a b c d e f

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                              images

                                                                                                                                                              (a) (b) (c)

                                                                                                                                                              Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                              significant bit of each pixel (c) the difference between the two images

                                                                                                                                                              Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                              Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                              images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                              difference between images (a) and (b)

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                              h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                              intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                              source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                              bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                              anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                              images after injection of the contrast medium

                                                                                                                                                              In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                              Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                              propagates through the various arteries in the area being observed

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              An important application of image multiplication (and division) is shading correction

                                                                                                                                                              Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                              f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                              When the shading function is known

                                                                                                                                                              ( )( )( )

                                                                                                                                                              g x yf x yh x y

                                                                                                                                                              h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                              approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                              sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              (a) (b) (c)

                                                                                                                                                              Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                              operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                              1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                              image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                              (a) (b) (c)

                                                                                                                                                              Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                              min( )mf f f

                                                                                                                                                              0 ( 255)max( )

                                                                                                                                                              ms

                                                                                                                                                              m

                                                                                                                                                              ff K K K

                                                                                                                                                              f

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Spatial Operations

                                                                                                                                                              - are performed directly on the pixels of a given image

                                                                                                                                                              There are three categories of spatial operations

                                                                                                                                                              single-pixel operations

                                                                                                                                                              neighborhood operations

                                                                                                                                                              geometric spatial transformations

                                                                                                                                                              Single-pixel operations

                                                                                                                                                              - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                              where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                              corresponding pixel in the processed image

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Neighborhood operations

                                                                                                                                                              Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                              in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                              based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                              rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                              intensity by computing the average value of the pixels in Sxy

                                                                                                                                                              ( )

                                                                                                                                                              1( ) ( )xyr c S

                                                                                                                                                              g x y f r cm n

                                                                                                                                                              The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                              used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                              largest region of an image

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Geometric spatial transformations and image registration

                                                                                                                                                              - modify the spatial relationship between pixels in an image

                                                                                                                                                              - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                              printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                              predefined set of rules

                                                                                                                                                              A geometric transformation consists of 2 basic operations

                                                                                                                                                              1 a spatial transformation of coordinates

                                                                                                                                                              2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                              pixels

                                                                                                                                                              The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                              (v w) ndash pixel coordinates in the original image

                                                                                                                                                              (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                              Affine transform

                                                                                                                                                              11 1211 21 31

                                                                                                                                                              21 2212 22 33

                                                                                                                                                              31 32

                                                                                                                                                              0[ 1] [ 1] [ 1] 0

                                                                                                                                                              1

                                                                                                                                                              t tx t v t w t

                                                                                                                                                              x y v w T v w t ty t v t w t

                                                                                                                                                              t t

                                                                                                                                                              (AT)

                                                                                                                                                              This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                              on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                              result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                              scaling rotation and translation matrices from Table 1

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Affine transformations

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                              the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                              using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                              In practice we can use equation (AT) in two basic ways

                                                                                                                                                              forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                              location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                              Problems

                                                                                                                                                              - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                              the same location in the output image

                                                                                                                                                              - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                              assignment)

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                              computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                              It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                              pixel value

                                                                                                                                                              Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                              numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Image registration ndash align two or more images of the same scene

                                                                                                                                                              In image registration we have available the input and output images but the specific

                                                                                                                                                              transformation that produced the output image from the input is generally unknown

                                                                                                                                                              The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                              images

                                                                                                                                                              - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                              same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                              - align images of a given location taken by the same instrument at different moments

                                                                                                                                                              of time (satellite images)

                                                                                                                                                              Solving the problem using tie points (also called control points) which are

                                                                                                                                                              corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                              image

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              How to select tie points

                                                                                                                                                              - interactively selecting them

                                                                                                                                                              - use of algorithms that try to detect these points

                                                                                                                                                              - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                              the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                              marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                              for establishing tie points

                                                                                                                                                              The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                              of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                              a bilinear approximation is given by

                                                                                                                                                              1 2 3 4

                                                                                                                                                              5 6 7 8

                                                                                                                                                              x c v c w c v w cy c v c w c v w c

                                                                                                                                                              (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                              frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                              subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                              subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                              The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                              problem depend on the severity of the geometrical distortion

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Probabilistic Methods

                                                                                                                                                              zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                              p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                              ( ) kk

                                                                                                                                                              np zM N

                                                                                                                                                              nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                              pixels in the image) 1

                                                                                                                                                              0( ) 1

                                                                                                                                                              L

                                                                                                                                                              kk

                                                                                                                                                              p z

                                                                                                                                                              The mean (average) intensity of an image is given by 1

                                                                                                                                                              0( )

                                                                                                                                                              L

                                                                                                                                                              k kk

                                                                                                                                                              m z p z

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              The variance of the intensities is 1

                                                                                                                                                              2 2

                                                                                                                                                              0( ) ( )

                                                                                                                                                              L

                                                                                                                                                              k kk

                                                                                                                                                              z m p z

                                                                                                                                                              The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                              measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                              ( ) is used

                                                                                                                                                              The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                              0( ) ( ) ( )

                                                                                                                                                              Ln

                                                                                                                                                              n k kk

                                                                                                                                                              z z m p z

                                                                                                                                                              ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                              3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                              ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                              mean

                                                                                                                                                              Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                              Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                              Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                              Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Intensity Transformations and Spatial Filtering

                                                                                                                                                              ( ) ( )g x y T f x y

                                                                                                                                                              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                              neighborhood of (x y)

                                                                                                                                                              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                              and much smaller in size than the image

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                              called spatial filter (spatial mask kernel template or window)

                                                                                                                                                              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                              ( )s T r

                                                                                                                                                              s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                              Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                              darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                              is called contrast stretching

                                                                                                                                                              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

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                                                                                                                                                              Week 1

                                                                                                                                                              Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                              thresholding function

                                                                                                                                                              Some Basic Intensity Transformation Functions

                                                                                                                                                              Image Negatives

                                                                                                                                                              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                              - equivalent of a photographic negative

                                                                                                                                                              - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                              image

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Original Negative image

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                              Some basic intensity transformation functions

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                              range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                              while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                              transformation The log functions compress the dynamic range of images with large

                                                                                                                                                              variations in pixel values

                                                                                                                                                              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Power-Law (Gamma) Transformations

                                                                                                                                                              - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                              Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                              of output values with the opposite being true for higher values of input values The

                                                                                                                                                              curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                              1c - identity transformation

                                                                                                                                                              A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                              power law The process used to correct these power-law response phenomena is called

                                                                                                                                                              gamma correction

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Piecewise-Linear Transformations Functions

                                                                                                                                                              Contrast stretching

                                                                                                                                                              - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                              intensity range of the recording tool or display device

                                                                                                                                                              a b c d Fig5

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              11

                                                                                                                                                              1

                                                                                                                                                              2 1 1 21 2

                                                                                                                                                              2 1 2 1

                                                                                                                                                              22

                                                                                                                                                              2

                                                                                                                                                              [0 ]

                                                                                                                                                              ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                              ( 1 ) [ 1]( 1 )

                                                                                                                                                              s r r rrs r r s r rT r r r r

                                                                                                                                                              r r r rs L r r r L

                                                                                                                                                              L r

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                              1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                              Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                              in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                              from their original range to the full range [0 L-1]

                                                                                                                                                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                              2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                              The original image on which these results are based is a scanning electron microscope

                                                                                                                                                              image of pollen magnified approximately 700 times

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Intensity-level slicing

                                                                                                                                                              - highlighting a specific range of intensities in an image

                                                                                                                                                              There are two approaches for intensity-level slicing

                                                                                                                                                              1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                              another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                              intensities in the image (Figure 311 (b))

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                              the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                              Highlights range [A B] and preserves all other intensities

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                              blockageshellip)

                                                                                                                                                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                              image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                              Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Bit-plane slicing

                                                                                                                                                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                              This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              Digital Image Processing

                                                                                                                                                              Week 1

                                                                                                                                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                              • DIP 1 2017
                                                                                                                                                              • DIP 02 (2017)

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Bilinear interpolation ndash assign for the new (x y) location the following intensity ( )v x y a x b y c x y d

                                                                                                                                                                where the four coefficients are determined from the 4 equations in 4 unknowns that can

                                                                                                                                                                be written using the 4 nearest neighbors of point (x y)

                                                                                                                                                                Bilinear interpolation gives much better results than nearest neighbor interpolation with a

                                                                                                                                                                modest increase in computational effort

                                                                                                                                                                Bicubic interpolation ndash assign for the new (x y) location an intensity that involves the 16

                                                                                                                                                                nearest neighbors of the point 3 3

                                                                                                                                                                0 0

                                                                                                                                                                ( ) i ji j

                                                                                                                                                                i jv x y c x y

                                                                                                                                                                The coefficients cij are obtained solving a 16x16 linear system

                                                                                                                                                                intensity levels of the 16 nearest neighbors of 3 3

                                                                                                                                                                0 0

                                                                                                                                                                ( )i ji j

                                                                                                                                                                i jc x y x y

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                                                bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                                                programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                                                Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                                                the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                                                then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                                                neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                                                Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                                                interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                                                from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Neighbors of a Pixel

                                                                                                                                                                A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                                                This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                                                The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                                                and are denoted ND(p)

                                                                                                                                                                The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                                                N8 (p)

                                                                                                                                                                If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                                                fall outside the image

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Adjacency Connectivity Regions Boundaries

                                                                                                                                                                Denote by V the set of intensity levels used to define adjacency

                                                                                                                                                                - in a binary image V 01 (V=0 V=1)

                                                                                                                                                                - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                                                We consider 3 types of adjacency

                                                                                                                                                                (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                                                (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                                                (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                                                m-adjacent if

                                                                                                                                                                4( )q N p or

                                                                                                                                                                ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                                                Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                                                ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                binary image

                                                                                                                                                                0 1 1 0 1 1 0 1 1

                                                                                                                                                                1 0 1 0 0 1 0 0 1 0

                                                                                                                                                                0 0 1 0 0 1 0 0 1

                                                                                                                                                                V

                                                                                                                                                                The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                                                8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                                                m-adjacency

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                                is a sequence of distinct pixels with coordinates

                                                                                                                                                                and are adjacent 0 0 1 1

                                                                                                                                                                1 1

                                                                                                                                                                ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                                n n

                                                                                                                                                                i i i i

                                                                                                                                                                x y x y x y x y s tx y x y i n

                                                                                                                                                                The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                                Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                                Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                                in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                                S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                                Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                                Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                                that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                                8-adjacency are considered

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                                touches the image border

                                                                                                                                                                the complement of 1

                                                                                                                                                                ( )K

                                                                                                                                                                cu k u u

                                                                                                                                                                k

                                                                                                                                                                R R R R

                                                                                                                                                                We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                                background of the image

                                                                                                                                                                The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                                points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                                region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                                inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                                border in the background

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Distance measures

                                                                                                                                                                For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                                function or metric if

                                                                                                                                                                (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                                (b) D(p q) = D(q p)

                                                                                                                                                                (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                                The Euclidean distance between p and q is defined as 1

                                                                                                                                                                2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                                The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                                centered at (x y)

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                                4( ) | | | |D p q x s y t

                                                                                                                                                                The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                                4

                                                                                                                                                                22 1 2

                                                                                                                                                                2 2 1 0 1 22 1 2

                                                                                                                                                                2

                                                                                                                                                                D

                                                                                                                                                                The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                                The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                                8( ) max| | | |D p q x s y t

                                                                                                                                                                The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                8

                                                                                                                                                                2 2 2 2 22 1 1 1 2

                                                                                                                                                                2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                                D

                                                                                                                                                                The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                                D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                                because these distances involve only the coordinates of the point

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Array versus Matrix Operations

                                                                                                                                                                An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                                11 12 11 12

                                                                                                                                                                21 22 21 22

                                                                                                                                                                a a b ba a b b

                                                                                                                                                                Array product

                                                                                                                                                                11 12 11 12 11 11 12 12

                                                                                                                                                                21 22 21 22 21 21 22 21

                                                                                                                                                                a a b b a b a ba a b b a b a b

                                                                                                                                                                Matrix product

                                                                                                                                                                11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                                21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                                a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                                We assume array operations unless stated otherwise

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Linear versus Nonlinear Operations

                                                                                                                                                                One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                                linear or nonlinear

                                                                                                                                                                ( ) ( )H f x y g x y

                                                                                                                                                                H is said to be a linear operator if

                                                                                                                                                                images1 2 1 2

                                                                                                                                                                1 2

                                                                                                                                                                ( ) ( ) ( ) ( )

                                                                                                                                                                H a f x y b f x y a H f x y b H f x y

                                                                                                                                                                a b f f

                                                                                                                                                                Example of nonlinear operator

                                                                                                                                                                the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                                1 2

                                                                                                                                                                0 2 6 5 1 1

                                                                                                                                                                2 3 4 7f f a b

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                1 2

                                                                                                                                                                0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                                2 3 4 7 2 4a f b f

                                                                                                                                                                0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                                2 3 4 7

                                                                                                                                                                Arithmetic Operations in Image Processing

                                                                                                                                                                Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                                f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                                For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                                value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                                The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                used in image enhancement)

                                                                                                                                                                1

                                                                                                                                                                1( ) ( )K

                                                                                                                                                                ii

                                                                                                                                                                g x y g x yK

                                                                                                                                                                If the noise satisfies the properties stated above we have

                                                                                                                                                                2 2( ) ( )

                                                                                                                                                                1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                the average image is

                                                                                                                                                                ( ) ( )1

                                                                                                                                                                g x y x yK

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                averaging process increases

                                                                                                                                                                An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                images respectively

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                100 noisy images

                                                                                                                                                                a b c d e f

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                images

                                                                                                                                                                (a) (b) (c)

                                                                                                                                                                Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                difference between images (a) and (b)

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                images after injection of the contrast medium

                                                                                                                                                                In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                propagates through the various arteries in the area being observed

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                An important application of image multiplication (and division) is shading correction

                                                                                                                                                                Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                When the shading function is known

                                                                                                                                                                ( )( )( )

                                                                                                                                                                g x yf x yh x y

                                                                                                                                                                h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                (a) (b) (c)

                                                                                                                                                                Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                (a) (b) (c)

                                                                                                                                                                Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                min( )mf f f

                                                                                                                                                                0 ( 255)max( )

                                                                                                                                                                ms

                                                                                                                                                                m

                                                                                                                                                                ff K K K

                                                                                                                                                                f

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Spatial Operations

                                                                                                                                                                - are performed directly on the pixels of a given image

                                                                                                                                                                There are three categories of spatial operations

                                                                                                                                                                single-pixel operations

                                                                                                                                                                neighborhood operations

                                                                                                                                                                geometric spatial transformations

                                                                                                                                                                Single-pixel operations

                                                                                                                                                                - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                corresponding pixel in the processed image

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Neighborhood operations

                                                                                                                                                                Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                ( )

                                                                                                                                                                1( ) ( )xyr c S

                                                                                                                                                                g x y f r cm n

                                                                                                                                                                The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                largest region of an image

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Geometric spatial transformations and image registration

                                                                                                                                                                - modify the spatial relationship between pixels in an image

                                                                                                                                                                - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                predefined set of rules

                                                                                                                                                                A geometric transformation consists of 2 basic operations

                                                                                                                                                                1 a spatial transformation of coordinates

                                                                                                                                                                2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                pixels

                                                                                                                                                                The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                (v w) ndash pixel coordinates in the original image

                                                                                                                                                                (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                Affine transform

                                                                                                                                                                11 1211 21 31

                                                                                                                                                                21 2212 22 33

                                                                                                                                                                31 32

                                                                                                                                                                0[ 1] [ 1] [ 1] 0

                                                                                                                                                                1

                                                                                                                                                                t tx t v t w t

                                                                                                                                                                x y v w T v w t ty t v t w t

                                                                                                                                                                t t

                                                                                                                                                                (AT)

                                                                                                                                                                This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                scaling rotation and translation matrices from Table 1

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Affine transformations

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                In practice we can use equation (AT) in two basic ways

                                                                                                                                                                forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                Problems

                                                                                                                                                                - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                the same location in the output image

                                                                                                                                                                - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                assignment)

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                pixel value

                                                                                                                                                                Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Image registration ndash align two or more images of the same scene

                                                                                                                                                                In image registration we have available the input and output images but the specific

                                                                                                                                                                transformation that produced the output image from the input is generally unknown

                                                                                                                                                                The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                images

                                                                                                                                                                - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                of time (satellite images)

                                                                                                                                                                Solving the problem using tie points (also called control points) which are

                                                                                                                                                                corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                image

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                How to select tie points

                                                                                                                                                                - interactively selecting them

                                                                                                                                                                - use of algorithms that try to detect these points

                                                                                                                                                                - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                for establishing tie points

                                                                                                                                                                The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                a bilinear approximation is given by

                                                                                                                                                                1 2 3 4

                                                                                                                                                                5 6 7 8

                                                                                                                                                                x c v c w c v w cy c v c w c v w c

                                                                                                                                                                (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                problem depend on the severity of the geometrical distortion

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Probabilistic Methods

                                                                                                                                                                zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                ( ) kk

                                                                                                                                                                np zM N

                                                                                                                                                                nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                pixels in the image) 1

                                                                                                                                                                0( ) 1

                                                                                                                                                                L

                                                                                                                                                                kk

                                                                                                                                                                p z

                                                                                                                                                                The mean (average) intensity of an image is given by 1

                                                                                                                                                                0( )

                                                                                                                                                                L

                                                                                                                                                                k kk

                                                                                                                                                                m z p z

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                The variance of the intensities is 1

                                                                                                                                                                2 2

                                                                                                                                                                0( ) ( )

                                                                                                                                                                L

                                                                                                                                                                k kk

                                                                                                                                                                z m p z

                                                                                                                                                                The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                ( ) is used

                                                                                                                                                                The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                0( ) ( ) ( )

                                                                                                                                                                Ln

                                                                                                                                                                n k kk

                                                                                                                                                                z z m p z

                                                                                                                                                                ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                mean

                                                                                                                                                                Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Intensity Transformations and Spatial Filtering

                                                                                                                                                                ( ) ( )g x y T f x y

                                                                                                                                                                f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                neighborhood of (x y)

                                                                                                                                                                - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                and much smaller in size than the image

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                ( )s T r

                                                                                                                                                                s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                is called contrast stretching

                                                                                                                                                                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                thresholding function

                                                                                                                                                                Some Basic Intensity Transformation Functions

                                                                                                                                                                Image Negatives

                                                                                                                                                                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                - equivalent of a photographic negative

                                                                                                                                                                - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                image

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Original Negative image

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                Some basic intensity transformation functions

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                variations in pixel values

                                                                                                                                                                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Power-Law (Gamma) Transformations

                                                                                                                                                                - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                of output values with the opposite being true for higher values of input values The

                                                                                                                                                                curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                1c - identity transformation

                                                                                                                                                                A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                gamma correction

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Piecewise-Linear Transformations Functions

                                                                                                                                                                Contrast stretching

                                                                                                                                                                - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                intensity range of the recording tool or display device

                                                                                                                                                                a b c d Fig5

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                11

                                                                                                                                                                1

                                                                                                                                                                2 1 1 21 2

                                                                                                                                                                2 1 2 1

                                                                                                                                                                22

                                                                                                                                                                2

                                                                                                                                                                [0 ]

                                                                                                                                                                ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                ( 1 ) [ 1]( 1 )

                                                                                                                                                                s r r rrs r r s r rT r r r r

                                                                                                                                                                r r r rs L r r r L

                                                                                                                                                                L r

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                from their original range to the full range [0 L-1]

                                                                                                                                                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                image of pollen magnified approximately 700 times

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Intensity-level slicing

                                                                                                                                                                - highlighting a specific range of intensities in an image

                                                                                                                                                                There are two approaches for intensity-level slicing

                                                                                                                                                                1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                intensities in the image (Figure 311 (b))

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                Highlights range [A B] and preserves all other intensities

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                blockageshellip)

                                                                                                                                                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Bit-plane slicing

                                                                                                                                                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                Digital Image Processing

                                                                                                                                                                Week 1

                                                                                                                                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                • DIP 1 2017
                                                                                                                                                                • DIP 02 (2017)

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Generally bicubic interpolation does a better job of preserving fine detail than the

                                                                                                                                                                  bilinear technique Bicubic interpolation is the standard used in commercial image editing

                                                                                                                                                                  programs such as Adobe Photoshop and Corel Photopaint

                                                                                                                                                                  Figure 2 (a) is the same as Fig 1 (d) which was obtained by reducing the resolution of

                                                                                                                                                                  the 1250 dpi in Fig 1(a) to 72 dpi (the size shrank from 3692 times 2812 to 213 times 162) and

                                                                                                                                                                  then zooming the reduced image back to its original size To generate Fig 1(d) nearest

                                                                                                                                                                  neighbor interpolation was used (both for shrinking and zooming)

                                                                                                                                                                  Figures 2(b) and (c) were generated using the same steps but using bilinear and bicubic

                                                                                                                                                                  interpolation respectively Figures 2(d)+(e)+(f) were obtained by reducing the resolution

                                                                                                                                                                  from 1250 dpi to 150 dpi (instead of 72 dpi)

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Neighbors of a Pixel

                                                                                                                                                                  A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                                                  This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                                                  The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                                                  and are denoted ND(p)

                                                                                                                                                                  The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                                                  N8 (p)

                                                                                                                                                                  If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                                                  fall outside the image

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Adjacency Connectivity Regions Boundaries

                                                                                                                                                                  Denote by V the set of intensity levels used to define adjacency

                                                                                                                                                                  - in a binary image V 01 (V=0 V=1)

                                                                                                                                                                  - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                                                  We consider 3 types of adjacency

                                                                                                                                                                  (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                                                  (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                                                  (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                                                  m-adjacent if

                                                                                                                                                                  4( )q N p or

                                                                                                                                                                  ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                                                  Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                                                  ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  binary image

                                                                                                                                                                  0 1 1 0 1 1 0 1 1

                                                                                                                                                                  1 0 1 0 0 1 0 0 1 0

                                                                                                                                                                  0 0 1 0 0 1 0 0 1

                                                                                                                                                                  V

                                                                                                                                                                  The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                                                  8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                                                  m-adjacency

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                                  is a sequence of distinct pixels with coordinates

                                                                                                                                                                  and are adjacent 0 0 1 1

                                                                                                                                                                  1 1

                                                                                                                                                                  ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                                  n n

                                                                                                                                                                  i i i i

                                                                                                                                                                  x y x y x y x y s tx y x y i n

                                                                                                                                                                  The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                                  Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                                  Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                                  in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                                  S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                                  Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                                  Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                                  that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                                  8-adjacency are considered

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                                  touches the image border

                                                                                                                                                                  the complement of 1

                                                                                                                                                                  ( )K

                                                                                                                                                                  cu k u u

                                                                                                                                                                  k

                                                                                                                                                                  R R R R

                                                                                                                                                                  We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                                  background of the image

                                                                                                                                                                  The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                                  points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                                  region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                                  inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                                  border in the background

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Distance measures

                                                                                                                                                                  For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                                  function or metric if

                                                                                                                                                                  (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                                  (b) D(p q) = D(q p)

                                                                                                                                                                  (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                                  The Euclidean distance between p and q is defined as 1

                                                                                                                                                                  2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                                  The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                                  centered at (x y)

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                                  4( ) | | | |D p q x s y t

                                                                                                                                                                  The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                                  4

                                                                                                                                                                  22 1 2

                                                                                                                                                                  2 2 1 0 1 22 1 2

                                                                                                                                                                  2

                                                                                                                                                                  D

                                                                                                                                                                  The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                                  The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                                  8( ) max| | | |D p q x s y t

                                                                                                                                                                  The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  8

                                                                                                                                                                  2 2 2 2 22 1 1 1 2

                                                                                                                                                                  2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                                  D

                                                                                                                                                                  The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                                  D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                                  because these distances involve only the coordinates of the point

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Array versus Matrix Operations

                                                                                                                                                                  An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                                  11 12 11 12

                                                                                                                                                                  21 22 21 22

                                                                                                                                                                  a a b ba a b b

                                                                                                                                                                  Array product

                                                                                                                                                                  11 12 11 12 11 11 12 12

                                                                                                                                                                  21 22 21 22 21 21 22 21

                                                                                                                                                                  a a b b a b a ba a b b a b a b

                                                                                                                                                                  Matrix product

                                                                                                                                                                  11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                                  21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                                  a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                                  We assume array operations unless stated otherwise

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Linear versus Nonlinear Operations

                                                                                                                                                                  One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                                  linear or nonlinear

                                                                                                                                                                  ( ) ( )H f x y g x y

                                                                                                                                                                  H is said to be a linear operator if

                                                                                                                                                                  images1 2 1 2

                                                                                                                                                                  1 2

                                                                                                                                                                  ( ) ( ) ( ) ( )

                                                                                                                                                                  H a f x y b f x y a H f x y b H f x y

                                                                                                                                                                  a b f f

                                                                                                                                                                  Example of nonlinear operator

                                                                                                                                                                  the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                                  1 2

                                                                                                                                                                  0 2 6 5 1 1

                                                                                                                                                                  2 3 4 7f f a b

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  1 2

                                                                                                                                                                  0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                                  2 3 4 7 2 4a f b f

                                                                                                                                                                  0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                                  2 3 4 7

                                                                                                                                                                  Arithmetic Operations in Image Processing

                                                                                                                                                                  Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                                  f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                                  For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                                  value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                                  The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                  used in image enhancement)

                                                                                                                                                                  1

                                                                                                                                                                  1( ) ( )K

                                                                                                                                                                  ii

                                                                                                                                                                  g x y g x yK

                                                                                                                                                                  If the noise satisfies the properties stated above we have

                                                                                                                                                                  2 2( ) ( )

                                                                                                                                                                  1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                  ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                  and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                  the average image is

                                                                                                                                                                  ( ) ( )1

                                                                                                                                                                  g x y x yK

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                                                                                                                                                                  Week 1

                                                                                                                                                                  As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                  the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                  means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                  averaging process increases

                                                                                                                                                                  An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                  under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                  virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                  was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                  64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                  images respectively

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                                                                                                                                                                  Week 1

                                                                                                                                                                  Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                  100 noisy images

                                                                                                                                                                  a b c d e f

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                  images

                                                                                                                                                                  (a) (b) (c)

                                                                                                                                                                  Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                  significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                  Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                  Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                  images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                  difference between images (a) and (b)

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                  h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                  intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                  source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                  bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                  anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                  images after injection of the contrast medium

                                                                                                                                                                  In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                  Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                  propagates through the various arteries in the area being observed

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  An important application of image multiplication (and division) is shading correction

                                                                                                                                                                  Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                  f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                  When the shading function is known

                                                                                                                                                                  ( )( )( )

                                                                                                                                                                  g x yf x yh x y

                                                                                                                                                                  h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                  approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                  sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  (a) (b) (c)

                                                                                                                                                                  Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                  operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                  1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                  image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                  (a) (b) (c)

                                                                                                                                                                  Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                  min( )mf f f

                                                                                                                                                                  0 ( 255)max( )

                                                                                                                                                                  ms

                                                                                                                                                                  m

                                                                                                                                                                  ff K K K

                                                                                                                                                                  f

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Spatial Operations

                                                                                                                                                                  - are performed directly on the pixels of a given image

                                                                                                                                                                  There are three categories of spatial operations

                                                                                                                                                                  single-pixel operations

                                                                                                                                                                  neighborhood operations

                                                                                                                                                                  geometric spatial transformations

                                                                                                                                                                  Single-pixel operations

                                                                                                                                                                  - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                  where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                  corresponding pixel in the processed image

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Neighborhood operations

                                                                                                                                                                  Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                  in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                  based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                  rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                  intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                  ( )

                                                                                                                                                                  1( ) ( )xyr c S

                                                                                                                                                                  g x y f r cm n

                                                                                                                                                                  The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                  used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                  largest region of an image

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Geometric spatial transformations and image registration

                                                                                                                                                                  - modify the spatial relationship between pixels in an image

                                                                                                                                                                  - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                  printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                  predefined set of rules

                                                                                                                                                                  A geometric transformation consists of 2 basic operations

                                                                                                                                                                  1 a spatial transformation of coordinates

                                                                                                                                                                  2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                  pixels

                                                                                                                                                                  The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                  (v w) ndash pixel coordinates in the original image

                                                                                                                                                                  (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                  Affine transform

                                                                                                                                                                  11 1211 21 31

                                                                                                                                                                  21 2212 22 33

                                                                                                                                                                  31 32

                                                                                                                                                                  0[ 1] [ 1] [ 1] 0

                                                                                                                                                                  1

                                                                                                                                                                  t tx t v t w t

                                                                                                                                                                  x y v w T v w t ty t v t w t

                                                                                                                                                                  t t

                                                                                                                                                                  (AT)

                                                                                                                                                                  This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                  on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                  result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                  scaling rotation and translation matrices from Table 1

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Affine transformations

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                  the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                  using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                  In practice we can use equation (AT) in two basic ways

                                                                                                                                                                  forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                  location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                  Problems

                                                                                                                                                                  - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                  the same location in the output image

                                                                                                                                                                  - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                  assignment)

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                  computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                  It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                  pixel value

                                                                                                                                                                  Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                  numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Image registration ndash align two or more images of the same scene

                                                                                                                                                                  In image registration we have available the input and output images but the specific

                                                                                                                                                                  transformation that produced the output image from the input is generally unknown

                                                                                                                                                                  The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                  images

                                                                                                                                                                  - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                  same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                  - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                  of time (satellite images)

                                                                                                                                                                  Solving the problem using tie points (also called control points) which are

                                                                                                                                                                  corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                  image

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  How to select tie points

                                                                                                                                                                  - interactively selecting them

                                                                                                                                                                  - use of algorithms that try to detect these points

                                                                                                                                                                  - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                  the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                  marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                  for establishing tie points

                                                                                                                                                                  The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                  of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                  a bilinear approximation is given by

                                                                                                                                                                  1 2 3 4

                                                                                                                                                                  5 6 7 8

                                                                                                                                                                  x c v c w c v w cy c v c w c v w c

                                                                                                                                                                  (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                  frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                  subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                  subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                  The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                  problem depend on the severity of the geometrical distortion

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Probabilistic Methods

                                                                                                                                                                  zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                  p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                  ( ) kk

                                                                                                                                                                  np zM N

                                                                                                                                                                  nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                  pixels in the image) 1

                                                                                                                                                                  0( ) 1

                                                                                                                                                                  L

                                                                                                                                                                  kk

                                                                                                                                                                  p z

                                                                                                                                                                  The mean (average) intensity of an image is given by 1

                                                                                                                                                                  0( )

                                                                                                                                                                  L

                                                                                                                                                                  k kk

                                                                                                                                                                  m z p z

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                                                                                                                                                                  Week 1

                                                                                                                                                                  The variance of the intensities is 1

                                                                                                                                                                  2 2

                                                                                                                                                                  0( ) ( )

                                                                                                                                                                  L

                                                                                                                                                                  k kk

                                                                                                                                                                  z m p z

                                                                                                                                                                  The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                  measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                  ( ) is used

                                                                                                                                                                  The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                  0( ) ( ) ( )

                                                                                                                                                                  Ln

                                                                                                                                                                  n k kk

                                                                                                                                                                  z z m p z

                                                                                                                                                                  ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                  3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                  ( 3( ) 0z the intensities are biased to values lower than the mean

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                                                                                                                                                                  Week 1

                                                                                                                                                                  3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                  mean

                                                                                                                                                                  Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                  Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                  Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                  Figure 1(c) ndash standard deviation 492 (variance = 24206)

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                                                                                                                                                                  Week 1

                                                                                                                                                                  Intensity Transformations and Spatial Filtering

                                                                                                                                                                  ( ) ( )g x y T f x y

                                                                                                                                                                  f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                  neighborhood of (x y)

                                                                                                                                                                  - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                  and much smaller in size than the image

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                                                                                                                                                                  Week 1

                                                                                                                                                                  - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                  called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                  ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                  ( )s T r

                                                                                                                                                                  s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                  Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                  darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                  is called contrast stretching

                                                                                                                                                                  Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

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                                                                                                                                                                  Week 1

                                                                                                                                                                  Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                  thresholding function

                                                                                                                                                                  Some Basic Intensity Transformation Functions

                                                                                                                                                                  Image Negatives

                                                                                                                                                                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                  - equivalent of a photographic negative

                                                                                                                                                                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                  image

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                                                                                                                                                                  Week 1

                                                                                                                                                                  Original Negative image

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                  Some basic intensity transformation functions

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                  range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                  while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                  transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                  variations in pixel values

                                                                                                                                                                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Power-Law (Gamma) Transformations

                                                                                                                                                                  - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                  Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                  of output values with the opposite being true for higher values of input values The

                                                                                                                                                                  curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                  1c - identity transformation

                                                                                                                                                                  A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                  power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                  gamma correction

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Piecewise-Linear Transformations Functions

                                                                                                                                                                  Contrast stretching

                                                                                                                                                                  - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                  intensity range of the recording tool or display device

                                                                                                                                                                  a b c d Fig5

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  11

                                                                                                                                                                  1

                                                                                                                                                                  2 1 1 21 2

                                                                                                                                                                  2 1 2 1

                                                                                                                                                                  22

                                                                                                                                                                  2

                                                                                                                                                                  [0 ]

                                                                                                                                                                  ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                  ( 1 ) [ 1]( 1 )

                                                                                                                                                                  s r r rrs r r s r rT r r r r

                                                                                                                                                                  r r r rs L r r r L

                                                                                                                                                                  L r

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                  1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                  Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                  in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                  from their original range to the full range [0 L-1]

                                                                                                                                                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                  2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                  The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                  image of pollen magnified approximately 700 times

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Intensity-level slicing

                                                                                                                                                                  - highlighting a specific range of intensities in an image

                                                                                                                                                                  There are two approaches for intensity-level slicing

                                                                                                                                                                  1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                  another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                  intensities in the image (Figure 311 (b))

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                                                                                                                                                                  Week 1

                                                                                                                                                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                  the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                  Highlights range [A B] and preserves all other intensities

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                  blockageshellip)

                                                                                                                                                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                  image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                  Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Bit-plane slicing

                                                                                                                                                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                  This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                  Week 1

                                                                                                                                                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                  • DIP 1 2017
                                                                                                                                                                  • DIP 02 (2017)

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Fig 2 ndash Interpolation examples for zooming and shrinking (nearest neighbor linear bicubic)

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Neighbors of a Pixel

                                                                                                                                                                    A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                                                    This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                                                    The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                                                    and are denoted ND(p)

                                                                                                                                                                    The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                                                    N8 (p)

                                                                                                                                                                    If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                                                    fall outside the image

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Adjacency Connectivity Regions Boundaries

                                                                                                                                                                    Denote by V the set of intensity levels used to define adjacency

                                                                                                                                                                    - in a binary image V 01 (V=0 V=1)

                                                                                                                                                                    - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                                                    We consider 3 types of adjacency

                                                                                                                                                                    (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                                                    (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                                                    (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                                                    m-adjacent if

                                                                                                                                                                    4( )q N p or

                                                                                                                                                                    ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                                                    Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                                                    ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    binary image

                                                                                                                                                                    0 1 1 0 1 1 0 1 1

                                                                                                                                                                    1 0 1 0 0 1 0 0 1 0

                                                                                                                                                                    0 0 1 0 0 1 0 0 1

                                                                                                                                                                    V

                                                                                                                                                                    The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                                                    8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                                                    m-adjacency

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                                    is a sequence of distinct pixels with coordinates

                                                                                                                                                                    and are adjacent 0 0 1 1

                                                                                                                                                                    1 1

                                                                                                                                                                    ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                                    n n

                                                                                                                                                                    i i i i

                                                                                                                                                                    x y x y x y x y s tx y x y i n

                                                                                                                                                                    The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                                    Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                                    Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                                    in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                                    S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                                    Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                                    Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                                    that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                                    8-adjacency are considered

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                                    touches the image border

                                                                                                                                                                    the complement of 1

                                                                                                                                                                    ( )K

                                                                                                                                                                    cu k u u

                                                                                                                                                                    k

                                                                                                                                                                    R R R R

                                                                                                                                                                    We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                                    background of the image

                                                                                                                                                                    The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                                    points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                                    region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                                    inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                                    border in the background

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Distance measures

                                                                                                                                                                    For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                                    function or metric if

                                                                                                                                                                    (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                                    (b) D(p q) = D(q p)

                                                                                                                                                                    (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                                    The Euclidean distance between p and q is defined as 1

                                                                                                                                                                    2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                                    The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                                    centered at (x y)

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                                    4( ) | | | |D p q x s y t

                                                                                                                                                                    The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                                    4

                                                                                                                                                                    22 1 2

                                                                                                                                                                    2 2 1 0 1 22 1 2

                                                                                                                                                                    2

                                                                                                                                                                    D

                                                                                                                                                                    The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                                    The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                                    8( ) max| | | |D p q x s y t

                                                                                                                                                                    The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    8

                                                                                                                                                                    2 2 2 2 22 1 1 1 2

                                                                                                                                                                    2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                                    D

                                                                                                                                                                    The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                                    D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                                    because these distances involve only the coordinates of the point

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Array versus Matrix Operations

                                                                                                                                                                    An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                                    11 12 11 12

                                                                                                                                                                    21 22 21 22

                                                                                                                                                                    a a b ba a b b

                                                                                                                                                                    Array product

                                                                                                                                                                    11 12 11 12 11 11 12 12

                                                                                                                                                                    21 22 21 22 21 21 22 21

                                                                                                                                                                    a a b b a b a ba a b b a b a b

                                                                                                                                                                    Matrix product

                                                                                                                                                                    11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                                    21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                                    a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                                    We assume array operations unless stated otherwise

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Linear versus Nonlinear Operations

                                                                                                                                                                    One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                                    linear or nonlinear

                                                                                                                                                                    ( ) ( )H f x y g x y

                                                                                                                                                                    H is said to be a linear operator if

                                                                                                                                                                    images1 2 1 2

                                                                                                                                                                    1 2

                                                                                                                                                                    ( ) ( ) ( ) ( )

                                                                                                                                                                    H a f x y b f x y a H f x y b H f x y

                                                                                                                                                                    a b f f

                                                                                                                                                                    Example of nonlinear operator

                                                                                                                                                                    the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                                    1 2

                                                                                                                                                                    0 2 6 5 1 1

                                                                                                                                                                    2 3 4 7f f a b

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    1 2

                                                                                                                                                                    0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                                    2 3 4 7 2 4a f b f

                                                                                                                                                                    0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                                    2 3 4 7

                                                                                                                                                                    Arithmetic Operations in Image Processing

                                                                                                                                                                    Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                                    f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                                    For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                                    value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                                    The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                    used in image enhancement)

                                                                                                                                                                    1

                                                                                                                                                                    1( ) ( )K

                                                                                                                                                                    ii

                                                                                                                                                                    g x y g x yK

                                                                                                                                                                    If the noise satisfies the properties stated above we have

                                                                                                                                                                    2 2( ) ( )

                                                                                                                                                                    1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                    ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                    and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                    the average image is

                                                                                                                                                                    ( ) ( )1

                                                                                                                                                                    g x y x yK

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                    the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                    means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                    averaging process increases

                                                                                                                                                                    An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                    under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                    virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                    was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                    64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                    images respectively

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                    100 noisy images

                                                                                                                                                                    a b c d e f

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                    images

                                                                                                                                                                    (a) (b) (c)

                                                                                                                                                                    Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                    significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                    Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                    Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                    images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                    difference between images (a) and (b)

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                    h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                    intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                    source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                    bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                    anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                    images after injection of the contrast medium

                                                                                                                                                                    In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                    Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                    propagates through the various arteries in the area being observed

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    An important application of image multiplication (and division) is shading correction

                                                                                                                                                                    Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                    f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                    When the shading function is known

                                                                                                                                                                    ( )( )( )

                                                                                                                                                                    g x yf x yh x y

                                                                                                                                                                    h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                    approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                    sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    (a) (b) (c)

                                                                                                                                                                    Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                    operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                    1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                    image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                    (a) (b) (c)

                                                                                                                                                                    Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                    min( )mf f f

                                                                                                                                                                    0 ( 255)max( )

                                                                                                                                                                    ms

                                                                                                                                                                    m

                                                                                                                                                                    ff K K K

                                                                                                                                                                    f

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Spatial Operations

                                                                                                                                                                    - are performed directly on the pixels of a given image

                                                                                                                                                                    There are three categories of spatial operations

                                                                                                                                                                    single-pixel operations

                                                                                                                                                                    neighborhood operations

                                                                                                                                                                    geometric spatial transformations

                                                                                                                                                                    Single-pixel operations

                                                                                                                                                                    - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                    where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                    corresponding pixel in the processed image

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Neighborhood operations

                                                                                                                                                                    Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                    in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                    based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                    rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                    intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                    ( )

                                                                                                                                                                    1( ) ( )xyr c S

                                                                                                                                                                    g x y f r cm n

                                                                                                                                                                    The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                    used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                    largest region of an image

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Geometric spatial transformations and image registration

                                                                                                                                                                    - modify the spatial relationship between pixels in an image

                                                                                                                                                                    - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                    printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                    predefined set of rules

                                                                                                                                                                    A geometric transformation consists of 2 basic operations

                                                                                                                                                                    1 a spatial transformation of coordinates

                                                                                                                                                                    2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                    pixels

                                                                                                                                                                    The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                    (v w) ndash pixel coordinates in the original image

                                                                                                                                                                    (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                    Affine transform

                                                                                                                                                                    11 1211 21 31

                                                                                                                                                                    21 2212 22 33

                                                                                                                                                                    31 32

                                                                                                                                                                    0[ 1] [ 1] [ 1] 0

                                                                                                                                                                    1

                                                                                                                                                                    t tx t v t w t

                                                                                                                                                                    x y v w T v w t ty t v t w t

                                                                                                                                                                    t t

                                                                                                                                                                    (AT)

                                                                                                                                                                    This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                    on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                    result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                    scaling rotation and translation matrices from Table 1

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Affine transformations

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                    the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                    using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                    In practice we can use equation (AT) in two basic ways

                                                                                                                                                                    forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                    location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                    Problems

                                                                                                                                                                    - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                    the same location in the output image

                                                                                                                                                                    - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                    assignment)

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                    computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                    It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                    pixel value

                                                                                                                                                                    Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                    numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Image registration ndash align two or more images of the same scene

                                                                                                                                                                    In image registration we have available the input and output images but the specific

                                                                                                                                                                    transformation that produced the output image from the input is generally unknown

                                                                                                                                                                    The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                    images

                                                                                                                                                                    - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                    same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                    - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                    of time (satellite images)

                                                                                                                                                                    Solving the problem using tie points (also called control points) which are

                                                                                                                                                                    corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                    image

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    How to select tie points

                                                                                                                                                                    - interactively selecting them

                                                                                                                                                                    - use of algorithms that try to detect these points

                                                                                                                                                                    - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                    the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                    marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                    for establishing tie points

                                                                                                                                                                    The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                    of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                    a bilinear approximation is given by

                                                                                                                                                                    1 2 3 4

                                                                                                                                                                    5 6 7 8

                                                                                                                                                                    x c v c w c v w cy c v c w c v w c

                                                                                                                                                                    (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                    frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                    subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                    subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                    The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                    problem depend on the severity of the geometrical distortion

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Probabilistic Methods

                                                                                                                                                                    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                    p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                    ( ) kk

                                                                                                                                                                    np zM N

                                                                                                                                                                    nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                    pixels in the image) 1

                                                                                                                                                                    0( ) 1

                                                                                                                                                                    L

                                                                                                                                                                    kk

                                                                                                                                                                    p z

                                                                                                                                                                    The mean (average) intensity of an image is given by 1

                                                                                                                                                                    0( )

                                                                                                                                                                    L

                                                                                                                                                                    k kk

                                                                                                                                                                    m z p z

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    The variance of the intensities is 1

                                                                                                                                                                    2 2

                                                                                                                                                                    0( ) ( )

                                                                                                                                                                    L

                                                                                                                                                                    k kk

                                                                                                                                                                    z m p z

                                                                                                                                                                    The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                    measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                    ( ) is used

                                                                                                                                                                    The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                    0( ) ( ) ( )

                                                                                                                                                                    Ln

                                                                                                                                                                    n k kk

                                                                                                                                                                    z z m p z

                                                                                                                                                                    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                    3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                    ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                    mean

                                                                                                                                                                    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                    Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                    Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                    Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Intensity Transformations and Spatial Filtering

                                                                                                                                                                    ( ) ( )g x y T f x y

                                                                                                                                                                    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                    neighborhood of (x y)

                                                                                                                                                                    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                    and much smaller in size than the image

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                    called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                    ( )s T r

                                                                                                                                                                    s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                    Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                    darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                    is called contrast stretching

                                                                                                                                                                    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                    thresholding function

                                                                                                                                                                    Some Basic Intensity Transformation Functions

                                                                                                                                                                    Image Negatives

                                                                                                                                                                    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                    - equivalent of a photographic negative

                                                                                                                                                                    - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                    image

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Original Negative image

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                    Some basic intensity transformation functions

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                    range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                    while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                    transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                    variations in pixel values

                                                                                                                                                                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Power-Law (Gamma) Transformations

                                                                                                                                                                    - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                    Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                    of output values with the opposite being true for higher values of input values The

                                                                                                                                                                    curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                    1c - identity transformation

                                                                                                                                                                    A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                    power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                    gamma correction

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Piecewise-Linear Transformations Functions

                                                                                                                                                                    Contrast stretching

                                                                                                                                                                    - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                    intensity range of the recording tool or display device

                                                                                                                                                                    a b c d Fig5

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    11

                                                                                                                                                                    1

                                                                                                                                                                    2 1 1 21 2

                                                                                                                                                                    2 1 2 1

                                                                                                                                                                    22

                                                                                                                                                                    2

                                                                                                                                                                    [0 ]

                                                                                                                                                                    ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                    ( 1 ) [ 1]( 1 )

                                                                                                                                                                    s r r rrs r r s r rT r r r r

                                                                                                                                                                    r r r rs L r r r L

                                                                                                                                                                    L r

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                    1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                    Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                    in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                    from their original range to the full range [0 L-1]

                                                                                                                                                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                    2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                    The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                    image of pollen magnified approximately 700 times

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Intensity-level slicing

                                                                                                                                                                    - highlighting a specific range of intensities in an image

                                                                                                                                                                    There are two approaches for intensity-level slicing

                                                                                                                                                                    1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                    another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                    intensities in the image (Figure 311 (b))

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                    the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                    Highlights range [A B] and preserves all other intensities

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                    blockageshellip)

                                                                                                                                                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                    image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                    Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Bit-plane slicing

                                                                                                                                                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                    This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                    Week 1

                                                                                                                                                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                    • DIP 1 2017
                                                                                                                                                                    • DIP 02 (2017)

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Neighbors of a Pixel

                                                                                                                                                                      A pixel p at coordinates (x y) has 4 horizontal and vertical neighbors horizontal vertical( 1 ) ( 1 ) ( 1) ( 1)x y x y x y x y

                                                                                                                                                                      This set of pixels called the 4-neighbors of p denoted by N4 (p)

                                                                                                                                                                      The 4 diagonal neighbors of p have coordinates ( 1 1) ( 1 1) ( 1 1) ( 1 1)x y x y x y x y

                                                                                                                                                                      and are denoted ND(p)

                                                                                                                                                                      The horizontal vertical and diagonal neighbors are called the 8-neighbors of p denoted

                                                                                                                                                                      N8 (p)

                                                                                                                                                                      If (xy) is on the border of the image some of the neighbor locations in ND(p) and N8(p)

                                                                                                                                                                      fall outside the image

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Adjacency Connectivity Regions Boundaries

                                                                                                                                                                      Denote by V the set of intensity levels used to define adjacency

                                                                                                                                                                      - in a binary image V 01 (V=0 V=1)

                                                                                                                                                                      - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                                                      We consider 3 types of adjacency

                                                                                                                                                                      (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                                                      (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                                                      (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                                                      m-adjacent if

                                                                                                                                                                      4( )q N p or

                                                                                                                                                                      ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                                                      Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                                                      ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      binary image

                                                                                                                                                                      0 1 1 0 1 1 0 1 1

                                                                                                                                                                      1 0 1 0 0 1 0 0 1 0

                                                                                                                                                                      0 0 1 0 0 1 0 0 1

                                                                                                                                                                      V

                                                                                                                                                                      The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                                                      8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                                                      m-adjacency

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                                      is a sequence of distinct pixels with coordinates

                                                                                                                                                                      and are adjacent 0 0 1 1

                                                                                                                                                                      1 1

                                                                                                                                                                      ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                                      n n

                                                                                                                                                                      i i i i

                                                                                                                                                                      x y x y x y x y s tx y x y i n

                                                                                                                                                                      The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                                      Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                                      Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                                      in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                                      S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                                      Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                                      Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                                      that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                                      8-adjacency are considered

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                                      touches the image border

                                                                                                                                                                      the complement of 1

                                                                                                                                                                      ( )K

                                                                                                                                                                      cu k u u

                                                                                                                                                                      k

                                                                                                                                                                      R R R R

                                                                                                                                                                      We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                                      background of the image

                                                                                                                                                                      The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                                      points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                                      region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                                      inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                                      border in the background

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Distance measures

                                                                                                                                                                      For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                                      function or metric if

                                                                                                                                                                      (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                                      (b) D(p q) = D(q p)

                                                                                                                                                                      (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                                      The Euclidean distance between p and q is defined as 1

                                                                                                                                                                      2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                                      The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                                      centered at (x y)

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                                      4( ) | | | |D p q x s y t

                                                                                                                                                                      The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                                      4

                                                                                                                                                                      22 1 2

                                                                                                                                                                      2 2 1 0 1 22 1 2

                                                                                                                                                                      2

                                                                                                                                                                      D

                                                                                                                                                                      The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                                      The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                                      8( ) max| | | |D p q x s y t

                                                                                                                                                                      The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      8

                                                                                                                                                                      2 2 2 2 22 1 1 1 2

                                                                                                                                                                      2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                                      D

                                                                                                                                                                      The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                                      D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                                      because these distances involve only the coordinates of the point

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Array versus Matrix Operations

                                                                                                                                                                      An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                                      11 12 11 12

                                                                                                                                                                      21 22 21 22

                                                                                                                                                                      a a b ba a b b

                                                                                                                                                                      Array product

                                                                                                                                                                      11 12 11 12 11 11 12 12

                                                                                                                                                                      21 22 21 22 21 21 22 21

                                                                                                                                                                      a a b b a b a ba a b b a b a b

                                                                                                                                                                      Matrix product

                                                                                                                                                                      11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                                      21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                                      a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                                      We assume array operations unless stated otherwise

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Linear versus Nonlinear Operations

                                                                                                                                                                      One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                                      linear or nonlinear

                                                                                                                                                                      ( ) ( )H f x y g x y

                                                                                                                                                                      H is said to be a linear operator if

                                                                                                                                                                      images1 2 1 2

                                                                                                                                                                      1 2

                                                                                                                                                                      ( ) ( ) ( ) ( )

                                                                                                                                                                      H a f x y b f x y a H f x y b H f x y

                                                                                                                                                                      a b f f

                                                                                                                                                                      Example of nonlinear operator

                                                                                                                                                                      the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                                      1 2

                                                                                                                                                                      0 2 6 5 1 1

                                                                                                                                                                      2 3 4 7f f a b

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      1 2

                                                                                                                                                                      0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                                      2 3 4 7 2 4a f b f

                                                                                                                                                                      0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                                      2 3 4 7

                                                                                                                                                                      Arithmetic Operations in Image Processing

                                                                                                                                                                      Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                                      f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                                      For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                                      value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                                      The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                      used in image enhancement)

                                                                                                                                                                      1

                                                                                                                                                                      1( ) ( )K

                                                                                                                                                                      ii

                                                                                                                                                                      g x y g x yK

                                                                                                                                                                      If the noise satisfies the properties stated above we have

                                                                                                                                                                      2 2( ) ( )

                                                                                                                                                                      1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                      ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                      and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                      the average image is

                                                                                                                                                                      ( ) ( )1

                                                                                                                                                                      g x y x yK

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                      the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                      means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                      averaging process increases

                                                                                                                                                                      An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                      under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                      virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                      was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                      64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                      images respectively

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                      100 noisy images

                                                                                                                                                                      a b c d e f

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                                                                                                                                                                      Week 1

                                                                                                                                                                      A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                      images

                                                                                                                                                                      (a) (b) (c)

                                                                                                                                                                      Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                      significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                      Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                      Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                      images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                      difference between images (a) and (b)

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                      h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                      intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                      source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                      bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                      anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                      images after injection of the contrast medium

                                                                                                                                                                      In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                      Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                      propagates through the various arteries in the area being observed

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      An important application of image multiplication (and division) is shading correction

                                                                                                                                                                      Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                      f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                      When the shading function is known

                                                                                                                                                                      ( )( )( )

                                                                                                                                                                      g x yf x yh x y

                                                                                                                                                                      h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                      approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                      sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      (a) (b) (c)

                                                                                                                                                                      Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                      operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                      1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                      image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                      (a) (b) (c)

                                                                                                                                                                      Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                      min( )mf f f

                                                                                                                                                                      0 ( 255)max( )

                                                                                                                                                                      ms

                                                                                                                                                                      m

                                                                                                                                                                      ff K K K

                                                                                                                                                                      f

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Spatial Operations

                                                                                                                                                                      - are performed directly on the pixels of a given image

                                                                                                                                                                      There are three categories of spatial operations

                                                                                                                                                                      single-pixel operations

                                                                                                                                                                      neighborhood operations

                                                                                                                                                                      geometric spatial transformations

                                                                                                                                                                      Single-pixel operations

                                                                                                                                                                      - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                      where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                      corresponding pixel in the processed image

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Neighborhood operations

                                                                                                                                                                      Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                      in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                      based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                      rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                      intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                      ( )

                                                                                                                                                                      1( ) ( )xyr c S

                                                                                                                                                                      g x y f r cm n

                                                                                                                                                                      The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                      used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                      largest region of an image

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Geometric spatial transformations and image registration

                                                                                                                                                                      - modify the spatial relationship between pixels in an image

                                                                                                                                                                      - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                      printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                      predefined set of rules

                                                                                                                                                                      A geometric transformation consists of 2 basic operations

                                                                                                                                                                      1 a spatial transformation of coordinates

                                                                                                                                                                      2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                      pixels

                                                                                                                                                                      The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                      (v w) ndash pixel coordinates in the original image

                                                                                                                                                                      (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                      Affine transform

                                                                                                                                                                      11 1211 21 31

                                                                                                                                                                      21 2212 22 33

                                                                                                                                                                      31 32

                                                                                                                                                                      0[ 1] [ 1] [ 1] 0

                                                                                                                                                                      1

                                                                                                                                                                      t tx t v t w t

                                                                                                                                                                      x y v w T v w t ty t v t w t

                                                                                                                                                                      t t

                                                                                                                                                                      (AT)

                                                                                                                                                                      This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                      on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                      result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                      scaling rotation and translation matrices from Table 1

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Affine transformations

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                      the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                      using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                      In practice we can use equation (AT) in two basic ways

                                                                                                                                                                      forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                      location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                      Problems

                                                                                                                                                                      - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                      the same location in the output image

                                                                                                                                                                      - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                      assignment)

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                      computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                      It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                      pixel value

                                                                                                                                                                      Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                      numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Image registration ndash align two or more images of the same scene

                                                                                                                                                                      In image registration we have available the input and output images but the specific

                                                                                                                                                                      transformation that produced the output image from the input is generally unknown

                                                                                                                                                                      The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                      images

                                                                                                                                                                      - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                      same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                      - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                      of time (satellite images)

                                                                                                                                                                      Solving the problem using tie points (also called control points) which are

                                                                                                                                                                      corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                      image

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      How to select tie points

                                                                                                                                                                      - interactively selecting them

                                                                                                                                                                      - use of algorithms that try to detect these points

                                                                                                                                                                      - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                      the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                      marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                      for establishing tie points

                                                                                                                                                                      The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                      of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                      a bilinear approximation is given by

                                                                                                                                                                      1 2 3 4

                                                                                                                                                                      5 6 7 8

                                                                                                                                                                      x c v c w c v w cy c v c w c v w c

                                                                                                                                                                      (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                      frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                      subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                      subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                      The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                      problem depend on the severity of the geometrical distortion

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Probabilistic Methods

                                                                                                                                                                      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                      p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                      ( ) kk

                                                                                                                                                                      np zM N

                                                                                                                                                                      nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                      pixels in the image) 1

                                                                                                                                                                      0( ) 1

                                                                                                                                                                      L

                                                                                                                                                                      kk

                                                                                                                                                                      p z

                                                                                                                                                                      The mean (average) intensity of an image is given by 1

                                                                                                                                                                      0( )

                                                                                                                                                                      L

                                                                                                                                                                      k kk

                                                                                                                                                                      m z p z

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      The variance of the intensities is 1

                                                                                                                                                                      2 2

                                                                                                                                                                      0( ) ( )

                                                                                                                                                                      L

                                                                                                                                                                      k kk

                                                                                                                                                                      z m p z

                                                                                                                                                                      The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                      measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                      ( ) is used

                                                                                                                                                                      The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                      0( ) ( ) ( )

                                                                                                                                                                      Ln

                                                                                                                                                                      n k kk

                                                                                                                                                                      z z m p z

                                                                                                                                                                      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                      3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                      ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                      mean

                                                                                                                                                                      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                      Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                      Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                      Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Intensity Transformations and Spatial Filtering

                                                                                                                                                                      ( ) ( )g x y T f x y

                                                                                                                                                                      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                      neighborhood of (x y)

                                                                                                                                                                      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                      and much smaller in size than the image

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                      called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                      ( )s T r

                                                                                                                                                                      s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                      Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                      darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                      is called contrast stretching

                                                                                                                                                                      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                      thresholding function

                                                                                                                                                                      Some Basic Intensity Transformation Functions

                                                                                                                                                                      Image Negatives

                                                                                                                                                                      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                      - equivalent of a photographic negative

                                                                                                                                                                      - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                      image

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Original Negative image

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                      Some basic intensity transformation functions

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                      range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                      while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                      transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                      variations in pixel values

                                                                                                                                                                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Power-Law (Gamma) Transformations

                                                                                                                                                                      - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                      Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                      of output values with the opposite being true for higher values of input values The

                                                                                                                                                                      curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                      1c - identity transformation

                                                                                                                                                                      A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                      power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                      gamma correction

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Piecewise-Linear Transformations Functions

                                                                                                                                                                      Contrast stretching

                                                                                                                                                                      - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                      intensity range of the recording tool or display device

                                                                                                                                                                      a b c d Fig5

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      11

                                                                                                                                                                      1

                                                                                                                                                                      2 1 1 21 2

                                                                                                                                                                      2 1 2 1

                                                                                                                                                                      22

                                                                                                                                                                      2

                                                                                                                                                                      [0 ]

                                                                                                                                                                      ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                      ( 1 ) [ 1]( 1 )

                                                                                                                                                                      s r r rrs r r s r rT r r r r

                                                                                                                                                                      r r r rs L r r r L

                                                                                                                                                                      L r

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                      1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                      Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                      in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                      from their original range to the full range [0 L-1]

                                                                                                                                                                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                      2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                      The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                      image of pollen magnified approximately 700 times

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Intensity-level slicing

                                                                                                                                                                      - highlighting a specific range of intensities in an image

                                                                                                                                                                      There are two approaches for intensity-level slicing

                                                                                                                                                                      1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                      another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                      intensities in the image (Figure 311 (b))

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                      the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                      Highlights range [A B] and preserves all other intensities

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                      blockageshellip)

                                                                                                                                                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                      image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                      Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Bit-plane slicing

                                                                                                                                                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                      This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                      Week 1

                                                                                                                                                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                      • DIP 1 2017
                                                                                                                                                                      • DIP 02 (2017)

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Adjacency Connectivity Regions Boundaries

                                                                                                                                                                        Denote by V the set of intensity levels used to define adjacency

                                                                                                                                                                        - in a binary image V 01 (V=0 V=1)

                                                                                                                                                                        - in a gray-scale image with 256 possible gray-levels V can be any subset of 0255

                                                                                                                                                                        We consider 3 types of adjacency

                                                                                                                                                                        (a) 4-adjacency two pixels p and q with values from V are 4-adjacent if 4( )q N p

                                                                                                                                                                        (b) 8-adjacency two pixels p and q with values from V are 8-adjacent if 8( )q N p

                                                                                                                                                                        (c) m-adjacency (mixed adjacency) two pixels p and q with values from V are

                                                                                                                                                                        m-adjacent if

                                                                                                                                                                        4( )q N p or

                                                                                                                                                                        ( )Dq N p and the set 4 4( ) ( )N p N q has no pixels whose values are from V

                                                                                                                                                                        Mixed adjacency is a modification of 8-adjacency It is introduced to eliminate the

                                                                                                                                                                        ambiguities that often arise when 8-adjacency is used Consider the example

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        binary image

                                                                                                                                                                        0 1 1 0 1 1 0 1 1

                                                                                                                                                                        1 0 1 0 0 1 0 0 1 0

                                                                                                                                                                        0 0 1 0 0 1 0 0 1

                                                                                                                                                                        V

                                                                                                                                                                        The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                                                        8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                                                        m-adjacency

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                                        is a sequence of distinct pixels with coordinates

                                                                                                                                                                        and are adjacent 0 0 1 1

                                                                                                                                                                        1 1

                                                                                                                                                                        ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                                        n n

                                                                                                                                                                        i i i i

                                                                                                                                                                        x y x y x y x y s tx y x y i n

                                                                                                                                                                        The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                                        Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                                        Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                                        in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                                        S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                                        Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                                        Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                                        that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                                        8-adjacency are considered

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                                        touches the image border

                                                                                                                                                                        the complement of 1

                                                                                                                                                                        ( )K

                                                                                                                                                                        cu k u u

                                                                                                                                                                        k

                                                                                                                                                                        R R R R

                                                                                                                                                                        We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                                        background of the image

                                                                                                                                                                        The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                                        points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                                        region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                                        inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                                        border in the background

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Distance measures

                                                                                                                                                                        For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                                        function or metric if

                                                                                                                                                                        (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                                        (b) D(p q) = D(q p)

                                                                                                                                                                        (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                                        The Euclidean distance between p and q is defined as 1

                                                                                                                                                                        2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                                        The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                                        centered at (x y)

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                                        4( ) | | | |D p q x s y t

                                                                                                                                                                        The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                                        4

                                                                                                                                                                        22 1 2

                                                                                                                                                                        2 2 1 0 1 22 1 2

                                                                                                                                                                        2

                                                                                                                                                                        D

                                                                                                                                                                        The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                                        The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                                        8( ) max| | | |D p q x s y t

                                                                                                                                                                        The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        8

                                                                                                                                                                        2 2 2 2 22 1 1 1 2

                                                                                                                                                                        2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                                        D

                                                                                                                                                                        The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                                        D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                                        because these distances involve only the coordinates of the point

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Array versus Matrix Operations

                                                                                                                                                                        An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                                        11 12 11 12

                                                                                                                                                                        21 22 21 22

                                                                                                                                                                        a a b ba a b b

                                                                                                                                                                        Array product

                                                                                                                                                                        11 12 11 12 11 11 12 12

                                                                                                                                                                        21 22 21 22 21 21 22 21

                                                                                                                                                                        a a b b a b a ba a b b a b a b

                                                                                                                                                                        Matrix product

                                                                                                                                                                        11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                                        21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                                        a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                                        We assume array operations unless stated otherwise

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Linear versus Nonlinear Operations

                                                                                                                                                                        One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                                        linear or nonlinear

                                                                                                                                                                        ( ) ( )H f x y g x y

                                                                                                                                                                        H is said to be a linear operator if

                                                                                                                                                                        images1 2 1 2

                                                                                                                                                                        1 2

                                                                                                                                                                        ( ) ( ) ( ) ( )

                                                                                                                                                                        H a f x y b f x y a H f x y b H f x y

                                                                                                                                                                        a b f f

                                                                                                                                                                        Example of nonlinear operator

                                                                                                                                                                        the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                                        1 2

                                                                                                                                                                        0 2 6 5 1 1

                                                                                                                                                                        2 3 4 7f f a b

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        1 2

                                                                                                                                                                        0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                                        2 3 4 7 2 4a f b f

                                                                                                                                                                        0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                                        2 3 4 7

                                                                                                                                                                        Arithmetic Operations in Image Processing

                                                                                                                                                                        Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                                        f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                                        For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                                        value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                                        The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                        used in image enhancement)

                                                                                                                                                                        1

                                                                                                                                                                        1( ) ( )K

                                                                                                                                                                        ii

                                                                                                                                                                        g x y g x yK

                                                                                                                                                                        If the noise satisfies the properties stated above we have

                                                                                                                                                                        2 2( ) ( )

                                                                                                                                                                        1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                        ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                        and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                        the average image is

                                                                                                                                                                        ( ) ( )1

                                                                                                                                                                        g x y x yK

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                        the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                        means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                        averaging process increases

                                                                                                                                                                        An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                        under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                        virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                        was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                        64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                        images respectively

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                        100 noisy images

                                                                                                                                                                        a b c d e f

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                        images

                                                                                                                                                                        (a) (b) (c)

                                                                                                                                                                        Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                        significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                        Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                        Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                        images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                        difference between images (a) and (b)

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                        h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                        intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                        source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                        bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                        anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                        images after injection of the contrast medium

                                                                                                                                                                        In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                        Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                        propagates through the various arteries in the area being observed

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        An important application of image multiplication (and division) is shading correction

                                                                                                                                                                        Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                        f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                        When the shading function is known

                                                                                                                                                                        ( )( )( )

                                                                                                                                                                        g x yf x yh x y

                                                                                                                                                                        h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                        approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                        sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        (a) (b) (c)

                                                                                                                                                                        Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                        operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                        1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                        image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                        (a) (b) (c)

                                                                                                                                                                        Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                        min( )mf f f

                                                                                                                                                                        0 ( 255)max( )

                                                                                                                                                                        ms

                                                                                                                                                                        m

                                                                                                                                                                        ff K K K

                                                                                                                                                                        f

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Spatial Operations

                                                                                                                                                                        - are performed directly on the pixels of a given image

                                                                                                                                                                        There are three categories of spatial operations

                                                                                                                                                                        single-pixel operations

                                                                                                                                                                        neighborhood operations

                                                                                                                                                                        geometric spatial transformations

                                                                                                                                                                        Single-pixel operations

                                                                                                                                                                        - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                        where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                        corresponding pixel in the processed image

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Neighborhood operations

                                                                                                                                                                        Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                        in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                        based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                        rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                        intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                        ( )

                                                                                                                                                                        1( ) ( )xyr c S

                                                                                                                                                                        g x y f r cm n

                                                                                                                                                                        The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                        used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                        largest region of an image

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Geometric spatial transformations and image registration

                                                                                                                                                                        - modify the spatial relationship between pixels in an image

                                                                                                                                                                        - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                        printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                        predefined set of rules

                                                                                                                                                                        A geometric transformation consists of 2 basic operations

                                                                                                                                                                        1 a spatial transformation of coordinates

                                                                                                                                                                        2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                        pixels

                                                                                                                                                                        The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                        (v w) ndash pixel coordinates in the original image

                                                                                                                                                                        (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                        Affine transform

                                                                                                                                                                        11 1211 21 31

                                                                                                                                                                        21 2212 22 33

                                                                                                                                                                        31 32

                                                                                                                                                                        0[ 1] [ 1] [ 1] 0

                                                                                                                                                                        1

                                                                                                                                                                        t tx t v t w t

                                                                                                                                                                        x y v w T v w t ty t v t w t

                                                                                                                                                                        t t

                                                                                                                                                                        (AT)

                                                                                                                                                                        This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                        on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                        result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                        scaling rotation and translation matrices from Table 1

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Affine transformations

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                        the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                        using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                        In practice we can use equation (AT) in two basic ways

                                                                                                                                                                        forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                        location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                        Problems

                                                                                                                                                                        - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                        the same location in the output image

                                                                                                                                                                        - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                        assignment)

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                        computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                        It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                        pixel value

                                                                                                                                                                        Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                        numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Image registration ndash align two or more images of the same scene

                                                                                                                                                                        In image registration we have available the input and output images but the specific

                                                                                                                                                                        transformation that produced the output image from the input is generally unknown

                                                                                                                                                                        The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                        images

                                                                                                                                                                        - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                        same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                        - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                        of time (satellite images)

                                                                                                                                                                        Solving the problem using tie points (also called control points) which are

                                                                                                                                                                        corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                        image

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        How to select tie points

                                                                                                                                                                        - interactively selecting them

                                                                                                                                                                        - use of algorithms that try to detect these points

                                                                                                                                                                        - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                        the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                        marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                        for establishing tie points

                                                                                                                                                                        The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                        of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                        a bilinear approximation is given by

                                                                                                                                                                        1 2 3 4

                                                                                                                                                                        5 6 7 8

                                                                                                                                                                        x c v c w c v w cy c v c w c v w c

                                                                                                                                                                        (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                        frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                        subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                        subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                        The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                        problem depend on the severity of the geometrical distortion

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Probabilistic Methods

                                                                                                                                                                        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                        p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                        ( ) kk

                                                                                                                                                                        np zM N

                                                                                                                                                                        nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                        pixels in the image) 1

                                                                                                                                                                        0( ) 1

                                                                                                                                                                        L

                                                                                                                                                                        kk

                                                                                                                                                                        p z

                                                                                                                                                                        The mean (average) intensity of an image is given by 1

                                                                                                                                                                        0( )

                                                                                                                                                                        L

                                                                                                                                                                        k kk

                                                                                                                                                                        m z p z

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        The variance of the intensities is 1

                                                                                                                                                                        2 2

                                                                                                                                                                        0( ) ( )

                                                                                                                                                                        L

                                                                                                                                                                        k kk

                                                                                                                                                                        z m p z

                                                                                                                                                                        The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                        measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                        ( ) is used

                                                                                                                                                                        The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                        0( ) ( ) ( )

                                                                                                                                                                        Ln

                                                                                                                                                                        n k kk

                                                                                                                                                                        z z m p z

                                                                                                                                                                        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                        3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                        ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                        mean

                                                                                                                                                                        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                        Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                        Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                        Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Intensity Transformations and Spatial Filtering

                                                                                                                                                                        ( ) ( )g x y T f x y

                                                                                                                                                                        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                        neighborhood of (x y)

                                                                                                                                                                        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                        and much smaller in size than the image

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                                                                                                                                                                        Week 1

                                                                                                                                                                        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                        called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                        ( )s T r

                                                                                                                                                                        s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                        Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                        darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                        is called contrast stretching

                                                                                                                                                                        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                        thresholding function

                                                                                                                                                                        Some Basic Intensity Transformation Functions

                                                                                                                                                                        Image Negatives

                                                                                                                                                                        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                        - equivalent of a photographic negative

                                                                                                                                                                        - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                        image

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Original Negative image

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                        Some basic intensity transformation functions

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                        range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                        while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                        transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                        variations in pixel values

                                                                                                                                                                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Power-Law (Gamma) Transformations

                                                                                                                                                                        - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                        Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                        of output values with the opposite being true for higher values of input values The

                                                                                                                                                                        curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                        1c - identity transformation

                                                                                                                                                                        A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                        power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                        gamma correction

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Piecewise-Linear Transformations Functions

                                                                                                                                                                        Contrast stretching

                                                                                                                                                                        - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                        intensity range of the recording tool or display device

                                                                                                                                                                        a b c d Fig5

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        11

                                                                                                                                                                        1

                                                                                                                                                                        2 1 1 21 2

                                                                                                                                                                        2 1 2 1

                                                                                                                                                                        22

                                                                                                                                                                        2

                                                                                                                                                                        [0 ]

                                                                                                                                                                        ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                        ( 1 ) [ 1]( 1 )

                                                                                                                                                                        s r r rrs r r s r rT r r r r

                                                                                                                                                                        r r r rs L r r r L

                                                                                                                                                                        L r

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                        1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                        Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                        in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                        from their original range to the full range [0 L-1]

                                                                                                                                                                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                        2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                        The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                        image of pollen magnified approximately 700 times

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Intensity-level slicing

                                                                                                                                                                        - highlighting a specific range of intensities in an image

                                                                                                                                                                        There are two approaches for intensity-level slicing

                                                                                                                                                                        1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                        another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                        intensities in the image (Figure 311 (b))

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                        the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                        Highlights range [A B] and preserves all other intensities

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                        blockageshellip)

                                                                                                                                                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                        image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                        Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Bit-plane slicing

                                                                                                                                                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                        This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                        Week 1

                                                                                                                                                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                        • DIP 1 2017
                                                                                                                                                                        • DIP 02 (2017)

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          binary image

                                                                                                                                                                          0 1 1 0 1 1 0 1 1

                                                                                                                                                                          1 0 1 0 0 1 0 0 1 0

                                                                                                                                                                          0 0 1 0 0 1 0 0 1

                                                                                                                                                                          V

                                                                                                                                                                          The three pixels at the top (first line) in the above example show multiple (ambiguous)

                                                                                                                                                                          8-adjacency as indicated by the dashed lines This ambiguity is removed by using

                                                                                                                                                                          m-adjacency

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                                          is a sequence of distinct pixels with coordinates

                                                                                                                                                                          and are adjacent 0 0 1 1

                                                                                                                                                                          1 1

                                                                                                                                                                          ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                                          n n

                                                                                                                                                                          i i i i

                                                                                                                                                                          x y x y x y x y s tx y x y i n

                                                                                                                                                                          The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                                          Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                                          Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                                          in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                                          S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                                          Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                                          Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                                          that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                                          8-adjacency are considered

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                                          touches the image border

                                                                                                                                                                          the complement of 1

                                                                                                                                                                          ( )K

                                                                                                                                                                          cu k u u

                                                                                                                                                                          k

                                                                                                                                                                          R R R R

                                                                                                                                                                          We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                                          background of the image

                                                                                                                                                                          The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                                          points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                                          region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                                          inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                                          border in the background

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          Distance measures

                                                                                                                                                                          For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                                          function or metric if

                                                                                                                                                                          (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                                          (b) D(p q) = D(q p)

                                                                                                                                                                          (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                                          The Euclidean distance between p and q is defined as 1

                                                                                                                                                                          2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                                          The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                                          centered at (x y)

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                                          4( ) | | | |D p q x s y t

                                                                                                                                                                          The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                                          4

                                                                                                                                                                          22 1 2

                                                                                                                                                                          2 2 1 0 1 22 1 2

                                                                                                                                                                          2

                                                                                                                                                                          D

                                                                                                                                                                          The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                                          The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                                          8( ) max| | | |D p q x s y t

                                                                                                                                                                          The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          8

                                                                                                                                                                          2 2 2 2 22 1 1 1 2

                                                                                                                                                                          2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                                          D

                                                                                                                                                                          The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                                          D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                                          because these distances involve only the coordinates of the point

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          Array versus Matrix Operations

                                                                                                                                                                          An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                                          11 12 11 12

                                                                                                                                                                          21 22 21 22

                                                                                                                                                                          a a b ba a b b

                                                                                                                                                                          Array product

                                                                                                                                                                          11 12 11 12 11 11 12 12

                                                                                                                                                                          21 22 21 22 21 21 22 21

                                                                                                                                                                          a a b b a b a ba a b b a b a b

                                                                                                                                                                          Matrix product

                                                                                                                                                                          11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                                          21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                                          a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                                          We assume array operations unless stated otherwise

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          Linear versus Nonlinear Operations

                                                                                                                                                                          One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                                          linear or nonlinear

                                                                                                                                                                          ( ) ( )H f x y g x y

                                                                                                                                                                          H is said to be a linear operator if

                                                                                                                                                                          images1 2 1 2

                                                                                                                                                                          1 2

                                                                                                                                                                          ( ) ( ) ( ) ( )

                                                                                                                                                                          H a f x y b f x y a H f x y b H f x y

                                                                                                                                                                          a b f f

                                                                                                                                                                          Example of nonlinear operator

                                                                                                                                                                          the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                                          1 2

                                                                                                                                                                          0 2 6 5 1 1

                                                                                                                                                                          2 3 4 7f f a b

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          1 2

                                                                                                                                                                          0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                                          2 3 4 7 2 4a f b f

                                                                                                                                                                          0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                                          2 3 4 7

                                                                                                                                                                          Arithmetic Operations in Image Processing

                                                                                                                                                                          Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                                          f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                                          For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                                          value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                                          The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                          used in image enhancement)

                                                                                                                                                                          1

                                                                                                                                                                          1( ) ( )K

                                                                                                                                                                          ii

                                                                                                                                                                          g x y g x yK

                                                                                                                                                                          If the noise satisfies the properties stated above we have

                                                                                                                                                                          2 2( ) ( )

                                                                                                                                                                          1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                          ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                          and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                          the average image is

                                                                                                                                                                          ( ) ( )1

                                                                                                                                                                          g x y x yK

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                          the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                          means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                          averaging process increases

                                                                                                                                                                          An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                          under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                          virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                          was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                          64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                          images respectively

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                          100 noisy images

                                                                                                                                                                          a b c d e f

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                          images

                                                                                                                                                                          (a) (b) (c)

                                                                                                                                                                          Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                          significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                          Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                          Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                          images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                          difference between images (a) and (b)

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                          h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                          intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                          source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                          bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                          anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                          images after injection of the contrast medium

                                                                                                                                                                          In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                          Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                          propagates through the various arteries in the area being observed

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          An important application of image multiplication (and division) is shading correction

                                                                                                                                                                          Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                          f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                          When the shading function is known

                                                                                                                                                                          ( )( )( )

                                                                                                                                                                          g x yf x yh x y

                                                                                                                                                                          h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                          approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                          sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          (a) (b) (c)

                                                                                                                                                                          Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                          operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                          1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                          image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                          (a) (b) (c)

                                                                                                                                                                          Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                          min( )mf f f

                                                                                                                                                                          0 ( 255)max( )

                                                                                                                                                                          ms

                                                                                                                                                                          m

                                                                                                                                                                          ff K K K

                                                                                                                                                                          f

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          Spatial Operations

                                                                                                                                                                          - are performed directly on the pixels of a given image

                                                                                                                                                                          There are three categories of spatial operations

                                                                                                                                                                          single-pixel operations

                                                                                                                                                                          neighborhood operations

                                                                                                                                                                          geometric spatial transformations

                                                                                                                                                                          Single-pixel operations

                                                                                                                                                                          - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                          where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                          corresponding pixel in the processed image

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          Neighborhood operations

                                                                                                                                                                          Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                          in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                          based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                          rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                          intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                          ( )

                                                                                                                                                                          1( ) ( )xyr c S

                                                                                                                                                                          g x y f r cm n

                                                                                                                                                                          The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                          used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                          largest region of an image

                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                          Week 1

                                                                                                                                                                          Geometric spatial transformations and image registration

                                                                                                                                                                          - modify the spatial relationship between pixels in an image

                                                                                                                                                                          - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                          printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                          predefined set of rules

                                                                                                                                                                          A geometric transformation consists of 2 basic operations

                                                                                                                                                                          1 a spatial transformation of coordinates

                                                                                                                                                                          2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                          pixels

                                                                                                                                                                          The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                          (v w) ndash pixel coordinates in the original image

                                                                                                                                                                          (x y) ndash pixel coordinates in the transformed image

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                                                                                                                                                                          [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                          Affine transform

                                                                                                                                                                          11 1211 21 31

                                                                                                                                                                          21 2212 22 33

                                                                                                                                                                          31 32

                                                                                                                                                                          0[ 1] [ 1] [ 1] 0

                                                                                                                                                                          1

                                                                                                                                                                          t tx t v t w t

                                                                                                                                                                          x y v w T v w t ty t v t w t

                                                                                                                                                                          t t

                                                                                                                                                                          (AT)

                                                                                                                                                                          This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                          on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                          result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                          scaling rotation and translation matrices from Table 1

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                                                                                                                                                                          Affine transformations

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                                                                                                                                                                          The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                          the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                          using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                          In practice we can use equation (AT) in two basic ways

                                                                                                                                                                          forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                          location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                          Problems

                                                                                                                                                                          - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                          the same location in the output image

                                                                                                                                                                          - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                          assignment)

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                                                                                                                                                                          inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                          computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                          It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                          pixel value

                                                                                                                                                                          Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                          numerous commercial implementations of spatial transformations (MATLAB for ex)

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                                                                                                                                                                          Digital Image Processing

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                                                                                                                                                                          Image registration ndash align two or more images of the same scene

                                                                                                                                                                          In image registration we have available the input and output images but the specific

                                                                                                                                                                          transformation that produced the output image from the input is generally unknown

                                                                                                                                                                          The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                          images

                                                                                                                                                                          - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                          same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                          - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                          of time (satellite images)

                                                                                                                                                                          Solving the problem using tie points (also called control points) which are

                                                                                                                                                                          corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                          image

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                                                                                                                                                                          How to select tie points

                                                                                                                                                                          - interactively selecting them

                                                                                                                                                                          - use of algorithms that try to detect these points

                                                                                                                                                                          - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                          the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                          marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                          for establishing tie points

                                                                                                                                                                          The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                          of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                          a bilinear approximation is given by

                                                                                                                                                                          1 2 3 4

                                                                                                                                                                          5 6 7 8

                                                                                                                                                                          x c v c w c v w cy c v c w c v w c

                                                                                                                                                                          (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

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                                                                                                                                                                          When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                          frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                          subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                          subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                          The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                          problem depend on the severity of the geometrical distortion

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                                                                                                                                                                          a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

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                                                                                                                                                                          Probabilistic Methods

                                                                                                                                                                          zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                          p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                          ( ) kk

                                                                                                                                                                          np zM N

                                                                                                                                                                          nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                          pixels in the image) 1

                                                                                                                                                                          0( ) 1

                                                                                                                                                                          L

                                                                                                                                                                          kk

                                                                                                                                                                          p z

                                                                                                                                                                          The mean (average) intensity of an image is given by 1

                                                                                                                                                                          0( )

                                                                                                                                                                          L

                                                                                                                                                                          k kk

                                                                                                                                                                          m z p z

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                                                                                                                                                                          The variance of the intensities is 1

                                                                                                                                                                          2 2

                                                                                                                                                                          0( ) ( )

                                                                                                                                                                          L

                                                                                                                                                                          k kk

                                                                                                                                                                          z m p z

                                                                                                                                                                          The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                          measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                          ( ) is used

                                                                                                                                                                          The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                          0( ) ( ) ( )

                                                                                                                                                                          Ln

                                                                                                                                                                          n k kk

                                                                                                                                                                          z z m p z

                                                                                                                                                                          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                          3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                          ( 3( ) 0z the intensities are biased to values lower than the mean

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                                                                                                                                                                          3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                          mean

                                                                                                                                                                          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                          Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                          Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                          Figure 1(c) ndash standard deviation 492 (variance = 24206)

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                                                                                                                                                                          Intensity Transformations and Spatial Filtering

                                                                                                                                                                          ( ) ( )g x y T f x y

                                                                                                                                                                          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                          neighborhood of (x y)

                                                                                                                                                                          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                          and much smaller in size than the image

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                                                                                                                                                                          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                          called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                          ( )s T r

                                                                                                                                                                          s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                          Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                          darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                          is called contrast stretching

                                                                                                                                                                          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

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                                                                                                                                                                          Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                          thresholding function

                                                                                                                                                                          Some Basic Intensity Transformation Functions

                                                                                                                                                                          Image Negatives

                                                                                                                                                                          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                          - equivalent of a photographic negative

                                                                                                                                                                          - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                          image

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                                                                                                                                                                          Original Negative image

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                                                                                                                                                                          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                          Some basic intensity transformation functions

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                                                                                                                                                                          This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                          range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                          while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                          transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                          variations in pixel values

                                                                                                                                                                          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

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                                                                                                                                                                          Power-Law (Gamma) Transformations

                                                                                                                                                                          - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                          Plots of gamma transformation for different values of γ (c=1)

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                                                                                                                                                                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                          of output values with the opposite being true for higher values of input values The

                                                                                                                                                                          curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                          1c - identity transformation

                                                                                                                                                                          A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                          power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                          gamma correction

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                                                                                                                                                                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

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                                                                                                                                                                          Piecewise-Linear Transformations Functions

                                                                                                                                                                          Contrast stretching

                                                                                                                                                                          - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                          intensity range of the recording tool or display device

                                                                                                                                                                          a b c d Fig5

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                                                                                                                                                                          11

                                                                                                                                                                          1

                                                                                                                                                                          2 1 1 21 2

                                                                                                                                                                          2 1 2 1

                                                                                                                                                                          22

                                                                                                                                                                          2

                                                                                                                                                                          [0 ]

                                                                                                                                                                          ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                          ( 1 ) [ 1]( 1 )

                                                                                                                                                                          s r r rrs r r s r rT r r r r

                                                                                                                                                                          r r r rs L r r r L

                                                                                                                                                                          L r

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                                                                                                                                                                          1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                          1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                          Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                          in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                          from their original range to the full range [0 L-1]

                                                                                                                                                                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                          2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                          The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                          image of pollen magnified approximately 700 times

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                                                                                                                                                                          Intensity-level slicing

                                                                                                                                                                          - highlighting a specific range of intensities in an image

                                                                                                                                                                          There are two approaches for intensity-level slicing

                                                                                                                                                                          1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                          another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                          intensities in the image (Figure 311 (b))

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                                                                                                                                                                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                          the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                          Highlights range [A B] and preserves all other intensities

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                                                                                                                                                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                          blockageshellip)

                                                                                                                                                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                          image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                          Fig 6 - Aortic angiogram and intensity sliced versions

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                                                                                                                                                                          Bit-plane slicing

                                                                                                                                                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                          This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

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                                                                                                                                                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                          • DIP 1 2017
                                                                                                                                                                          • DIP 02 (2017)

                                                                                                                                                                            Digital Image Processing

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                                                                                                                                                                            A (digital) path (or curve) from pixel p with coordinates (xy) to q with coordinates (st)

                                                                                                                                                                            is a sequence of distinct pixels with coordinates

                                                                                                                                                                            and are adjacent 0 0 1 1

                                                                                                                                                                            1 1

                                                                                                                                                                            ( ) ( ) ( ) ( ) ( )( ) ( ) 12

                                                                                                                                                                            n n

                                                                                                                                                                            i i i i

                                                                                                                                                                            x y x y x y x y s tx y x y i n

                                                                                                                                                                            The length of the path is n If 0 0( ) ( )n nx y x y the path is closed

                                                                                                                                                                            Depending on the type of adjacency considered the paths are 4- 8- or m-paths

                                                                                                                                                                            Let S denote a subset of pixels in an image Two pixels p and q are said to be connected

                                                                                                                                                                            in S if there exists a path between them consisting only of pixels from S

                                                                                                                                                                            S is a connected set if there is a path in S between any 2 pixels in S

                                                                                                                                                                            Let R be a subset of pixels in an image R is a region of the image if R is a connected set

                                                                                                                                                                            Two regions R1 and R2 are said to be adjacent if 1 2R R form a connected set Regions

                                                                                                                                                                            that are not adjacent are said to be disjoint When referring to regions only 4- and

                                                                                                                                                                            8-adjacency are considered

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                                                                                                                                                                            Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                                            touches the image border

                                                                                                                                                                            the complement of 1

                                                                                                                                                                            ( )K

                                                                                                                                                                            cu k u u

                                                                                                                                                                            k

                                                                                                                                                                            R R R R

                                                                                                                                                                            We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                                            background of the image

                                                                                                                                                                            The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                                            points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                                            region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                                            inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                                            border in the background

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                                                                                                                                                                            Distance measures

                                                                                                                                                                            For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                                            function or metric if

                                                                                                                                                                            (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                                            (b) D(p q) = D(q p)

                                                                                                                                                                            (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                                            The Euclidean distance between p and q is defined as 1

                                                                                                                                                                            2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                                            The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                                            centered at (x y)

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                                                                                                                                                                            The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                                            4( ) | | | |D p q x s y t

                                                                                                                                                                            The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                                            4

                                                                                                                                                                            22 1 2

                                                                                                                                                                            2 2 1 0 1 22 1 2

                                                                                                                                                                            2

                                                                                                                                                                            D

                                                                                                                                                                            The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                                            The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                                            8( ) max| | | |D p q x s y t

                                                                                                                                                                            The pixels q for which 8( )D p q r form a square centered at (x y)

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                                                                                                                                                                            8

                                                                                                                                                                            2 2 2 2 22 1 1 1 2

                                                                                                                                                                            2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                                            D

                                                                                                                                                                            The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                                            D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                                            because these distances involve only the coordinates of the point

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                                                                                                                                                                            Array versus Matrix Operations

                                                                                                                                                                            An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                                            11 12 11 12

                                                                                                                                                                            21 22 21 22

                                                                                                                                                                            a a b ba a b b

                                                                                                                                                                            Array product

                                                                                                                                                                            11 12 11 12 11 11 12 12

                                                                                                                                                                            21 22 21 22 21 21 22 21

                                                                                                                                                                            a a b b a b a ba a b b a b a b

                                                                                                                                                                            Matrix product

                                                                                                                                                                            11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                                            21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                                            a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                                            We assume array operations unless stated otherwise

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                                                                                                                                                                            Linear versus Nonlinear Operations

                                                                                                                                                                            One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                                            linear or nonlinear

                                                                                                                                                                            ( ) ( )H f x y g x y

                                                                                                                                                                            H is said to be a linear operator if

                                                                                                                                                                            images1 2 1 2

                                                                                                                                                                            1 2

                                                                                                                                                                            ( ) ( ) ( ) ( )

                                                                                                                                                                            H a f x y b f x y a H f x y b H f x y

                                                                                                                                                                            a b f f

                                                                                                                                                                            Example of nonlinear operator

                                                                                                                                                                            the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                                            1 2

                                                                                                                                                                            0 2 6 5 1 1

                                                                                                                                                                            2 3 4 7f f a b

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                                                                                                                                                                            1 2

                                                                                                                                                                            0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                                            2 3 4 7 2 4a f b f

                                                                                                                                                                            0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                                            2 3 4 7

                                                                                                                                                                            Arithmetic Operations in Image Processing

                                                                                                                                                                            Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                                            f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                                            For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                                            value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                                            The two random variables are uncorrelated when their covariance is 0

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                                                                                                                                                                            Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                            used in image enhancement)

                                                                                                                                                                            1

                                                                                                                                                                            1( ) ( )K

                                                                                                                                                                            ii

                                                                                                                                                                            g x y g x yK

                                                                                                                                                                            If the noise satisfies the properties stated above we have

                                                                                                                                                                            2 2( ) ( )

                                                                                                                                                                            1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                            ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                            and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                            the average image is

                                                                                                                                                                            ( ) ( )1

                                                                                                                                                                            g x y x yK

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                                                                                                                                                                            As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                            the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                            means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                            averaging process increases

                                                                                                                                                                            An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                            under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                            virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                            was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                            64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                            images respectively

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                            100 noisy images

                                                                                                                                                                            a b c d e f

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                            images

                                                                                                                                                                            (a) (b) (c)

                                                                                                                                                                            Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                            significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                            Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                            Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                            images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                            difference between images (a) and (b)

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                            h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                            intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                            source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                            bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                            anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                            images after injection of the contrast medium

                                                                                                                                                                            In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                            Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                            propagates through the various arteries in the area being observed

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            An important application of image multiplication (and division) is shading correction

                                                                                                                                                                            Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                            f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                            When the shading function is known

                                                                                                                                                                            ( )( )( )

                                                                                                                                                                            g x yf x yh x y

                                                                                                                                                                            h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                            approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                            sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            (a) (b) (c)

                                                                                                                                                                            Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                            operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                            1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                            image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                            (a) (b) (c)

                                                                                                                                                                            Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                            min( )mf f f

                                                                                                                                                                            0 ( 255)max( )

                                                                                                                                                                            ms

                                                                                                                                                                            m

                                                                                                                                                                            ff K K K

                                                                                                                                                                            f

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Spatial Operations

                                                                                                                                                                            - are performed directly on the pixels of a given image

                                                                                                                                                                            There are three categories of spatial operations

                                                                                                                                                                            single-pixel operations

                                                                                                                                                                            neighborhood operations

                                                                                                                                                                            geometric spatial transformations

                                                                                                                                                                            Single-pixel operations

                                                                                                                                                                            - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                            where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                            corresponding pixel in the processed image

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Neighborhood operations

                                                                                                                                                                            Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                            in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                            based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                            rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                            intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                            ( )

                                                                                                                                                                            1( ) ( )xyr c S

                                                                                                                                                                            g x y f r cm n

                                                                                                                                                                            The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                            used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                            largest region of an image

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Geometric spatial transformations and image registration

                                                                                                                                                                            - modify the spatial relationship between pixels in an image

                                                                                                                                                                            - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                            printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                            predefined set of rules

                                                                                                                                                                            A geometric transformation consists of 2 basic operations

                                                                                                                                                                            1 a spatial transformation of coordinates

                                                                                                                                                                            2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                            pixels

                                                                                                                                                                            The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                            (v w) ndash pixel coordinates in the original image

                                                                                                                                                                            (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                            Affine transform

                                                                                                                                                                            11 1211 21 31

                                                                                                                                                                            21 2212 22 33

                                                                                                                                                                            31 32

                                                                                                                                                                            0[ 1] [ 1] [ 1] 0

                                                                                                                                                                            1

                                                                                                                                                                            t tx t v t w t

                                                                                                                                                                            x y v w T v w t ty t v t w t

                                                                                                                                                                            t t

                                                                                                                                                                            (AT)

                                                                                                                                                                            This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                            on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                            result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                            scaling rotation and translation matrices from Table 1

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Affine transformations

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                            the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                            using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                            In practice we can use equation (AT) in two basic ways

                                                                                                                                                                            forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                            location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                            Problems

                                                                                                                                                                            - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                            the same location in the output image

                                                                                                                                                                            - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                            assignment)

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                            computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                            It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                            pixel value

                                                                                                                                                                            Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                            numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Image registration ndash align two or more images of the same scene

                                                                                                                                                                            In image registration we have available the input and output images but the specific

                                                                                                                                                                            transformation that produced the output image from the input is generally unknown

                                                                                                                                                                            The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                            images

                                                                                                                                                                            - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                            same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                            - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                            of time (satellite images)

                                                                                                                                                                            Solving the problem using tie points (also called control points) which are

                                                                                                                                                                            corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                            image

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            How to select tie points

                                                                                                                                                                            - interactively selecting them

                                                                                                                                                                            - use of algorithms that try to detect these points

                                                                                                                                                                            - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                            the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                            marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                            for establishing tie points

                                                                                                                                                                            The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                            of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                            a bilinear approximation is given by

                                                                                                                                                                            1 2 3 4

                                                                                                                                                                            5 6 7 8

                                                                                                                                                                            x c v c w c v w cy c v c w c v w c

                                                                                                                                                                            (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                            frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                            subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                            subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                            The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                            problem depend on the severity of the geometrical distortion

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Probabilistic Methods

                                                                                                                                                                            zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                            p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                            ( ) kk

                                                                                                                                                                            np zM N

                                                                                                                                                                            nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                            pixels in the image) 1

                                                                                                                                                                            0( ) 1

                                                                                                                                                                            L

                                                                                                                                                                            kk

                                                                                                                                                                            p z

                                                                                                                                                                            The mean (average) intensity of an image is given by 1

                                                                                                                                                                            0( )

                                                                                                                                                                            L

                                                                                                                                                                            k kk

                                                                                                                                                                            m z p z

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            The variance of the intensities is 1

                                                                                                                                                                            2 2

                                                                                                                                                                            0( ) ( )

                                                                                                                                                                            L

                                                                                                                                                                            k kk

                                                                                                                                                                            z m p z

                                                                                                                                                                            The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                            measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                            ( ) is used

                                                                                                                                                                            The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                            0( ) ( ) ( )

                                                                                                                                                                            Ln

                                                                                                                                                                            n k kk

                                                                                                                                                                            z z m p z

                                                                                                                                                                            ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                            3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                            ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                            mean

                                                                                                                                                                            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                            Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                            Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                            Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Intensity Transformations and Spatial Filtering

                                                                                                                                                                            ( ) ( )g x y T f x y

                                                                                                                                                                            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                            neighborhood of (x y)

                                                                                                                                                                            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                            and much smaller in size than the image

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                            called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                            ( )s T r

                                                                                                                                                                            s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                            Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                            darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                            is called contrast stretching

                                                                                                                                                                            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                            thresholding function

                                                                                                                                                                            Some Basic Intensity Transformation Functions

                                                                                                                                                                            Image Negatives

                                                                                                                                                                            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                            - equivalent of a photographic negative

                                                                                                                                                                            - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                            image

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Original Negative image

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                            Some basic intensity transformation functions

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                            range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                            while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                            transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                            variations in pixel values

                                                                                                                                                                            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Power-Law (Gamma) Transformations

                                                                                                                                                                            - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                            Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                            of output values with the opposite being true for higher values of input values The

                                                                                                                                                                            curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                            1c - identity transformation

                                                                                                                                                                            A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                            power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                            gamma correction

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Piecewise-Linear Transformations Functions

                                                                                                                                                                            Contrast stretching

                                                                                                                                                                            - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                            intensity range of the recording tool or display device

                                                                                                                                                                            a b c d Fig5

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            11

                                                                                                                                                                            1

                                                                                                                                                                            2 1 1 21 2

                                                                                                                                                                            2 1 2 1

                                                                                                                                                                            22

                                                                                                                                                                            2

                                                                                                                                                                            [0 ]

                                                                                                                                                                            ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                            ( 1 ) [ 1]( 1 )

                                                                                                                                                                            s r r rrs r r s r rT r r r r

                                                                                                                                                                            r r r rs L r r r L

                                                                                                                                                                            L r

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                            1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                            Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                            in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                            from their original range to the full range [0 L-1]

                                                                                                                                                                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                            2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                            The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                            image of pollen magnified approximately 700 times

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Intensity-level slicing

                                                                                                                                                                            - highlighting a specific range of intensities in an image

                                                                                                                                                                            There are two approaches for intensity-level slicing

                                                                                                                                                                            1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                            another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                            intensities in the image (Figure 311 (b))

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                            the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                            Highlights range [A B] and preserves all other intensities

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                            blockageshellip)

                                                                                                                                                                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                            image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                            Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Bit-plane slicing

                                                                                                                                                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                            This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                            Week 1

                                                                                                                                                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                            • DIP 1 2017
                                                                                                                                                                            • DIP 02 (2017)

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Suppose that an image contains K disjoint regions 1 kR k K none of which

                                                                                                                                                                              touches the image border

                                                                                                                                                                              the complement of 1

                                                                                                                                                                              ( )K

                                                                                                                                                                              cu k u u

                                                                                                                                                                              k

                                                                                                                                                                              R R R R

                                                                                                                                                                              We call all the points in Ru the foreground of the image and the points in ( )cuR the

                                                                                                                                                                              background of the image

                                                                                                                                                                              The boundary (border or contour) of a region R is the set of points that are adjacent to

                                                                                                                                                                              points in the complement of R (R)c The border of an image is the set of pixels in the

                                                                                                                                                                              region that have at least one background neighbor This definition is referred to as the

                                                                                                                                                                              inner border to distinguish it from the notion of outer border which is the corresponding

                                                                                                                                                                              border in the background

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Distance measures

                                                                                                                                                                              For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                                              function or metric if

                                                                                                                                                                              (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                                              (b) D(p q) = D(q p)

                                                                                                                                                                              (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                                              The Euclidean distance between p and q is defined as 1

                                                                                                                                                                              2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                                              The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                                              centered at (x y)

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                                              4( ) | | | |D p q x s y t

                                                                                                                                                                              The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                                              4

                                                                                                                                                                              22 1 2

                                                                                                                                                                              2 2 1 0 1 22 1 2

                                                                                                                                                                              2

                                                                                                                                                                              D

                                                                                                                                                                              The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                                              The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                                              8( ) max| | | |D p q x s y t

                                                                                                                                                                              The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              8

                                                                                                                                                                              2 2 2 2 22 1 1 1 2

                                                                                                                                                                              2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                                              D

                                                                                                                                                                              The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                                              D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                                              because these distances involve only the coordinates of the point

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Array versus Matrix Operations

                                                                                                                                                                              An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                                              11 12 11 12

                                                                                                                                                                              21 22 21 22

                                                                                                                                                                              a a b ba a b b

                                                                                                                                                                              Array product

                                                                                                                                                                              11 12 11 12 11 11 12 12

                                                                                                                                                                              21 22 21 22 21 21 22 21

                                                                                                                                                                              a a b b a b a ba a b b a b a b

                                                                                                                                                                              Matrix product

                                                                                                                                                                              11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                                              21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                                              a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                                              We assume array operations unless stated otherwise

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Linear versus Nonlinear Operations

                                                                                                                                                                              One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                                              linear or nonlinear

                                                                                                                                                                              ( ) ( )H f x y g x y

                                                                                                                                                                              H is said to be a linear operator if

                                                                                                                                                                              images1 2 1 2

                                                                                                                                                                              1 2

                                                                                                                                                                              ( ) ( ) ( ) ( )

                                                                                                                                                                              H a f x y b f x y a H f x y b H f x y

                                                                                                                                                                              a b f f

                                                                                                                                                                              Example of nonlinear operator

                                                                                                                                                                              the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                                              1 2

                                                                                                                                                                              0 2 6 5 1 1

                                                                                                                                                                              2 3 4 7f f a b

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              1 2

                                                                                                                                                                              0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                                              2 3 4 7 2 4a f b f

                                                                                                                                                                              0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                                              2 3 4 7

                                                                                                                                                                              Arithmetic Operations in Image Processing

                                                                                                                                                                              Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                                              f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                                              For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                                              value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                                              The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                              used in image enhancement)

                                                                                                                                                                              1

                                                                                                                                                                              1( ) ( )K

                                                                                                                                                                              ii

                                                                                                                                                                              g x y g x yK

                                                                                                                                                                              If the noise satisfies the properties stated above we have

                                                                                                                                                                              2 2( ) ( )

                                                                                                                                                                              1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                              ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                              and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                              the average image is

                                                                                                                                                                              ( ) ( )1

                                                                                                                                                                              g x y x yK

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                              the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                              means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                              averaging process increases

                                                                                                                                                                              An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                              under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                              virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                              was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                              64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                              images respectively

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                                                                                                                                                                              Week 1

                                                                                                                                                                              Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                              100 noisy images

                                                                                                                                                                              a b c d e f

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                                                                                                                                                                              Week 1

                                                                                                                                                                              A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                              images

                                                                                                                                                                              (a) (b) (c)

                                                                                                                                                                              Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                              significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                              Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                              Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                              images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                              difference between images (a) and (b)

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                              h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                              intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                              source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                              bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                              anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                              images after injection of the contrast medium

                                                                                                                                                                              In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                              Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                              propagates through the various arteries in the area being observed

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                                                                                                                                                                              Week 1

                                                                                                                                                                              a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              An important application of image multiplication (and division) is shading correction

                                                                                                                                                                              Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                              f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                              When the shading function is known

                                                                                                                                                                              ( )( )( )

                                                                                                                                                                              g x yf x yh x y

                                                                                                                                                                              h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                              approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                              sensor is not available often the shading pattern can be estimated from the image

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                                                                                                                                                                              Week 1

                                                                                                                                                                              (a) (b) (c)

                                                                                                                                                                              Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                              operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                              1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                              image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                              (a) (b) (c)

                                                                                                                                                                              Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                              min( )mf f f

                                                                                                                                                                              0 ( 255)max( )

                                                                                                                                                                              ms

                                                                                                                                                                              m

                                                                                                                                                                              ff K K K

                                                                                                                                                                              f

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Spatial Operations

                                                                                                                                                                              - are performed directly on the pixels of a given image

                                                                                                                                                                              There are three categories of spatial operations

                                                                                                                                                                              single-pixel operations

                                                                                                                                                                              neighborhood operations

                                                                                                                                                                              geometric spatial transformations

                                                                                                                                                                              Single-pixel operations

                                                                                                                                                                              - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                              where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                              corresponding pixel in the processed image

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Neighborhood operations

                                                                                                                                                                              Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                              in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                              based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                              rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                              intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                              ( )

                                                                                                                                                                              1( ) ( )xyr c S

                                                                                                                                                                              g x y f r cm n

                                                                                                                                                                              The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                              used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                              largest region of an image

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Geometric spatial transformations and image registration

                                                                                                                                                                              - modify the spatial relationship between pixels in an image

                                                                                                                                                                              - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                              printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                              predefined set of rules

                                                                                                                                                                              A geometric transformation consists of 2 basic operations

                                                                                                                                                                              1 a spatial transformation of coordinates

                                                                                                                                                                              2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                              pixels

                                                                                                                                                                              The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                              (v w) ndash pixel coordinates in the original image

                                                                                                                                                                              (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                              Affine transform

                                                                                                                                                                              11 1211 21 31

                                                                                                                                                                              21 2212 22 33

                                                                                                                                                                              31 32

                                                                                                                                                                              0[ 1] [ 1] [ 1] 0

                                                                                                                                                                              1

                                                                                                                                                                              t tx t v t w t

                                                                                                                                                                              x y v w T v w t ty t v t w t

                                                                                                                                                                              t t

                                                                                                                                                                              (AT)

                                                                                                                                                                              This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                              on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                              result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                              scaling rotation and translation matrices from Table 1

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Affine transformations

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                              the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                              using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                              In practice we can use equation (AT) in two basic ways

                                                                                                                                                                              forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                              location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                              Problems

                                                                                                                                                                              - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                              the same location in the output image

                                                                                                                                                                              - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                              assignment)

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                              computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                              It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                              pixel value

                                                                                                                                                                              Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                              numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Image registration ndash align two or more images of the same scene

                                                                                                                                                                              In image registration we have available the input and output images but the specific

                                                                                                                                                                              transformation that produced the output image from the input is generally unknown

                                                                                                                                                                              The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                              images

                                                                                                                                                                              - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                              same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                              - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                              of time (satellite images)

                                                                                                                                                                              Solving the problem using tie points (also called control points) which are

                                                                                                                                                                              corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                              image

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              How to select tie points

                                                                                                                                                                              - interactively selecting them

                                                                                                                                                                              - use of algorithms that try to detect these points

                                                                                                                                                                              - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                              the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                              marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                              for establishing tie points

                                                                                                                                                                              The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                              of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                              a bilinear approximation is given by

                                                                                                                                                                              1 2 3 4

                                                                                                                                                                              5 6 7 8

                                                                                                                                                                              x c v c w c v w cy c v c w c v w c

                                                                                                                                                                              (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                              frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                              subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                              subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                              The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                              problem depend on the severity of the geometrical distortion

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Probabilistic Methods

                                                                                                                                                                              zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                              p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                              ( ) kk

                                                                                                                                                                              np zM N

                                                                                                                                                                              nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                              pixels in the image) 1

                                                                                                                                                                              0( ) 1

                                                                                                                                                                              L

                                                                                                                                                                              kk

                                                                                                                                                                              p z

                                                                                                                                                                              The mean (average) intensity of an image is given by 1

                                                                                                                                                                              0( )

                                                                                                                                                                              L

                                                                                                                                                                              k kk

                                                                                                                                                                              m z p z

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              The variance of the intensities is 1

                                                                                                                                                                              2 2

                                                                                                                                                                              0( ) ( )

                                                                                                                                                                              L

                                                                                                                                                                              k kk

                                                                                                                                                                              z m p z

                                                                                                                                                                              The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                              measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                              ( ) is used

                                                                                                                                                                              The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                              0( ) ( ) ( )

                                                                                                                                                                              Ln

                                                                                                                                                                              n k kk

                                                                                                                                                                              z z m p z

                                                                                                                                                                              ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                              3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                              ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                              mean

                                                                                                                                                                              Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                              Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                              Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                              Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Intensity Transformations and Spatial Filtering

                                                                                                                                                                              ( ) ( )g x y T f x y

                                                                                                                                                                              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                              neighborhood of (x y)

                                                                                                                                                                              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                              and much smaller in size than the image

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                              called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                              ( )s T r

                                                                                                                                                                              s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                              Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                              darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                              is called contrast stretching

                                                                                                                                                                              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                              thresholding function

                                                                                                                                                                              Some Basic Intensity Transformation Functions

                                                                                                                                                                              Image Negatives

                                                                                                                                                                              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                              - equivalent of a photographic negative

                                                                                                                                                                              - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                              image

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Original Negative image

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                              Some basic intensity transformation functions

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                              range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                              while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                              transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                              variations in pixel values

                                                                                                                                                                              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Power-Law (Gamma) Transformations

                                                                                                                                                                              - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                              Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                              of output values with the opposite being true for higher values of input values The

                                                                                                                                                                              curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                              1c - identity transformation

                                                                                                                                                                              A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                              power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                              gamma correction

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Piecewise-Linear Transformations Functions

                                                                                                                                                                              Contrast stretching

                                                                                                                                                                              - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                              intensity range of the recording tool or display device

                                                                                                                                                                              a b c d Fig5

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              11

                                                                                                                                                                              1

                                                                                                                                                                              2 1 1 21 2

                                                                                                                                                                              2 1 2 1

                                                                                                                                                                              22

                                                                                                                                                                              2

                                                                                                                                                                              [0 ]

                                                                                                                                                                              ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                              ( 1 ) [ 1]( 1 )

                                                                                                                                                                              s r r rrs r r s r rT r r r r

                                                                                                                                                                              r r r rs L r r r L

                                                                                                                                                                              L r

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                              1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                              Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                              in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                              from their original range to the full range [0 L-1]

                                                                                                                                                                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                              2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                              The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                              image of pollen magnified approximately 700 times

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Intensity-level slicing

                                                                                                                                                                              - highlighting a specific range of intensities in an image

                                                                                                                                                                              There are two approaches for intensity-level slicing

                                                                                                                                                                              1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                              another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                              intensities in the image (Figure 311 (b))

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                              the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                              Highlights range [A B] and preserves all other intensities

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                              blockageshellip)

                                                                                                                                                                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                              image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                              Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Bit-plane slicing

                                                                                                                                                                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                              This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                              Week 1

                                                                                                                                                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                              • DIP 1 2017
                                                                                                                                                                              • DIP 02 (2017)

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Distance measures

                                                                                                                                                                                For pixels p q and z with coordinates (xy) (st) and (vw) respectively D is a distance

                                                                                                                                                                                function or metric if

                                                                                                                                                                                (a) D(p q) ge 0 D(p q) = 0 iff p=q

                                                                                                                                                                                (b) D(p q) = D(q p)

                                                                                                                                                                                (c) D(p z) le D(p q) + D(q z)

                                                                                                                                                                                The Euclidean distance between p and q is defined as 1

                                                                                                                                                                                2 2 2 22( ) ( ) ( ) ( ) ( )eD p q x s y t x s y t

                                                                                                                                                                                The pixels q for which ( )eD p q r are the points contained in a disk of radius r

                                                                                                                                                                                centered at (x y)

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                                                4( ) | | | |D p q x s y t

                                                                                                                                                                                The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                                                4

                                                                                                                                                                                22 1 2

                                                                                                                                                                                2 2 1 0 1 22 1 2

                                                                                                                                                                                2

                                                                                                                                                                                D

                                                                                                                                                                                The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                                                The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                                                8( ) max| | | |D p q x s y t

                                                                                                                                                                                The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                8

                                                                                                                                                                                2 2 2 2 22 1 1 1 2

                                                                                                                                                                                2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                                                D

                                                                                                                                                                                The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                                                D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                                                because these distances involve only the coordinates of the point

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Array versus Matrix Operations

                                                                                                                                                                                An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                                                11 12 11 12

                                                                                                                                                                                21 22 21 22

                                                                                                                                                                                a a b ba a b b

                                                                                                                                                                                Array product

                                                                                                                                                                                11 12 11 12 11 11 12 12

                                                                                                                                                                                21 22 21 22 21 21 22 21

                                                                                                                                                                                a a b b a b a ba a b b a b a b

                                                                                                                                                                                Matrix product

                                                                                                                                                                                11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                                                21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                                                a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                                                We assume array operations unless stated otherwise

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Linear versus Nonlinear Operations

                                                                                                                                                                                One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                                                linear or nonlinear

                                                                                                                                                                                ( ) ( )H f x y g x y

                                                                                                                                                                                H is said to be a linear operator if

                                                                                                                                                                                images1 2 1 2

                                                                                                                                                                                1 2

                                                                                                                                                                                ( ) ( ) ( ) ( )

                                                                                                                                                                                H a f x y b f x y a H f x y b H f x y

                                                                                                                                                                                a b f f

                                                                                                                                                                                Example of nonlinear operator

                                                                                                                                                                                the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                                                1 2

                                                                                                                                                                                0 2 6 5 1 1

                                                                                                                                                                                2 3 4 7f f a b

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                1 2

                                                                                                                                                                                0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                                                2 3 4 7 2 4a f b f

                                                                                                                                                                                0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                                                2 3 4 7

                                                                                                                                                                                Arithmetic Operations in Image Processing

                                                                                                                                                                                Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                                                f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                                                For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                                                value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                                                The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                                used in image enhancement)

                                                                                                                                                                                1

                                                                                                                                                                                1( ) ( )K

                                                                                                                                                                                ii

                                                                                                                                                                                g x y g x yK

                                                                                                                                                                                If the noise satisfies the properties stated above we have

                                                                                                                                                                                2 2( ) ( )

                                                                                                                                                                                1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                                ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                                and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                                the average image is

                                                                                                                                                                                ( ) ( )1

                                                                                                                                                                                g x y x yK

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                                the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                                means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                                averaging process increases

                                                                                                                                                                                An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                                under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                                virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                                was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                                64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                                images respectively

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                                100 noisy images

                                                                                                                                                                                a b c d e f

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                                images

                                                                                                                                                                                (a) (b) (c)

                                                                                                                                                                                Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                                significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                                Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                                Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                                images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                                difference between images (a) and (b)

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                                intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                                source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                                bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                                anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                                images after injection of the contrast medium

                                                                                                                                                                                In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                                Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                                propagates through the various arteries in the area being observed

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                An important application of image multiplication (and division) is shading correction

                                                                                                                                                                                Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                                When the shading function is known

                                                                                                                                                                                ( )( )( )

                                                                                                                                                                                g x yf x yh x y

                                                                                                                                                                                h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                                approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                                sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                (a) (b) (c)

                                                                                                                                                                                Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                (a) (b) (c)

                                                                                                                                                                                Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                min( )mf f f

                                                                                                                                                                                0 ( 255)max( )

                                                                                                                                                                                ms

                                                                                                                                                                                m

                                                                                                                                                                                ff K K K

                                                                                                                                                                                f

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Spatial Operations

                                                                                                                                                                                - are performed directly on the pixels of a given image

                                                                                                                                                                                There are three categories of spatial operations

                                                                                                                                                                                single-pixel operations

                                                                                                                                                                                neighborhood operations

                                                                                                                                                                                geometric spatial transformations

                                                                                                                                                                                Single-pixel operations

                                                                                                                                                                                - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                corresponding pixel in the processed image

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Neighborhood operations

                                                                                                                                                                                Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                ( )

                                                                                                                                                                                1( ) ( )xyr c S

                                                                                                                                                                                g x y f r cm n

                                                                                                                                                                                The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                largest region of an image

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Geometric spatial transformations and image registration

                                                                                                                                                                                - modify the spatial relationship between pixels in an image

                                                                                                                                                                                - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                predefined set of rules

                                                                                                                                                                                A geometric transformation consists of 2 basic operations

                                                                                                                                                                                1 a spatial transformation of coordinates

                                                                                                                                                                                2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                pixels

                                                                                                                                                                                The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                Affine transform

                                                                                                                                                                                11 1211 21 31

                                                                                                                                                                                21 2212 22 33

                                                                                                                                                                                31 32

                                                                                                                                                                                0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                1

                                                                                                                                                                                t tx t v t w t

                                                                                                                                                                                x y v w T v w t ty t v t w t

                                                                                                                                                                                t t

                                                                                                                                                                                (AT)

                                                                                                                                                                                This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                scaling rotation and translation matrices from Table 1

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Affine transformations

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                Problems

                                                                                                                                                                                - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                the same location in the output image

                                                                                                                                                                                - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                assignment)

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                pixel value

                                                                                                                                                                                Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Image registration ndash align two or more images of the same scene

                                                                                                                                                                                In image registration we have available the input and output images but the specific

                                                                                                                                                                                transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                images

                                                                                                                                                                                - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                of time (satellite images)

                                                                                                                                                                                Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                image

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                How to select tie points

                                                                                                                                                                                - interactively selecting them

                                                                                                                                                                                - use of algorithms that try to detect these points

                                                                                                                                                                                - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                for establishing tie points

                                                                                                                                                                                The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                a bilinear approximation is given by

                                                                                                                                                                                1 2 3 4

                                                                                                                                                                                5 6 7 8

                                                                                                                                                                                x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                problem depend on the severity of the geometrical distortion

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Probabilistic Methods

                                                                                                                                                                                zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                ( ) kk

                                                                                                                                                                                np zM N

                                                                                                                                                                                nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                pixels in the image) 1

                                                                                                                                                                                0( ) 1

                                                                                                                                                                                L

                                                                                                                                                                                kk

                                                                                                                                                                                p z

                                                                                                                                                                                The mean (average) intensity of an image is given by 1

                                                                                                                                                                                0( )

                                                                                                                                                                                L

                                                                                                                                                                                k kk

                                                                                                                                                                                m z p z

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                The variance of the intensities is 1

                                                                                                                                                                                2 2

                                                                                                                                                                                0( ) ( )

                                                                                                                                                                                L

                                                                                                                                                                                k kk

                                                                                                                                                                                z m p z

                                                                                                                                                                                The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                ( ) is used

                                                                                                                                                                                The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                0( ) ( ) ( )

                                                                                                                                                                                Ln

                                                                                                                                                                                n k kk

                                                                                                                                                                                z z m p z

                                                                                                                                                                                ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                mean

                                                                                                                                                                                Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Intensity Transformations and Spatial Filtering

                                                                                                                                                                                ( ) ( )g x y T f x y

                                                                                                                                                                                f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                neighborhood of (x y)

                                                                                                                                                                                - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                and much smaller in size than the image

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                ( )s T r

                                                                                                                                                                                s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                is called contrast stretching

                                                                                                                                                                                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                thresholding function

                                                                                                                                                                                Some Basic Intensity Transformation Functions

                                                                                                                                                                                Image Negatives

                                                                                                                                                                                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                - equivalent of a photographic negative

                                                                                                                                                                                - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                image

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Original Negative image

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                Some basic intensity transformation functions

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                variations in pixel values

                                                                                                                                                                                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Power-Law (Gamma) Transformations

                                                                                                                                                                                - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                1c - identity transformation

                                                                                                                                                                                A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                gamma correction

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Piecewise-Linear Transformations Functions

                                                                                                                                                                                Contrast stretching

                                                                                                                                                                                - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                intensity range of the recording tool or display device

                                                                                                                                                                                a b c d Fig5

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                11

                                                                                                                                                                                1

                                                                                                                                                                                2 1 1 21 2

                                                                                                                                                                                2 1 2 1

                                                                                                                                                                                22

                                                                                                                                                                                2

                                                                                                                                                                                [0 ]

                                                                                                                                                                                ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                ( 1 ) [ 1]( 1 )

                                                                                                                                                                                s r r rrs r r s r rT r r r r

                                                                                                                                                                                r r r rs L r r r L

                                                                                                                                                                                L r

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                from their original range to the full range [0 L-1]

                                                                                                                                                                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                image of pollen magnified approximately 700 times

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Intensity-level slicing

                                                                                                                                                                                - highlighting a specific range of intensities in an image

                                                                                                                                                                                There are two approaches for intensity-level slicing

                                                                                                                                                                                1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                intensities in the image (Figure 311 (b))

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                blockageshellip)

                                                                                                                                                                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Bit-plane slicing

                                                                                                                                                                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                Week 1

                                                                                                                                                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                • DIP 1 2017
                                                                                                                                                                                • DIP 02 (2017)

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  The D4 distance (also called city-block distance) between p and q is defined as

                                                                                                                                                                                  4( ) | | | |D p q x s y t

                                                                                                                                                                                  The pixels q for which 4( )D p q r form a diamond centered at (xy)

                                                                                                                                                                                  4

                                                                                                                                                                                  22 1 2

                                                                                                                                                                                  2 2 1 0 1 22 1 2

                                                                                                                                                                                  2

                                                                                                                                                                                  D

                                                                                                                                                                                  The pixels with D4 = 1 are the 4-neighbors of (x y)

                                                                                                                                                                                  The D8 distance (called the chessboard distance) between p and q is defined as

                                                                                                                                                                                  8( ) max| | | |D p q x s y t

                                                                                                                                                                                  The pixels q for which 8( )D p q r form a square centered at (x y)

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  8

                                                                                                                                                                                  2 2 2 2 22 1 1 1 2

                                                                                                                                                                                  2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                                                  D

                                                                                                                                                                                  The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                                                  D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                                                  because these distances involve only the coordinates of the point

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Array versus Matrix Operations

                                                                                                                                                                                  An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                                                  11 12 11 12

                                                                                                                                                                                  21 22 21 22

                                                                                                                                                                                  a a b ba a b b

                                                                                                                                                                                  Array product

                                                                                                                                                                                  11 12 11 12 11 11 12 12

                                                                                                                                                                                  21 22 21 22 21 21 22 21

                                                                                                                                                                                  a a b b a b a ba a b b a b a b

                                                                                                                                                                                  Matrix product

                                                                                                                                                                                  11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                                                  21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                                                  a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                                                  We assume array operations unless stated otherwise

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Linear versus Nonlinear Operations

                                                                                                                                                                                  One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                                                  linear or nonlinear

                                                                                                                                                                                  ( ) ( )H f x y g x y

                                                                                                                                                                                  H is said to be a linear operator if

                                                                                                                                                                                  images1 2 1 2

                                                                                                                                                                                  1 2

                                                                                                                                                                                  ( ) ( ) ( ) ( )

                                                                                                                                                                                  H a f x y b f x y a H f x y b H f x y

                                                                                                                                                                                  a b f f

                                                                                                                                                                                  Example of nonlinear operator

                                                                                                                                                                                  the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                                                  1 2

                                                                                                                                                                                  0 2 6 5 1 1

                                                                                                                                                                                  2 3 4 7f f a b

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  1 2

                                                                                                                                                                                  0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                                                  2 3 4 7 2 4a f b f

                                                                                                                                                                                  0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                                                  2 3 4 7

                                                                                                                                                                                  Arithmetic Operations in Image Processing

                                                                                                                                                                                  Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                                                  f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                                                  For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                                                  value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                                                  The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                                  used in image enhancement)

                                                                                                                                                                                  1

                                                                                                                                                                                  1( ) ( )K

                                                                                                                                                                                  ii

                                                                                                                                                                                  g x y g x yK

                                                                                                                                                                                  If the noise satisfies the properties stated above we have

                                                                                                                                                                                  2 2( ) ( )

                                                                                                                                                                                  1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                                  ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                                  and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                                  the average image is

                                                                                                                                                                                  ( ) ( )1

                                                                                                                                                                                  g x y x yK

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                                  the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                                  means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                                  averaging process increases

                                                                                                                                                                                  An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                                  under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                                  virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                                  was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                                  64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                                  images respectively

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                                  100 noisy images

                                                                                                                                                                                  a b c d e f

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                                  images

                                                                                                                                                                                  (a) (b) (c)

                                                                                                                                                                                  Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                                  significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                                  Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                                  Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                                  images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                                  difference between images (a) and (b)

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                  h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                                  intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                                  source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                                  bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                                  anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                                  images after injection of the contrast medium

                                                                                                                                                                                  In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                                  Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                                  propagates through the various arteries in the area being observed

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  An important application of image multiplication (and division) is shading correction

                                                                                                                                                                                  Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                  f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                                  When the shading function is known

                                                                                                                                                                                  ( )( )( )

                                                                                                                                                                                  g x yf x yh x y

                                                                                                                                                                                  h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                                  approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                                  sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  (a) (b) (c)

                                                                                                                                                                                  Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                  operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                  1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                  image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                  (a) (b) (c)

                                                                                                                                                                                  Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                  min( )mf f f

                                                                                                                                                                                  0 ( 255)max( )

                                                                                                                                                                                  ms

                                                                                                                                                                                  m

                                                                                                                                                                                  ff K K K

                                                                                                                                                                                  f

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Spatial Operations

                                                                                                                                                                                  - are performed directly on the pixels of a given image

                                                                                                                                                                                  There are three categories of spatial operations

                                                                                                                                                                                  single-pixel operations

                                                                                                                                                                                  neighborhood operations

                                                                                                                                                                                  geometric spatial transformations

                                                                                                                                                                                  Single-pixel operations

                                                                                                                                                                                  - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                  where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                  corresponding pixel in the processed image

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Neighborhood operations

                                                                                                                                                                                  Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                  in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                  based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                  rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                  intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                  ( )

                                                                                                                                                                                  1( ) ( )xyr c S

                                                                                                                                                                                  g x y f r cm n

                                                                                                                                                                                  The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                  used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                  largest region of an image

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Geometric spatial transformations and image registration

                                                                                                                                                                                  - modify the spatial relationship between pixels in an image

                                                                                                                                                                                  - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                  printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                  predefined set of rules

                                                                                                                                                                                  A geometric transformation consists of 2 basic operations

                                                                                                                                                                                  1 a spatial transformation of coordinates

                                                                                                                                                                                  2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                  pixels

                                                                                                                                                                                  The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                  (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                  (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                  Affine transform

                                                                                                                                                                                  11 1211 21 31

                                                                                                                                                                                  21 2212 22 33

                                                                                                                                                                                  31 32

                                                                                                                                                                                  0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                  1

                                                                                                                                                                                  t tx t v t w t

                                                                                                                                                                                  x y v w T v w t ty t v t w t

                                                                                                                                                                                  t t

                                                                                                                                                                                  (AT)

                                                                                                                                                                                  This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                  on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                  result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                  scaling rotation and translation matrices from Table 1

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Affine transformations

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                  the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                  using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                  In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                  forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                  location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                  Problems

                                                                                                                                                                                  - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                  the same location in the output image

                                                                                                                                                                                  - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                  assignment)

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                  computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                  It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                  pixel value

                                                                                                                                                                                  Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                  numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Image registration ndash align two or more images of the same scene

                                                                                                                                                                                  In image registration we have available the input and output images but the specific

                                                                                                                                                                                  transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                  The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                  images

                                                                                                                                                                                  - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                  same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                  - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                  of time (satellite images)

                                                                                                                                                                                  Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                  corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                  image

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  How to select tie points

                                                                                                                                                                                  - interactively selecting them

                                                                                                                                                                                  - use of algorithms that try to detect these points

                                                                                                                                                                                  - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                  the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                  marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                  for establishing tie points

                                                                                                                                                                                  The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                  of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                  a bilinear approximation is given by

                                                                                                                                                                                  1 2 3 4

                                                                                                                                                                                  5 6 7 8

                                                                                                                                                                                  x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                  (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                  frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                  subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                  subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                  The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                  problem depend on the severity of the geometrical distortion

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Probabilistic Methods

                                                                                                                                                                                  zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                  p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                  ( ) kk

                                                                                                                                                                                  np zM N

                                                                                                                                                                                  nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                  pixels in the image) 1

                                                                                                                                                                                  0( ) 1

                                                                                                                                                                                  L

                                                                                                                                                                                  kk

                                                                                                                                                                                  p z

                                                                                                                                                                                  The mean (average) intensity of an image is given by 1

                                                                                                                                                                                  0( )

                                                                                                                                                                                  L

                                                                                                                                                                                  k kk

                                                                                                                                                                                  m z p z

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  The variance of the intensities is 1

                                                                                                                                                                                  2 2

                                                                                                                                                                                  0( ) ( )

                                                                                                                                                                                  L

                                                                                                                                                                                  k kk

                                                                                                                                                                                  z m p z

                                                                                                                                                                                  The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                  measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                  ( ) is used

                                                                                                                                                                                  The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                  0( ) ( ) ( )

                                                                                                                                                                                  Ln

                                                                                                                                                                                  n k kk

                                                                                                                                                                                  z z m p z

                                                                                                                                                                                  ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                  3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                  ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                  mean

                                                                                                                                                                                  Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                  Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                  Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                  Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Intensity Transformations and Spatial Filtering

                                                                                                                                                                                  ( ) ( )g x y T f x y

                                                                                                                                                                                  f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                  neighborhood of (x y)

                                                                                                                                                                                  - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                  and much smaller in size than the image

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                  called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                  ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                  ( )s T r

                                                                                                                                                                                  s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                  Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                  darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                  is called contrast stretching

                                                                                                                                                                                  Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                  thresholding function

                                                                                                                                                                                  Some Basic Intensity Transformation Functions

                                                                                                                                                                                  Image Negatives

                                                                                                                                                                                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                  - equivalent of a photographic negative

                                                                                                                                                                                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                  image

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Original Negative image

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                  Some basic intensity transformation functions

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                  range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                  while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                  transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                  variations in pixel values

                                                                                                                                                                                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Power-Law (Gamma) Transformations

                                                                                                                                                                                  - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                  Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                  of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                  curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                  1c - identity transformation

                                                                                                                                                                                  A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                  power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                  gamma correction

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Piecewise-Linear Transformations Functions

                                                                                                                                                                                  Contrast stretching

                                                                                                                                                                                  - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                  intensity range of the recording tool or display device

                                                                                                                                                                                  a b c d Fig5

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  11

                                                                                                                                                                                  1

                                                                                                                                                                                  2 1 1 21 2

                                                                                                                                                                                  2 1 2 1

                                                                                                                                                                                  22

                                                                                                                                                                                  2

                                                                                                                                                                                  [0 ]

                                                                                                                                                                                  ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                  ( 1 ) [ 1]( 1 )

                                                                                                                                                                                  s r r rrs r r s r rT r r r r

                                                                                                                                                                                  r r r rs L r r r L

                                                                                                                                                                                  L r

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                  1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                  Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                  in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                  from their original range to the full range [0 L-1]

                                                                                                                                                                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                  2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                  The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                  image of pollen magnified approximately 700 times

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Intensity-level slicing

                                                                                                                                                                                  - highlighting a specific range of intensities in an image

                                                                                                                                                                                  There are two approaches for intensity-level slicing

                                                                                                                                                                                  1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                  another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                  intensities in the image (Figure 311 (b))

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                  the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                  Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                  blockageshellip)

                                                                                                                                                                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                  image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                  Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Bit-plane slicing

                                                                                                                                                                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                  This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                  Week 1

                                                                                                                                                                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                  • DIP 1 2017
                                                                                                                                                                                  • DIP 02 (2017)

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    8

                                                                                                                                                                                    2 2 2 2 22 1 1 1 2

                                                                                                                                                                                    2 2 1 0 1 22 1 1 1 22 2 2 2 2

                                                                                                                                                                                    D

                                                                                                                                                                                    The pixels with D8 = 1 are the 8-neighbors of (x y)

                                                                                                                                                                                    D4 and D8 distances are independent of any paths that might exist between p and q

                                                                                                                                                                                    because these distances involve only the coordinates of the point

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Array versus Matrix Operations

                                                                                                                                                                                    An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                                                    11 12 11 12

                                                                                                                                                                                    21 22 21 22

                                                                                                                                                                                    a a b ba a b b

                                                                                                                                                                                    Array product

                                                                                                                                                                                    11 12 11 12 11 11 12 12

                                                                                                                                                                                    21 22 21 22 21 21 22 21

                                                                                                                                                                                    a a b b a b a ba a b b a b a b

                                                                                                                                                                                    Matrix product

                                                                                                                                                                                    11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                                                    21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                                                    a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                                                    We assume array operations unless stated otherwise

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Linear versus Nonlinear Operations

                                                                                                                                                                                    One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                                                    linear or nonlinear

                                                                                                                                                                                    ( ) ( )H f x y g x y

                                                                                                                                                                                    H is said to be a linear operator if

                                                                                                                                                                                    images1 2 1 2

                                                                                                                                                                                    1 2

                                                                                                                                                                                    ( ) ( ) ( ) ( )

                                                                                                                                                                                    H a f x y b f x y a H f x y b H f x y

                                                                                                                                                                                    a b f f

                                                                                                                                                                                    Example of nonlinear operator

                                                                                                                                                                                    the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                                                    1 2

                                                                                                                                                                                    0 2 6 5 1 1

                                                                                                                                                                                    2 3 4 7f f a b

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    1 2

                                                                                                                                                                                    0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                                                    2 3 4 7 2 4a f b f

                                                                                                                                                                                    0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                                                    2 3 4 7

                                                                                                                                                                                    Arithmetic Operations in Image Processing

                                                                                                                                                                                    Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                                                    f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                                                    For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                                                    value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                                                    The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                                    used in image enhancement)

                                                                                                                                                                                    1

                                                                                                                                                                                    1( ) ( )K

                                                                                                                                                                                    ii

                                                                                                                                                                                    g x y g x yK

                                                                                                                                                                                    If the noise satisfies the properties stated above we have

                                                                                                                                                                                    2 2( ) ( )

                                                                                                                                                                                    1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                                    ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                                    and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                                    the average image is

                                                                                                                                                                                    ( ) ( )1

                                                                                                                                                                                    g x y x yK

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                                    the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                                    means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                                    averaging process increases

                                                                                                                                                                                    An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                                    under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                                    virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                                    was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                                    64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                                    images respectively

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                                    100 noisy images

                                                                                                                                                                                    a b c d e f

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                                    images

                                                                                                                                                                                    (a) (b) (c)

                                                                                                                                                                                    Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                                    significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                                    Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                                    Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                                    images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                                    difference between images (a) and (b)

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                    h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                                    intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                                    source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                                    bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                                    anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                                    images after injection of the contrast medium

                                                                                                                                                                                    In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                                    Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                                    propagates through the various arteries in the area being observed

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    An important application of image multiplication (and division) is shading correction

                                                                                                                                                                                    Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                    f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                                    When the shading function is known

                                                                                                                                                                                    ( )( )( )

                                                                                                                                                                                    g x yf x yh x y

                                                                                                                                                                                    h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                                    approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                                    sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    (a) (b) (c)

                                                                                                                                                                                    Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                    operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                    1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                    image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                    (a) (b) (c)

                                                                                                                                                                                    Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                    min( )mf f f

                                                                                                                                                                                    0 ( 255)max( )

                                                                                                                                                                                    ms

                                                                                                                                                                                    m

                                                                                                                                                                                    ff K K K

                                                                                                                                                                                    f

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Spatial Operations

                                                                                                                                                                                    - are performed directly on the pixels of a given image

                                                                                                                                                                                    There are three categories of spatial operations

                                                                                                                                                                                    single-pixel operations

                                                                                                                                                                                    neighborhood operations

                                                                                                                                                                                    geometric spatial transformations

                                                                                                                                                                                    Single-pixel operations

                                                                                                                                                                                    - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                    where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                    corresponding pixel in the processed image

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Neighborhood operations

                                                                                                                                                                                    Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                    in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                    based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                    rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                    intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                    ( )

                                                                                                                                                                                    1( ) ( )xyr c S

                                                                                                                                                                                    g x y f r cm n

                                                                                                                                                                                    The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                    used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                    largest region of an image

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Geometric spatial transformations and image registration

                                                                                                                                                                                    - modify the spatial relationship between pixels in an image

                                                                                                                                                                                    - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                    printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                    predefined set of rules

                                                                                                                                                                                    A geometric transformation consists of 2 basic operations

                                                                                                                                                                                    1 a spatial transformation of coordinates

                                                                                                                                                                                    2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                    pixels

                                                                                                                                                                                    The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                    (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                    (x y) ndash pixel coordinates in the transformed image

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                                                                                                                                                                                    Week 1

                                                                                                                                                                                    [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                    Affine transform

                                                                                                                                                                                    11 1211 21 31

                                                                                                                                                                                    21 2212 22 33

                                                                                                                                                                                    31 32

                                                                                                                                                                                    0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                    1

                                                                                                                                                                                    t tx t v t w t

                                                                                                                                                                                    x y v w T v w t ty t v t w t

                                                                                                                                                                                    t t

                                                                                                                                                                                    (AT)

                                                                                                                                                                                    This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                    on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                    result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                    scaling rotation and translation matrices from Table 1

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                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Affine transformations

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                                                                                                                                                                                    Week 1

                                                                                                                                                                                    The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                    the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                    using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                    In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                    forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                    location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                    Problems

                                                                                                                                                                                    - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                    the same location in the output image

                                                                                                                                                                                    - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                    assignment)

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                                                                                                                                                                                    Week 1

                                                                                                                                                                                    inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                    computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                    It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                    pixel value

                                                                                                                                                                                    Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                    numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Image registration ndash align two or more images of the same scene

                                                                                                                                                                                    In image registration we have available the input and output images but the specific

                                                                                                                                                                                    transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                    The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                    images

                                                                                                                                                                                    - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                    same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                    - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                    of time (satellite images)

                                                                                                                                                                                    Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                    corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                    image

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    How to select tie points

                                                                                                                                                                                    - interactively selecting them

                                                                                                                                                                                    - use of algorithms that try to detect these points

                                                                                                                                                                                    - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                    the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                    marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                    for establishing tie points

                                                                                                                                                                                    The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                    of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                    a bilinear approximation is given by

                                                                                                                                                                                    1 2 3 4

                                                                                                                                                                                    5 6 7 8

                                                                                                                                                                                    x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                    (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                    frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                    subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                    subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                    The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                    problem depend on the severity of the geometrical distortion

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Probabilistic Methods

                                                                                                                                                                                    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                    p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                    ( ) kk

                                                                                                                                                                                    np zM N

                                                                                                                                                                                    nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                    pixels in the image) 1

                                                                                                                                                                                    0( ) 1

                                                                                                                                                                                    L

                                                                                                                                                                                    kk

                                                                                                                                                                                    p z

                                                                                                                                                                                    The mean (average) intensity of an image is given by 1

                                                                                                                                                                                    0( )

                                                                                                                                                                                    L

                                                                                                                                                                                    k kk

                                                                                                                                                                                    m z p z

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    The variance of the intensities is 1

                                                                                                                                                                                    2 2

                                                                                                                                                                                    0( ) ( )

                                                                                                                                                                                    L

                                                                                                                                                                                    k kk

                                                                                                                                                                                    z m p z

                                                                                                                                                                                    The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                    measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                    ( ) is used

                                                                                                                                                                                    The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                    0( ) ( ) ( )

                                                                                                                                                                                    Ln

                                                                                                                                                                                    n k kk

                                                                                                                                                                                    z z m p z

                                                                                                                                                                                    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                    3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                    ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                    mean

                                                                                                                                                                                    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                    Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                    Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                    Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Intensity Transformations and Spatial Filtering

                                                                                                                                                                                    ( ) ( )g x y T f x y

                                                                                                                                                                                    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                    neighborhood of (x y)

                                                                                                                                                                                    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                    and much smaller in size than the image

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                    called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                    ( )s T r

                                                                                                                                                                                    s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                    Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                    darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                    is called contrast stretching

                                                                                                                                                                                    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                    thresholding function

                                                                                                                                                                                    Some Basic Intensity Transformation Functions

                                                                                                                                                                                    Image Negatives

                                                                                                                                                                                    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                    - equivalent of a photographic negative

                                                                                                                                                                                    - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                    image

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Original Negative image

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                    Some basic intensity transformation functions

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                    range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                    while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                    transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                    variations in pixel values

                                                                                                                                                                                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Power-Law (Gamma) Transformations

                                                                                                                                                                                    - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                    Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                    of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                    curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                    1c - identity transformation

                                                                                                                                                                                    A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                    power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                    gamma correction

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Piecewise-Linear Transformations Functions

                                                                                                                                                                                    Contrast stretching

                                                                                                                                                                                    - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                    intensity range of the recording tool or display device

                                                                                                                                                                                    a b c d Fig5

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    11

                                                                                                                                                                                    1

                                                                                                                                                                                    2 1 1 21 2

                                                                                                                                                                                    2 1 2 1

                                                                                                                                                                                    22

                                                                                                                                                                                    2

                                                                                                                                                                                    [0 ]

                                                                                                                                                                                    ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                    ( 1 ) [ 1]( 1 )

                                                                                                                                                                                    s r r rrs r r s r rT r r r r

                                                                                                                                                                                    r r r rs L r r r L

                                                                                                                                                                                    L r

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                    1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                    Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                    in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                    from their original range to the full range [0 L-1]

                                                                                                                                                                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                    2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                    The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                    image of pollen magnified approximately 700 times

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Intensity-level slicing

                                                                                                                                                                                    - highlighting a specific range of intensities in an image

                                                                                                                                                                                    There are two approaches for intensity-level slicing

                                                                                                                                                                                    1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                    another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                    intensities in the image (Figure 311 (b))

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                    the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                    Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                    blockageshellip)

                                                                                                                                                                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                    image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                    Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Bit-plane slicing

                                                                                                                                                                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                    This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                    Week 1

                                                                                                                                                                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                    • DIP 1 2017
                                                                                                                                                                                    • DIP 02 (2017)

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Array versus Matrix Operations

                                                                                                                                                                                      An array operation involving one or more images is carried out on a pixel-by-pixel basis

                                                                                                                                                                                      11 12 11 12

                                                                                                                                                                                      21 22 21 22

                                                                                                                                                                                      a a b ba a b b

                                                                                                                                                                                      Array product

                                                                                                                                                                                      11 12 11 12 11 11 12 12

                                                                                                                                                                                      21 22 21 22 21 21 22 21

                                                                                                                                                                                      a a b b a b a ba a b b a b a b

                                                                                                                                                                                      Matrix product

                                                                                                                                                                                      11 12 11 12 11 11 12 21 11 12 12 21

                                                                                                                                                                                      21 22 21 22 21 11 22 21 21 12 22 22

                                                                                                                                                                                      a a b b a b a b a b a ba a b b a b a b a b a b

                                                                                                                                                                                      We assume array operations unless stated otherwise

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Linear versus Nonlinear Operations

                                                                                                                                                                                      One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                                                      linear or nonlinear

                                                                                                                                                                                      ( ) ( )H f x y g x y

                                                                                                                                                                                      H is said to be a linear operator if

                                                                                                                                                                                      images1 2 1 2

                                                                                                                                                                                      1 2

                                                                                                                                                                                      ( ) ( ) ( ) ( )

                                                                                                                                                                                      H a f x y b f x y a H f x y b H f x y

                                                                                                                                                                                      a b f f

                                                                                                                                                                                      Example of nonlinear operator

                                                                                                                                                                                      the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                                                      1 2

                                                                                                                                                                                      0 2 6 5 1 1

                                                                                                                                                                                      2 3 4 7f f a b

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      1 2

                                                                                                                                                                                      0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                                                      2 3 4 7 2 4a f b f

                                                                                                                                                                                      0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                                                      2 3 4 7

                                                                                                                                                                                      Arithmetic Operations in Image Processing

                                                                                                                                                                                      Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                                                      f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                                                      For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                                                      value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                                                      The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                                      used in image enhancement)

                                                                                                                                                                                      1

                                                                                                                                                                                      1( ) ( )K

                                                                                                                                                                                      ii

                                                                                                                                                                                      g x y g x yK

                                                                                                                                                                                      If the noise satisfies the properties stated above we have

                                                                                                                                                                                      2 2( ) ( )

                                                                                                                                                                                      1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                                      ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                                      and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                                      the average image is

                                                                                                                                                                                      ( ) ( )1

                                                                                                                                                                                      g x y x yK

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                                      the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                                      means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                                      averaging process increases

                                                                                                                                                                                      An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                                      under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                                      virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                                      was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                                      64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                                      images respectively

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                                      100 noisy images

                                                                                                                                                                                      a b c d e f

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                                      images

                                                                                                                                                                                      (a) (b) (c)

                                                                                                                                                                                      Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                                      significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                                      Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                                      Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                                      images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                                      difference between images (a) and (b)

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                      h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                                      intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                                      source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                                      bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                                      anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                                      images after injection of the contrast medium

                                                                                                                                                                                      In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                                      Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                                      propagates through the various arteries in the area being observed

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      An important application of image multiplication (and division) is shading correction

                                                                                                                                                                                      Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                      f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                                      When the shading function is known

                                                                                                                                                                                      ( )( )( )

                                                                                                                                                                                      g x yf x yh x y

                                                                                                                                                                                      h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                                      approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                                      sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      (a) (b) (c)

                                                                                                                                                                                      Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                      operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                      1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                      image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                      (a) (b) (c)

                                                                                                                                                                                      Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                      min( )mf f f

                                                                                                                                                                                      0 ( 255)max( )

                                                                                                                                                                                      ms

                                                                                                                                                                                      m

                                                                                                                                                                                      ff K K K

                                                                                                                                                                                      f

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Spatial Operations

                                                                                                                                                                                      - are performed directly on the pixels of a given image

                                                                                                                                                                                      There are three categories of spatial operations

                                                                                                                                                                                      single-pixel operations

                                                                                                                                                                                      neighborhood operations

                                                                                                                                                                                      geometric spatial transformations

                                                                                                                                                                                      Single-pixel operations

                                                                                                                                                                                      - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                      where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                      corresponding pixel in the processed image

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Neighborhood operations

                                                                                                                                                                                      Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                      in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                      based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                      rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                      intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                      ( )

                                                                                                                                                                                      1( ) ( )xyr c S

                                                                                                                                                                                      g x y f r cm n

                                                                                                                                                                                      The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                      used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                      largest region of an image

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Geometric spatial transformations and image registration

                                                                                                                                                                                      - modify the spatial relationship between pixels in an image

                                                                                                                                                                                      - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                      printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                      predefined set of rules

                                                                                                                                                                                      A geometric transformation consists of 2 basic operations

                                                                                                                                                                                      1 a spatial transformation of coordinates

                                                                                                                                                                                      2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                      pixels

                                                                                                                                                                                      The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                      (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                      (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                      Affine transform

                                                                                                                                                                                      11 1211 21 31

                                                                                                                                                                                      21 2212 22 33

                                                                                                                                                                                      31 32

                                                                                                                                                                                      0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                      1

                                                                                                                                                                                      t tx t v t w t

                                                                                                                                                                                      x y v w T v w t ty t v t w t

                                                                                                                                                                                      t t

                                                                                                                                                                                      (AT)

                                                                                                                                                                                      This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                      on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                      result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                      scaling rotation and translation matrices from Table 1

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Affine transformations

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                      the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                      using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                      In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                      forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                      location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                      Problems

                                                                                                                                                                                      - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                      the same location in the output image

                                                                                                                                                                                      - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                      assignment)

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                      computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                      It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                      pixel value

                                                                                                                                                                                      Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                      numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Image registration ndash align two or more images of the same scene

                                                                                                                                                                                      In image registration we have available the input and output images but the specific

                                                                                                                                                                                      transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                      The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                      images

                                                                                                                                                                                      - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                      same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                      - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                      of time (satellite images)

                                                                                                                                                                                      Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                      corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                      image

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      How to select tie points

                                                                                                                                                                                      - interactively selecting them

                                                                                                                                                                                      - use of algorithms that try to detect these points

                                                                                                                                                                                      - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                      the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                      marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                      for establishing tie points

                                                                                                                                                                                      The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                      of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                      a bilinear approximation is given by

                                                                                                                                                                                      1 2 3 4

                                                                                                                                                                                      5 6 7 8

                                                                                                                                                                                      x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                      (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                      frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                      subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                      subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                      The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                      problem depend on the severity of the geometrical distortion

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Probabilistic Methods

                                                                                                                                                                                      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                      p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                      ( ) kk

                                                                                                                                                                                      np zM N

                                                                                                                                                                                      nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                      pixels in the image) 1

                                                                                                                                                                                      0( ) 1

                                                                                                                                                                                      L

                                                                                                                                                                                      kk

                                                                                                                                                                                      p z

                                                                                                                                                                                      The mean (average) intensity of an image is given by 1

                                                                                                                                                                                      0( )

                                                                                                                                                                                      L

                                                                                                                                                                                      k kk

                                                                                                                                                                                      m z p z

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      The variance of the intensities is 1

                                                                                                                                                                                      2 2

                                                                                                                                                                                      0( ) ( )

                                                                                                                                                                                      L

                                                                                                                                                                                      k kk

                                                                                                                                                                                      z m p z

                                                                                                                                                                                      The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                      measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                      ( ) is used

                                                                                                                                                                                      The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                      0( ) ( ) ( )

                                                                                                                                                                                      Ln

                                                                                                                                                                                      n k kk

                                                                                                                                                                                      z z m p z

                                                                                                                                                                                      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                      3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                      ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                      mean

                                                                                                                                                                                      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                      Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                      Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                      Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Intensity Transformations and Spatial Filtering

                                                                                                                                                                                      ( ) ( )g x y T f x y

                                                                                                                                                                                      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                      neighborhood of (x y)

                                                                                                                                                                                      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                      and much smaller in size than the image

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                      called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                      ( )s T r

                                                                                                                                                                                      s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                      Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                      darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                      is called contrast stretching

                                                                                                                                                                                      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                      thresholding function

                                                                                                                                                                                      Some Basic Intensity Transformation Functions

                                                                                                                                                                                      Image Negatives

                                                                                                                                                                                      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                      - equivalent of a photographic negative

                                                                                                                                                                                      - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                      image

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Original Negative image

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                      Some basic intensity transformation functions

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                      range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                      while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                      transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                      variations in pixel values

                                                                                                                                                                                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Power-Law (Gamma) Transformations

                                                                                                                                                                                      - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                      Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                      of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                      curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                      1c - identity transformation

                                                                                                                                                                                      A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                      power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                      gamma correction

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Piecewise-Linear Transformations Functions

                                                                                                                                                                                      Contrast stretching

                                                                                                                                                                                      - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                      intensity range of the recording tool or display device

                                                                                                                                                                                      a b c d Fig5

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      11

                                                                                                                                                                                      1

                                                                                                                                                                                      2 1 1 21 2

                                                                                                                                                                                      2 1 2 1

                                                                                                                                                                                      22

                                                                                                                                                                                      2

                                                                                                                                                                                      [0 ]

                                                                                                                                                                                      ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                      ( 1 ) [ 1]( 1 )

                                                                                                                                                                                      s r r rrs r r s r rT r r r r

                                                                                                                                                                                      r r r rs L r r r L

                                                                                                                                                                                      L r

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                      1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                      Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                      in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                      from their original range to the full range [0 L-1]

                                                                                                                                                                                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                      2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                      The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                      image of pollen magnified approximately 700 times

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Intensity-level slicing

                                                                                                                                                                                      - highlighting a specific range of intensities in an image

                                                                                                                                                                                      There are two approaches for intensity-level slicing

                                                                                                                                                                                      1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                      another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                      intensities in the image (Figure 311 (b))

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                      the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                      Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                      blockageshellip)

                                                                                                                                                                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                      image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                      Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Bit-plane slicing

                                                                                                                                                                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                      This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                      Week 1

                                                                                                                                                                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                      • DIP 1 2017
                                                                                                                                                                                      • DIP 02 (2017)

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Linear versus Nonlinear Operations

                                                                                                                                                                                        One of the most important classifications of image-processing methods is whether it is

                                                                                                                                                                                        linear or nonlinear

                                                                                                                                                                                        ( ) ( )H f x y g x y

                                                                                                                                                                                        H is said to be a linear operator if

                                                                                                                                                                                        images1 2 1 2

                                                                                                                                                                                        1 2

                                                                                                                                                                                        ( ) ( ) ( ) ( )

                                                                                                                                                                                        H a f x y b f x y a H f x y b H f x y

                                                                                                                                                                                        a b f f

                                                                                                                                                                                        Example of nonlinear operator

                                                                                                                                                                                        the maximum value of the pixels of image max ( )H f f x y f

                                                                                                                                                                                        1 2

                                                                                                                                                                                        0 2 6 5 1 1

                                                                                                                                                                                        2 3 4 7f f a b

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        1 2

                                                                                                                                                                                        0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                                                        2 3 4 7 2 4a f b f

                                                                                                                                                                                        0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                                                        2 3 4 7

                                                                                                                                                                                        Arithmetic Operations in Image Processing

                                                                                                                                                                                        Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                                                        f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                                                        For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                                                        value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                                                        The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                                        used in image enhancement)

                                                                                                                                                                                        1

                                                                                                                                                                                        1( ) ( )K

                                                                                                                                                                                        ii

                                                                                                                                                                                        g x y g x yK

                                                                                                                                                                                        If the noise satisfies the properties stated above we have

                                                                                                                                                                                        2 2( ) ( )

                                                                                                                                                                                        1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                                        ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                                        and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                                        the average image is

                                                                                                                                                                                        ( ) ( )1

                                                                                                                                                                                        g x y x yK

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                                        the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                                        means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                                        averaging process increases

                                                                                                                                                                                        An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                                        under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                                        virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                                        was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                                        64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                                        images respectively

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                                        100 noisy images

                                                                                                                                                                                        a b c d e f

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                                        images

                                                                                                                                                                                        (a) (b) (c)

                                                                                                                                                                                        Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                                        significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                                        Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                                        Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                                        images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                                        difference between images (a) and (b)

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                        h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                                        intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                                        source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                                        bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                                        anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                                        images after injection of the contrast medium

                                                                                                                                                                                        In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                                        Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                                        propagates through the various arteries in the area being observed

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        An important application of image multiplication (and division) is shading correction

                                                                                                                                                                                        Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                        f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                                        When the shading function is known

                                                                                                                                                                                        ( )( )( )

                                                                                                                                                                                        g x yf x yh x y

                                                                                                                                                                                        h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                                        approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                                        sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        (a) (b) (c)

                                                                                                                                                                                        Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                        operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                        1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                        image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                        (a) (b) (c)

                                                                                                                                                                                        Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                        min( )mf f f

                                                                                                                                                                                        0 ( 255)max( )

                                                                                                                                                                                        ms

                                                                                                                                                                                        m

                                                                                                                                                                                        ff K K K

                                                                                                                                                                                        f

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Spatial Operations

                                                                                                                                                                                        - are performed directly on the pixels of a given image

                                                                                                                                                                                        There are three categories of spatial operations

                                                                                                                                                                                        single-pixel operations

                                                                                                                                                                                        neighborhood operations

                                                                                                                                                                                        geometric spatial transformations

                                                                                                                                                                                        Single-pixel operations

                                                                                                                                                                                        - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                        where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                        corresponding pixel in the processed image

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Neighborhood operations

                                                                                                                                                                                        Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                        in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                        based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                        rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                        intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                        ( )

                                                                                                                                                                                        1( ) ( )xyr c S

                                                                                                                                                                                        g x y f r cm n

                                                                                                                                                                                        The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                        used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                        largest region of an image

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Geometric spatial transformations and image registration

                                                                                                                                                                                        - modify the spatial relationship between pixels in an image

                                                                                                                                                                                        - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                        printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                        predefined set of rules

                                                                                                                                                                                        A geometric transformation consists of 2 basic operations

                                                                                                                                                                                        1 a spatial transformation of coordinates

                                                                                                                                                                                        2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                        pixels

                                                                                                                                                                                        The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                        (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                        (x y) ndash pixel coordinates in the transformed image

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                                                                                                                                                                                        Week 1

                                                                                                                                                                                        [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                        Affine transform

                                                                                                                                                                                        11 1211 21 31

                                                                                                                                                                                        21 2212 22 33

                                                                                                                                                                                        31 32

                                                                                                                                                                                        0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                        1

                                                                                                                                                                                        t tx t v t w t

                                                                                                                                                                                        x y v w T v w t ty t v t w t

                                                                                                                                                                                        t t

                                                                                                                                                                                        (AT)

                                                                                                                                                                                        This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                        on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                        result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                        scaling rotation and translation matrices from Table 1

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Affine transformations

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                        the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                        using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                        In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                        forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                        location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                        Problems

                                                                                                                                                                                        - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                        the same location in the output image

                                                                                                                                                                                        - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                        assignment)

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                        computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                        It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                        pixel value

                                                                                                                                                                                        Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                        numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Image registration ndash align two or more images of the same scene

                                                                                                                                                                                        In image registration we have available the input and output images but the specific

                                                                                                                                                                                        transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                        The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                        images

                                                                                                                                                                                        - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                        same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                        - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                        of time (satellite images)

                                                                                                                                                                                        Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                        corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                        image

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        How to select tie points

                                                                                                                                                                                        - interactively selecting them

                                                                                                                                                                                        - use of algorithms that try to detect these points

                                                                                                                                                                                        - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                        the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                        marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                        for establishing tie points

                                                                                                                                                                                        The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                        of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                        a bilinear approximation is given by

                                                                                                                                                                                        1 2 3 4

                                                                                                                                                                                        5 6 7 8

                                                                                                                                                                                        x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                        (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                        frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                        subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                        subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                        The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                        problem depend on the severity of the geometrical distortion

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Probabilistic Methods

                                                                                                                                                                                        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                        p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                        ( ) kk

                                                                                                                                                                                        np zM N

                                                                                                                                                                                        nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                        pixels in the image) 1

                                                                                                                                                                                        0( ) 1

                                                                                                                                                                                        L

                                                                                                                                                                                        kk

                                                                                                                                                                                        p z

                                                                                                                                                                                        The mean (average) intensity of an image is given by 1

                                                                                                                                                                                        0( )

                                                                                                                                                                                        L

                                                                                                                                                                                        k kk

                                                                                                                                                                                        m z p z

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        The variance of the intensities is 1

                                                                                                                                                                                        2 2

                                                                                                                                                                                        0( ) ( )

                                                                                                                                                                                        L

                                                                                                                                                                                        k kk

                                                                                                                                                                                        z m p z

                                                                                                                                                                                        The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                        measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                        ( ) is used

                                                                                                                                                                                        The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                        0( ) ( ) ( )

                                                                                                                                                                                        Ln

                                                                                                                                                                                        n k kk

                                                                                                                                                                                        z z m p z

                                                                                                                                                                                        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                        3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                        ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                        mean

                                                                                                                                                                                        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                        Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                        Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                        Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Intensity Transformations and Spatial Filtering

                                                                                                                                                                                        ( ) ( )g x y T f x y

                                                                                                                                                                                        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                        neighborhood of (x y)

                                                                                                                                                                                        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                        and much smaller in size than the image

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                        called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                        ( )s T r

                                                                                                                                                                                        s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                        Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                        darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                        is called contrast stretching

                                                                                                                                                                                        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                        thresholding function

                                                                                                                                                                                        Some Basic Intensity Transformation Functions

                                                                                                                                                                                        Image Negatives

                                                                                                                                                                                        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                        - equivalent of a photographic negative

                                                                                                                                                                                        - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                        image

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Original Negative image

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                        Some basic intensity transformation functions

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                        range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                        while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                        transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                        variations in pixel values

                                                                                                                                                                                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Power-Law (Gamma) Transformations

                                                                                                                                                                                        - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                        Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                        of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                        curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                        1c - identity transformation

                                                                                                                                                                                        A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                        power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                        gamma correction

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Piecewise-Linear Transformations Functions

                                                                                                                                                                                        Contrast stretching

                                                                                                                                                                                        - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                        intensity range of the recording tool or display device

                                                                                                                                                                                        a b c d Fig5

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        11

                                                                                                                                                                                        1

                                                                                                                                                                                        2 1 1 21 2

                                                                                                                                                                                        2 1 2 1

                                                                                                                                                                                        22

                                                                                                                                                                                        2

                                                                                                                                                                                        [0 ]

                                                                                                                                                                                        ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                        ( 1 ) [ 1]( 1 )

                                                                                                                                                                                        s r r rrs r r s r rT r r r r

                                                                                                                                                                                        r r r rs L r r r L

                                                                                                                                                                                        L r

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                        1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                        Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                        in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                        from their original range to the full range [0 L-1]

                                                                                                                                                                                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                        2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                        The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                        image of pollen magnified approximately 700 times

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Intensity-level slicing

                                                                                                                                                                                        - highlighting a specific range of intensities in an image

                                                                                                                                                                                        There are two approaches for intensity-level slicing

                                                                                                                                                                                        1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                        another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                        intensities in the image (Figure 311 (b))

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                        the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                        Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                        blockageshellip)

                                                                                                                                                                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                        image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                        Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Bit-plane slicing

                                                                                                                                                                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                        This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                        Week 1

                                                                                                                                                                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                        • DIP 1 2017
                                                                                                                                                                                        • DIP 02 (2017)

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          1 2

                                                                                                                                                                                          0 2 6 5 6 3max max 1 ( 1) max 2

                                                                                                                                                                                          2 3 4 7 2 4a f b f

                                                                                                                                                                                          0 2 6 51 max ( 1) max 3 ( 1)7 4

                                                                                                                                                                                          2 3 4 7

                                                                                                                                                                                          Arithmetic Operations in Image Processing

                                                                                                                                                                                          Let g(xy) denote a corrupted image formed by the addition of noise ( ) ( ) ( )g x y f x y x y

                                                                                                                                                                                          f(xy) ndash the noiseless image η(xy) the noise uncorrelated and has 0 average value

                                                                                                                                                                                          For a random variable z with mean m E[(z-m)2] is the variance (E( ∙ ) is the expected

                                                                                                                                                                                          value) The covariance of two random variables z1 and z2 is defined as E[(z1-m1) (z2-m2)]

                                                                                                                                                                                          The two random variables are uncorrelated when their covariance is 0

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                                          used in image enhancement)

                                                                                                                                                                                          1

                                                                                                                                                                                          1( ) ( )K

                                                                                                                                                                                          ii

                                                                                                                                                                                          g x y g x yK

                                                                                                                                                                                          If the noise satisfies the properties stated above we have

                                                                                                                                                                                          2 2( ) ( )

                                                                                                                                                                                          1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                                          ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                                          and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                                          the average image is

                                                                                                                                                                                          ( ) ( )1

                                                                                                                                                                                          g x y x yK

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                                          the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                                          means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                                          averaging process increases

                                                                                                                                                                                          An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                                          under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                                          virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                                          was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                                          64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                                          images respectively

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                                          100 noisy images

                                                                                                                                                                                          a b c d e f

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                                          images

                                                                                                                                                                                          (a) (b) (c)

                                                                                                                                                                                          Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                                          significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                                          Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                                          Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                                          images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                                          difference between images (a) and (b)

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                          h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                                          intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                                          source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                                          bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                                          anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                                          images after injection of the contrast medium

                                                                                                                                                                                          In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                                          Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                                          propagates through the various arteries in the area being observed

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          An important application of image multiplication (and division) is shading correction

                                                                                                                                                                                          Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                          f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                                          When the shading function is known

                                                                                                                                                                                          ( )( )( )

                                                                                                                                                                                          g x yf x yh x y

                                                                                                                                                                                          h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                                          approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                                          sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          (a) (b) (c)

                                                                                                                                                                                          Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                          operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                          1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                          image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                          (a) (b) (c)

                                                                                                                                                                                          Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                          min( )mf f f

                                                                                                                                                                                          0 ( 255)max( )

                                                                                                                                                                                          ms

                                                                                                                                                                                          m

                                                                                                                                                                                          ff K K K

                                                                                                                                                                                          f

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Spatial Operations

                                                                                                                                                                                          - are performed directly on the pixels of a given image

                                                                                                                                                                                          There are three categories of spatial operations

                                                                                                                                                                                          single-pixel operations

                                                                                                                                                                                          neighborhood operations

                                                                                                                                                                                          geometric spatial transformations

                                                                                                                                                                                          Single-pixel operations

                                                                                                                                                                                          - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                          where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                          corresponding pixel in the processed image

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Neighborhood operations

                                                                                                                                                                                          Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                          in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                          based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                          rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                          intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                          ( )

                                                                                                                                                                                          1( ) ( )xyr c S

                                                                                                                                                                                          g x y f r cm n

                                                                                                                                                                                          The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                          used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                          largest region of an image

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Geometric spatial transformations and image registration

                                                                                                                                                                                          - modify the spatial relationship between pixels in an image

                                                                                                                                                                                          - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                          printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                          predefined set of rules

                                                                                                                                                                                          A geometric transformation consists of 2 basic operations

                                                                                                                                                                                          1 a spatial transformation of coordinates

                                                                                                                                                                                          2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                          pixels

                                                                                                                                                                                          The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                          (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                          (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                          Affine transform

                                                                                                                                                                                          11 1211 21 31

                                                                                                                                                                                          21 2212 22 33

                                                                                                                                                                                          31 32

                                                                                                                                                                                          0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                          1

                                                                                                                                                                                          t tx t v t w t

                                                                                                                                                                                          x y v w T v w t ty t v t w t

                                                                                                                                                                                          t t

                                                                                                                                                                                          (AT)

                                                                                                                                                                                          This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                          on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                          result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                          scaling rotation and translation matrices from Table 1

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Affine transformations

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                          the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                          using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                          In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                          forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                          location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                          Problems

                                                                                                                                                                                          - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                          the same location in the output image

                                                                                                                                                                                          - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                          assignment)

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                          computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                          It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                          pixel value

                                                                                                                                                                                          Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                          numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Image registration ndash align two or more images of the same scene

                                                                                                                                                                                          In image registration we have available the input and output images but the specific

                                                                                                                                                                                          transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                          The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                          images

                                                                                                                                                                                          - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                          same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                          - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                          of time (satellite images)

                                                                                                                                                                                          Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                          corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                          image

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          How to select tie points

                                                                                                                                                                                          - interactively selecting them

                                                                                                                                                                                          - use of algorithms that try to detect these points

                                                                                                                                                                                          - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                          the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                          marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                          for establishing tie points

                                                                                                                                                                                          The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                          of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                          a bilinear approximation is given by

                                                                                                                                                                                          1 2 3 4

                                                                                                                                                                                          5 6 7 8

                                                                                                                                                                                          x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                          (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                          frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                          subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                          subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                          The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                          problem depend on the severity of the geometrical distortion

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Probabilistic Methods

                                                                                                                                                                                          zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                          p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                          ( ) kk

                                                                                                                                                                                          np zM N

                                                                                                                                                                                          nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                          pixels in the image) 1

                                                                                                                                                                                          0( ) 1

                                                                                                                                                                                          L

                                                                                                                                                                                          kk

                                                                                                                                                                                          p z

                                                                                                                                                                                          The mean (average) intensity of an image is given by 1

                                                                                                                                                                                          0( )

                                                                                                                                                                                          L

                                                                                                                                                                                          k kk

                                                                                                                                                                                          m z p z

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          The variance of the intensities is 1

                                                                                                                                                                                          2 2

                                                                                                                                                                                          0( ) ( )

                                                                                                                                                                                          L

                                                                                                                                                                                          k kk

                                                                                                                                                                                          z m p z

                                                                                                                                                                                          The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                          measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                          ( ) is used

                                                                                                                                                                                          The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                          0( ) ( ) ( )

                                                                                                                                                                                          Ln

                                                                                                                                                                                          n k kk

                                                                                                                                                                                          z z m p z

                                                                                                                                                                                          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                          3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                          ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                          mean

                                                                                                                                                                                          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                          Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                          Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                          Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Intensity Transformations and Spatial Filtering

                                                                                                                                                                                          ( ) ( )g x y T f x y

                                                                                                                                                                                          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                          neighborhood of (x y)

                                                                                                                                                                                          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                          and much smaller in size than the image

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                          called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                          ( )s T r

                                                                                                                                                                                          s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                          Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                          darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                          is called contrast stretching

                                                                                                                                                                                          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                          thresholding function

                                                                                                                                                                                          Some Basic Intensity Transformation Functions

                                                                                                                                                                                          Image Negatives

                                                                                                                                                                                          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                          - equivalent of a photographic negative

                                                                                                                                                                                          - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                          image

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Original Negative image

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                          Some basic intensity transformation functions

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                          range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                          while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                          transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                          variations in pixel values

                                                                                                                                                                                          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Power-Law (Gamma) Transformations

                                                                                                                                                                                          - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                          Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                          of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                          curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                          1c - identity transformation

                                                                                                                                                                                          A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                          power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                          gamma correction

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Piecewise-Linear Transformations Functions

                                                                                                                                                                                          Contrast stretching

                                                                                                                                                                                          - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                          intensity range of the recording tool or display device

                                                                                                                                                                                          a b c d Fig5

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          11

                                                                                                                                                                                          1

                                                                                                                                                                                          2 1 1 21 2

                                                                                                                                                                                          2 1 2 1

                                                                                                                                                                                          22

                                                                                                                                                                                          2

                                                                                                                                                                                          [0 ]

                                                                                                                                                                                          ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                          ( 1 ) [ 1]( 1 )

                                                                                                                                                                                          s r r rrs r r s r rT r r r r

                                                                                                                                                                                          r r r rs L r r r L

                                                                                                                                                                                          L r

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                          1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                          Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                          in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                          from their original range to the full range [0 L-1]

                                                                                                                                                                                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                          2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                          The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                          image of pollen magnified approximately 700 times

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Intensity-level slicing

                                                                                                                                                                                          - highlighting a specific range of intensities in an image

                                                                                                                                                                                          There are two approaches for intensity-level slicing

                                                                                                                                                                                          1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                          another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                          intensities in the image (Figure 311 (b))

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                          the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                          Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                          blockageshellip)

                                                                                                                                                                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                          image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                          Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Bit-plane slicing

                                                                                                                                                                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                          This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                          Week 1

                                                                                                                                                                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                          • DIP 1 2017
                                                                                                                                                                                          • DIP 02 (2017)

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Objective reduce noise by adding a set of noisy images ( )ig x y (technique frequently

                                                                                                                                                                                            used in image enhancement)

                                                                                                                                                                                            1

                                                                                                                                                                                            1( ) ( )K

                                                                                                                                                                                            ii

                                                                                                                                                                                            g x y g x yK

                                                                                                                                                                                            If the noise satisfies the properties stated above we have

                                                                                                                                                                                            2 2( ) ( )

                                                                                                                                                                                            1( ) ( ) g x y x yE g x y f x yK

                                                                                                                                                                                            ( ( ))E g x y is the expected value of g and and 2 2( ) ( )g x y x y are the variances of

                                                                                                                                                                                            and g respectively The standard deviation (square root of the variance) at any point in

                                                                                                                                                                                            the average image is

                                                                                                                                                                                            ( ) ( )1

                                                                                                                                                                                            g x y x yK

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                                            the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                                            means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                                            averaging process increases

                                                                                                                                                                                            An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                                            under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                                            virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                                            was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                                            64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                                            images respectively

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                                            100 noisy images

                                                                                                                                                                                            a b c d e f

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                                            images

                                                                                                                                                                                            (a) (b) (c)

                                                                                                                                                                                            Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                                            significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                                            Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                                            Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                                            images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                                            difference between images (a) and (b)

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                            h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                                            intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                                            source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                                            bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                                            anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                                            images after injection of the contrast medium

                                                                                                                                                                                            In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                                            Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                                            propagates through the various arteries in the area being observed

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            An important application of image multiplication (and division) is shading correction

                                                                                                                                                                                            Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                            f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                                            When the shading function is known

                                                                                                                                                                                            ( )( )( )

                                                                                                                                                                                            g x yf x yh x y

                                                                                                                                                                                            h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                                            approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                                            sensor is not available often the shading pattern can be estimated from the image

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                                                                                                                                                                                            Week 1

                                                                                                                                                                                            (a) (b) (c)

                                                                                                                                                                                            Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                            operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                            1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                            image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                            (a) (b) (c)

                                                                                                                                                                                            Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                            min( )mf f f

                                                                                                                                                                                            0 ( 255)max( )

                                                                                                                                                                                            ms

                                                                                                                                                                                            m

                                                                                                                                                                                            ff K K K

                                                                                                                                                                                            f

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Spatial Operations

                                                                                                                                                                                            - are performed directly on the pixels of a given image

                                                                                                                                                                                            There are three categories of spatial operations

                                                                                                                                                                                            single-pixel operations

                                                                                                                                                                                            neighborhood operations

                                                                                                                                                                                            geometric spatial transformations

                                                                                                                                                                                            Single-pixel operations

                                                                                                                                                                                            - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                            where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                            corresponding pixel in the processed image

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Neighborhood operations

                                                                                                                                                                                            Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                            in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                            based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                            rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                            intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                            ( )

                                                                                                                                                                                            1( ) ( )xyr c S

                                                                                                                                                                                            g x y f r cm n

                                                                                                                                                                                            The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                            used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                            largest region of an image

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Geometric spatial transformations and image registration

                                                                                                                                                                                            - modify the spatial relationship between pixels in an image

                                                                                                                                                                                            - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                            printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                            predefined set of rules

                                                                                                                                                                                            A geometric transformation consists of 2 basic operations

                                                                                                                                                                                            1 a spatial transformation of coordinates

                                                                                                                                                                                            2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                            pixels

                                                                                                                                                                                            The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                            (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                            (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                            Affine transform

                                                                                                                                                                                            11 1211 21 31

                                                                                                                                                                                            21 2212 22 33

                                                                                                                                                                                            31 32

                                                                                                                                                                                            0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                            1

                                                                                                                                                                                            t tx t v t w t

                                                                                                                                                                                            x y v w T v w t ty t v t w t

                                                                                                                                                                                            t t

                                                                                                                                                                                            (AT)

                                                                                                                                                                                            This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                            on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                            result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                            scaling rotation and translation matrices from Table 1

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Affine transformations

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                            the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                            using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                            In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                            forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                            location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                            Problems

                                                                                                                                                                                            - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                            the same location in the output image

                                                                                                                                                                                            - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                            assignment)

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                            computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                            It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                            pixel value

                                                                                                                                                                                            Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                            numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Image registration ndash align two or more images of the same scene

                                                                                                                                                                                            In image registration we have available the input and output images but the specific

                                                                                                                                                                                            transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                            The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                            images

                                                                                                                                                                                            - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                            same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                            - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                            of time (satellite images)

                                                                                                                                                                                            Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                            corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                            image

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            How to select tie points

                                                                                                                                                                                            - interactively selecting them

                                                                                                                                                                                            - use of algorithms that try to detect these points

                                                                                                                                                                                            - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                            the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                            marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                            for establishing tie points

                                                                                                                                                                                            The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                            of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                            a bilinear approximation is given by

                                                                                                                                                                                            1 2 3 4

                                                                                                                                                                                            5 6 7 8

                                                                                                                                                                                            x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                            (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                            frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                            subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                            subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                            The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                            problem depend on the severity of the geometrical distortion

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Probabilistic Methods

                                                                                                                                                                                            zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                            p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                            ( ) kk

                                                                                                                                                                                            np zM N

                                                                                                                                                                                            nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                            pixels in the image) 1

                                                                                                                                                                                            0( ) 1

                                                                                                                                                                                            L

                                                                                                                                                                                            kk

                                                                                                                                                                                            p z

                                                                                                                                                                                            The mean (average) intensity of an image is given by 1

                                                                                                                                                                                            0( )

                                                                                                                                                                                            L

                                                                                                                                                                                            k kk

                                                                                                                                                                                            m z p z

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            The variance of the intensities is 1

                                                                                                                                                                                            2 2

                                                                                                                                                                                            0( ) ( )

                                                                                                                                                                                            L

                                                                                                                                                                                            k kk

                                                                                                                                                                                            z m p z

                                                                                                                                                                                            The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                            measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                            ( ) is used

                                                                                                                                                                                            The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                            0( ) ( ) ( )

                                                                                                                                                                                            Ln

                                                                                                                                                                                            n k kk

                                                                                                                                                                                            z z m p z

                                                                                                                                                                                            ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                            3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                            ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                            mean

                                                                                                                                                                                            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                            Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                            Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                            Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Intensity Transformations and Spatial Filtering

                                                                                                                                                                                            ( ) ( )g x y T f x y

                                                                                                                                                                                            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                            neighborhood of (x y)

                                                                                                                                                                                            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                            and much smaller in size than the image

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                            called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                            ( )s T r

                                                                                                                                                                                            s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                            Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                            darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                            is called contrast stretching

                                                                                                                                                                                            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                            thresholding function

                                                                                                                                                                                            Some Basic Intensity Transformation Functions

                                                                                                                                                                                            Image Negatives

                                                                                                                                                                                            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                            - equivalent of a photographic negative

                                                                                                                                                                                            - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                            image

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Original Negative image

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                            Some basic intensity transformation functions

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                            range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                            while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                            transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                            variations in pixel values

                                                                                                                                                                                            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Power-Law (Gamma) Transformations

                                                                                                                                                                                            - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                            Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                            of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                            curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                            1c - identity transformation

                                                                                                                                                                                            A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                            power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                            gamma correction

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Piecewise-Linear Transformations Functions

                                                                                                                                                                                            Contrast stretching

                                                                                                                                                                                            - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                            intensity range of the recording tool or display device

                                                                                                                                                                                            a b c d Fig5

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            11

                                                                                                                                                                                            1

                                                                                                                                                                                            2 1 1 21 2

                                                                                                                                                                                            2 1 2 1

                                                                                                                                                                                            22

                                                                                                                                                                                            2

                                                                                                                                                                                            [0 ]

                                                                                                                                                                                            ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                            ( 1 ) [ 1]( 1 )

                                                                                                                                                                                            s r r rrs r r s r rT r r r r

                                                                                                                                                                                            r r r rs L r r r L

                                                                                                                                                                                            L r

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                            1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                            Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                            in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                            from their original range to the full range [0 L-1]

                                                                                                                                                                                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                            2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                            The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                            image of pollen magnified approximately 700 times

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Intensity-level slicing

                                                                                                                                                                                            - highlighting a specific range of intensities in an image

                                                                                                                                                                                            There are two approaches for intensity-level slicing

                                                                                                                                                                                            1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                            another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                            intensities in the image (Figure 311 (b))

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                            the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                            Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                            blockageshellip)

                                                                                                                                                                                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                            image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                            Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Bit-plane slicing

                                                                                                                                                                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                            This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                            Week 1

                                                                                                                                                                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                            • DIP 1 2017
                                                                                                                                                                                            • DIP 02 (2017)

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              As K increases the variability (as measured by the variance or the standard deviation) of

                                                                                                                                                                                              the pixel values at each location (x y) decreases Because ( ) ( )E g x y f x y this

                                                                                                                                                                                              means that ( )g x y approaches f(x y) as the number of noisy images used in the

                                                                                                                                                                                              averaging process increases

                                                                                                                                                                                              An important application of image averaging is in the field of astronomy where imaging

                                                                                                                                                                                              under very low light levels frequently causes sensor noise to render single images

                                                                                                                                                                                              virtually useless for analysis Figure 226(a) shows an 8-bit image in which corruption

                                                                                                                                                                                              was simulated by adding to it Gaussian noise with zero mean and a standard deviation of

                                                                                                                                                                                              64 intensity levels Figures 226(b)-(f) show the result of averaging 5 10 20 50 and 100

                                                                                                                                                                                              images respectively

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                                              100 noisy images

                                                                                                                                                                                              a b c d e f

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                                              images

                                                                                                                                                                                              (a) (b) (c)

                                                                                                                                                                                              Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                                              significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                                              Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                                              Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                                              images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                                              difference between images (a) and (b)

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                              h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                                              intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                                              source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                                              bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                                              anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                                              images after injection of the contrast medium

                                                                                                                                                                                              In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                                              Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                                              propagates through the various arteries in the area being observed

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              An important application of image multiplication (and division) is shading correction

                                                                                                                                                                                              Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                              f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                                              When the shading function is known

                                                                                                                                                                                              ( )( )( )

                                                                                                                                                                                              g x yf x yh x y

                                                                                                                                                                                              h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                                              approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                                              sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              (a) (b) (c)

                                                                                                                                                                                              Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                              operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                              1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                              image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                              (a) (b) (c)

                                                                                                                                                                                              Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                              min( )mf f f

                                                                                                                                                                                              0 ( 255)max( )

                                                                                                                                                                                              ms

                                                                                                                                                                                              m

                                                                                                                                                                                              ff K K K

                                                                                                                                                                                              f

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Spatial Operations

                                                                                                                                                                                              - are performed directly on the pixels of a given image

                                                                                                                                                                                              There are three categories of spatial operations

                                                                                                                                                                                              single-pixel operations

                                                                                                                                                                                              neighborhood operations

                                                                                                                                                                                              geometric spatial transformations

                                                                                                                                                                                              Single-pixel operations

                                                                                                                                                                                              - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                              where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                              corresponding pixel in the processed image

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Neighborhood operations

                                                                                                                                                                                              Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                              in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                              based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                              rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                              intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                              ( )

                                                                                                                                                                                              1( ) ( )xyr c S

                                                                                                                                                                                              g x y f r cm n

                                                                                                                                                                                              The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                              used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                              largest region of an image

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Geometric spatial transformations and image registration

                                                                                                                                                                                              - modify the spatial relationship between pixels in an image

                                                                                                                                                                                              - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                              printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                              predefined set of rules

                                                                                                                                                                                              A geometric transformation consists of 2 basic operations

                                                                                                                                                                                              1 a spatial transformation of coordinates

                                                                                                                                                                                              2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                              pixels

                                                                                                                                                                                              The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                              (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                              (x y) ndash pixel coordinates in the transformed image

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                                                                                                                                                                                              Week 1

                                                                                                                                                                                              [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                              Affine transform

                                                                                                                                                                                              11 1211 21 31

                                                                                                                                                                                              21 2212 22 33

                                                                                                                                                                                              31 32

                                                                                                                                                                                              0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                              1

                                                                                                                                                                                              t tx t v t w t

                                                                                                                                                                                              x y v w T v w t ty t v t w t

                                                                                                                                                                                              t t

                                                                                                                                                                                              (AT)

                                                                                                                                                                                              This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                              on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                              result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                              scaling rotation and translation matrices from Table 1

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Affine transformations

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                              the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                              using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                              In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                              forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                              location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                              Problems

                                                                                                                                                                                              - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                              the same location in the output image

                                                                                                                                                                                              - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                              assignment)

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                              computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                              It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                              pixel value

                                                                                                                                                                                              Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                              numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Image registration ndash align two or more images of the same scene

                                                                                                                                                                                              In image registration we have available the input and output images but the specific

                                                                                                                                                                                              transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                              The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                              images

                                                                                                                                                                                              - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                              same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                              - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                              of time (satellite images)

                                                                                                                                                                                              Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                              corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                              image

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              How to select tie points

                                                                                                                                                                                              - interactively selecting them

                                                                                                                                                                                              - use of algorithms that try to detect these points

                                                                                                                                                                                              - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                              the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                              marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                              for establishing tie points

                                                                                                                                                                                              The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                              of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                              a bilinear approximation is given by

                                                                                                                                                                                              1 2 3 4

                                                                                                                                                                                              5 6 7 8

                                                                                                                                                                                              x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                              (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                              frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                              subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                              subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                              The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                              problem depend on the severity of the geometrical distortion

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Probabilistic Methods

                                                                                                                                                                                              zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                              p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                              ( ) kk

                                                                                                                                                                                              np zM N

                                                                                                                                                                                              nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                              pixels in the image) 1

                                                                                                                                                                                              0( ) 1

                                                                                                                                                                                              L

                                                                                                                                                                                              kk

                                                                                                                                                                                              p z

                                                                                                                                                                                              The mean (average) intensity of an image is given by 1

                                                                                                                                                                                              0( )

                                                                                                                                                                                              L

                                                                                                                                                                                              k kk

                                                                                                                                                                                              m z p z

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              The variance of the intensities is 1

                                                                                                                                                                                              2 2

                                                                                                                                                                                              0( ) ( )

                                                                                                                                                                                              L

                                                                                                                                                                                              k kk

                                                                                                                                                                                              z m p z

                                                                                                                                                                                              The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                              measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                              ( ) is used

                                                                                                                                                                                              The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                              0( ) ( ) ( )

                                                                                                                                                                                              Ln

                                                                                                                                                                                              n k kk

                                                                                                                                                                                              z z m p z

                                                                                                                                                                                              ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                              3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                              ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                              mean

                                                                                                                                                                                              Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                              Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                              Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                              Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Intensity Transformations and Spatial Filtering

                                                                                                                                                                                              ( ) ( )g x y T f x y

                                                                                                                                                                                              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                              neighborhood of (x y)

                                                                                                                                                                                              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                              and much smaller in size than the image

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                              called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                              ( )s T r

                                                                                                                                                                                              s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                              Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                              darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                              is called contrast stretching

                                                                                                                                                                                              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                              thresholding function

                                                                                                                                                                                              Some Basic Intensity Transformation Functions

                                                                                                                                                                                              Image Negatives

                                                                                                                                                                                              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                              - equivalent of a photographic negative

                                                                                                                                                                                              - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                              image

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Original Negative image

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                              Some basic intensity transformation functions

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                              range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                              while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                              transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                              variations in pixel values

                                                                                                                                                                                              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Power-Law (Gamma) Transformations

                                                                                                                                                                                              - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                              Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                              of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                              curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                              1c - identity transformation

                                                                                                                                                                                              A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                              power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                              gamma correction

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Piecewise-Linear Transformations Functions

                                                                                                                                                                                              Contrast stretching

                                                                                                                                                                                              - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                              intensity range of the recording tool or display device

                                                                                                                                                                                              a b c d Fig5

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              11

                                                                                                                                                                                              1

                                                                                                                                                                                              2 1 1 21 2

                                                                                                                                                                                              2 1 2 1

                                                                                                                                                                                              22

                                                                                                                                                                                              2

                                                                                                                                                                                              [0 ]

                                                                                                                                                                                              ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                              ( 1 ) [ 1]( 1 )

                                                                                                                                                                                              s r r rrs r r s r rT r r r r

                                                                                                                                                                                              r r r rs L r r r L

                                                                                                                                                                                              L r

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                              1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                              Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                              in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                              from their original range to the full range [0 L-1]

                                                                                                                                                                                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                              2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                              The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                              image of pollen magnified approximately 700 times

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Intensity-level slicing

                                                                                                                                                                                              - highlighting a specific range of intensities in an image

                                                                                                                                                                                              There are two approaches for intensity-level slicing

                                                                                                                                                                                              1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                              another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                              intensities in the image (Figure 311 (b))

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                              the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                              Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                              blockageshellip)

                                                                                                                                                                                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                              image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                              Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Bit-plane slicing

                                                                                                                                                                                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                              This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                              Week 1

                                                                                                                                                                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                              • DIP 1 2017
                                                                                                                                                                                              • DIP 02 (2017)

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Fig 3 Image of Galaxy Pair NGC 3314 corrupted by additive Gaussian noise (left corner) Results of averaging 5 10 20 50

                                                                                                                                                                                                100 noisy images

                                                                                                                                                                                                a b c d e f

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                                                images

                                                                                                                                                                                                (a) (b) (c)

                                                                                                                                                                                                Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                                                significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                                                Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                                                Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                                                images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                                                difference between images (a) and (b)

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                                h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                                                intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                                                source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                                                bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                                                anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                                                images after injection of the contrast medium

                                                                                                                                                                                                In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                                                Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                                                propagates through the various arteries in the area being observed

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                An important application of image multiplication (and division) is shading correction

                                                                                                                                                                                                Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                                f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                                                When the shading function is known

                                                                                                                                                                                                ( )( )( )

                                                                                                                                                                                                g x yf x yh x y

                                                                                                                                                                                                h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                                                approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                                                sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                (a) (b) (c)

                                                                                                                                                                                                Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                                operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                                1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                                image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                                (a) (b) (c)

                                                                                                                                                                                                Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                                min( )mf f f

                                                                                                                                                                                                0 ( 255)max( )

                                                                                                                                                                                                ms

                                                                                                                                                                                                m

                                                                                                                                                                                                ff K K K

                                                                                                                                                                                                f

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Spatial Operations

                                                                                                                                                                                                - are performed directly on the pixels of a given image

                                                                                                                                                                                                There are three categories of spatial operations

                                                                                                                                                                                                single-pixel operations

                                                                                                                                                                                                neighborhood operations

                                                                                                                                                                                                geometric spatial transformations

                                                                                                                                                                                                Single-pixel operations

                                                                                                                                                                                                - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                                where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                                corresponding pixel in the processed image

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Neighborhood operations

                                                                                                                                                                                                Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                                in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                                based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                                rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                                intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                                ( )

                                                                                                                                                                                                1( ) ( )xyr c S

                                                                                                                                                                                                g x y f r cm n

                                                                                                                                                                                                The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                                used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                                largest region of an image

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Geometric spatial transformations and image registration

                                                                                                                                                                                                - modify the spatial relationship between pixels in an image

                                                                                                                                                                                                - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                                printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                                predefined set of rules

                                                                                                                                                                                                A geometric transformation consists of 2 basic operations

                                                                                                                                                                                                1 a spatial transformation of coordinates

                                                                                                                                                                                                2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                                pixels

                                                                                                                                                                                                The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                                (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                                (x y) ndash pixel coordinates in the transformed image

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                                                                                                                                                                                                Week 1

                                                                                                                                                                                                [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                                Affine transform

                                                                                                                                                                                                11 1211 21 31

                                                                                                                                                                                                21 2212 22 33

                                                                                                                                                                                                31 32

                                                                                                                                                                                                0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                                1

                                                                                                                                                                                                t tx t v t w t

                                                                                                                                                                                                x y v w T v w t ty t v t w t

                                                                                                                                                                                                t t

                                                                                                                                                                                                (AT)

                                                                                                                                                                                                This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                                on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                                result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                                scaling rotation and translation matrices from Table 1

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Affine transformations

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                                the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                                using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                                In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                                forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                                location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                                Problems

                                                                                                                                                                                                - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                                the same location in the output image

                                                                                                                                                                                                - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                                assignment)

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                pixel value

                                                                                                                                                                                                Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                In image registration we have available the input and output images but the specific

                                                                                                                                                                                                transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                images

                                                                                                                                                                                                - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                of time (satellite images)

                                                                                                                                                                                                Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                image

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                How to select tie points

                                                                                                                                                                                                - interactively selecting them

                                                                                                                                                                                                - use of algorithms that try to detect these points

                                                                                                                                                                                                - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                for establishing tie points

                                                                                                                                                                                                The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                a bilinear approximation is given by

                                                                                                                                                                                                1 2 3 4

                                                                                                                                                                                                5 6 7 8

                                                                                                                                                                                                x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Probabilistic Methods

                                                                                                                                                                                                zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                ( ) kk

                                                                                                                                                                                                np zM N

                                                                                                                                                                                                nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                pixels in the image) 1

                                                                                                                                                                                                0( ) 1

                                                                                                                                                                                                L

                                                                                                                                                                                                kk

                                                                                                                                                                                                p z

                                                                                                                                                                                                The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                0( )

                                                                                                                                                                                                L

                                                                                                                                                                                                k kk

                                                                                                                                                                                                m z p z

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                The variance of the intensities is 1

                                                                                                                                                                                                2 2

                                                                                                                                                                                                0( ) ( )

                                                                                                                                                                                                L

                                                                                                                                                                                                k kk

                                                                                                                                                                                                z m p z

                                                                                                                                                                                                The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                ( ) is used

                                                                                                                                                                                                The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                0( ) ( ) ( )

                                                                                                                                                                                                Ln

                                                                                                                                                                                                n k kk

                                                                                                                                                                                                z z m p z

                                                                                                                                                                                                ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                mean

                                                                                                                                                                                                Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                ( ) ( )g x y T f x y

                                                                                                                                                                                                f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                neighborhood of (x y)

                                                                                                                                                                                                - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                and much smaller in size than the image

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                ( )s T r

                                                                                                                                                                                                s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                is called contrast stretching

                                                                                                                                                                                                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                thresholding function

                                                                                                                                                                                                Some Basic Intensity Transformation Functions

                                                                                                                                                                                                Image Negatives

                                                                                                                                                                                                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                - equivalent of a photographic negative

                                                                                                                                                                                                - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                image

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Original Negative image

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                Some basic intensity transformation functions

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                variations in pixel values

                                                                                                                                                                                                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Power-Law (Gamma) Transformations

                                                                                                                                                                                                - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                1c - identity transformation

                                                                                                                                                                                                A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                gamma correction

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Piecewise-Linear Transformations Functions

                                                                                                                                                                                                Contrast stretching

                                                                                                                                                                                                - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                intensity range of the recording tool or display device

                                                                                                                                                                                                a b c d Fig5

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                11

                                                                                                                                                                                                1

                                                                                                                                                                                                2 1 1 21 2

                                                                                                                                                                                                2 1 2 1

                                                                                                                                                                                                22

                                                                                                                                                                                                2

                                                                                                                                                                                                [0 ]

                                                                                                                                                                                                ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                s r r rrs r r s r rT r r r r

                                                                                                                                                                                                r r r rs L r r r L

                                                                                                                                                                                                L r

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                from their original range to the full range [0 L-1]

                                                                                                                                                                                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                image of pollen magnified approximately 700 times

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Intensity-level slicing

                                                                                                                                                                                                - highlighting a specific range of intensities in an image

                                                                                                                                                                                                There are two approaches for intensity-level slicing

                                                                                                                                                                                                1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                intensities in the image (Figure 311 (b))

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                blockageshellip)

                                                                                                                                                                                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Bit-plane slicing

                                                                                                                                                                                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                Week 1

                                                                                                                                                                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                • DIP 1 2017
                                                                                                                                                                                                • DIP 02 (2017)

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  A frequent application of image subtraction is in the enhancement of differences between

                                                                                                                                                                                                  images

                                                                                                                                                                                                  (a) (b) (c)

                                                                                                                                                                                                  Fig 4 (a) Infrared image of Washington DC area (b) Image obtained from (a) by setting to zero the least

                                                                                                                                                                                                  significant bit of each pixel (c) the difference between the two images

                                                                                                                                                                                                  Figure 4(b) was obtained by setting to zero the least-significant bit of every pixel in

                                                                                                                                                                                                  Figure 4(a) The two images seem almost the same Figure 4(c) is the difference between

                                                                                                                                                                                                  images (a) and (b) Black (0) values in Figure (c) indicate locations where there is no

                                                                                                                                                                                                  difference between images (a) and (b)

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                                  h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                                                  intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                                                  source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                                                  bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                                                  anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                                                  images after injection of the contrast medium

                                                                                                                                                                                                  In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                                                  Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                                                  propagates through the various arteries in the area being observed

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  An important application of image multiplication (and division) is shading correction

                                                                                                                                                                                                  Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                                  f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                                                  When the shading function is known

                                                                                                                                                                                                  ( )( )( )

                                                                                                                                                                                                  g x yf x yh x y

                                                                                                                                                                                                  h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                                                  approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                                                  sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  (a) (b) (c)

                                                                                                                                                                                                  Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                                  operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                                  1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                                  image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                                  (a) (b) (c)

                                                                                                                                                                                                  Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                                  min( )mf f f

                                                                                                                                                                                                  0 ( 255)max( )

                                                                                                                                                                                                  ms

                                                                                                                                                                                                  m

                                                                                                                                                                                                  ff K K K

                                                                                                                                                                                                  f

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Spatial Operations

                                                                                                                                                                                                  - are performed directly on the pixels of a given image

                                                                                                                                                                                                  There are three categories of spatial operations

                                                                                                                                                                                                  single-pixel operations

                                                                                                                                                                                                  neighborhood operations

                                                                                                                                                                                                  geometric spatial transformations

                                                                                                                                                                                                  Single-pixel operations

                                                                                                                                                                                                  - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                                  where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                                  corresponding pixel in the processed image

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Neighborhood operations

                                                                                                                                                                                                  Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                                  in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                                  based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                                  rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                                  intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                                  ( )

                                                                                                                                                                                                  1( ) ( )xyr c S

                                                                                                                                                                                                  g x y f r cm n

                                                                                                                                                                                                  The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                                  used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                                  largest region of an image

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Geometric spatial transformations and image registration

                                                                                                                                                                                                  - modify the spatial relationship between pixels in an image

                                                                                                                                                                                                  - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                                  printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                                  predefined set of rules

                                                                                                                                                                                                  A geometric transformation consists of 2 basic operations

                                                                                                                                                                                                  1 a spatial transformation of coordinates

                                                                                                                                                                                                  2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                                  pixels

                                                                                                                                                                                                  The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                                  (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                                  (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                                  Affine transform

                                                                                                                                                                                                  11 1211 21 31

                                                                                                                                                                                                  21 2212 22 33

                                                                                                                                                                                                  31 32

                                                                                                                                                                                                  0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                                  1

                                                                                                                                                                                                  t tx t v t w t

                                                                                                                                                                                                  x y v w T v w t ty t v t w t

                                                                                                                                                                                                  t t

                                                                                                                                                                                                  (AT)

                                                                                                                                                                                                  This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                                  on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                                  result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                                  scaling rotation and translation matrices from Table 1

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Affine transformations

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                                  the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                                  using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                                  In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                                  forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                                  location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                                  Problems

                                                                                                                                                                                                  - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                                  the same location in the output image

                                                                                                                                                                                                  - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                                  assignment)

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                  computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                  It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                  pixel value

                                                                                                                                                                                                  Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                  numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                  In image registration we have available the input and output images but the specific

                                                                                                                                                                                                  transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                  The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                  images

                                                                                                                                                                                                  - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                  same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                  - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                  of time (satellite images)

                                                                                                                                                                                                  Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                  corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                  image

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  How to select tie points

                                                                                                                                                                                                  - interactively selecting them

                                                                                                                                                                                                  - use of algorithms that try to detect these points

                                                                                                                                                                                                  - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                  the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                  marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                  for establishing tie points

                                                                                                                                                                                                  The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                  of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                  a bilinear approximation is given by

                                                                                                                                                                                                  1 2 3 4

                                                                                                                                                                                                  5 6 7 8

                                                                                                                                                                                                  x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                  (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                  frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                  subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                  subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                  The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                  problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Probabilistic Methods

                                                                                                                                                                                                  zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                  p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                  ( ) kk

                                                                                                                                                                                                  np zM N

                                                                                                                                                                                                  nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                  pixels in the image) 1

                                                                                                                                                                                                  0( ) 1

                                                                                                                                                                                                  L

                                                                                                                                                                                                  kk

                                                                                                                                                                                                  p z

                                                                                                                                                                                                  The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                  0( )

                                                                                                                                                                                                  L

                                                                                                                                                                                                  k kk

                                                                                                                                                                                                  m z p z

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  The variance of the intensities is 1

                                                                                                                                                                                                  2 2

                                                                                                                                                                                                  0( ) ( )

                                                                                                                                                                                                  L

                                                                                                                                                                                                  k kk

                                                                                                                                                                                                  z m p z

                                                                                                                                                                                                  The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                  measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                  ( ) is used

                                                                                                                                                                                                  The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                  0( ) ( ) ( )

                                                                                                                                                                                                  Ln

                                                                                                                                                                                                  n k kk

                                                                                                                                                                                                  z z m p z

                                                                                                                                                                                                  ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                  3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                  ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                  mean

                                                                                                                                                                                                  Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                  Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                  Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                  Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                  ( ) ( )g x y T f x y

                                                                                                                                                                                                  f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                  neighborhood of (x y)

                                                                                                                                                                                                  - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                  and much smaller in size than the image

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                  called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                  ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                  ( )s T r

                                                                                                                                                                                                  s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                  Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                  darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                  is called contrast stretching

                                                                                                                                                                                                  Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                  thresholding function

                                                                                                                                                                                                  Some Basic Intensity Transformation Functions

                                                                                                                                                                                                  Image Negatives

                                                                                                                                                                                                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                  - equivalent of a photographic negative

                                                                                                                                                                                                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                  image

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Original Negative image

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                  Some basic intensity transformation functions

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                  range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                  while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                  transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                  variations in pixel values

                                                                                                                                                                                                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Power-Law (Gamma) Transformations

                                                                                                                                                                                                  - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                  Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                  of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                  curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                  1c - identity transformation

                                                                                                                                                                                                  A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                  power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                  gamma correction

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Piecewise-Linear Transformations Functions

                                                                                                                                                                                                  Contrast stretching

                                                                                                                                                                                                  - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                  intensity range of the recording tool or display device

                                                                                                                                                                                                  a b c d Fig5

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  11

                                                                                                                                                                                                  1

                                                                                                                                                                                                  2 1 1 21 2

                                                                                                                                                                                                  2 1 2 1

                                                                                                                                                                                                  22

                                                                                                                                                                                                  2

                                                                                                                                                                                                  [0 ]

                                                                                                                                                                                                  ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                  ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                  s r r rrs r r s r rT r r r r

                                                                                                                                                                                                  r r r rs L r r r L

                                                                                                                                                                                                  L r

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                  1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                  Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                  in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                  from their original range to the full range [0 L-1]

                                                                                                                                                                                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                  2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                  The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                  image of pollen magnified approximately 700 times

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Intensity-level slicing

                                                                                                                                                                                                  - highlighting a specific range of intensities in an image

                                                                                                                                                                                                  There are two approaches for intensity-level slicing

                                                                                                                                                                                                  1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                  another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                  intensities in the image (Figure 311 (b))

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                  the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                  Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                  blockageshellip)

                                                                                                                                                                                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                  image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                  Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Bit-plane slicing

                                                                                                                                                                                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                  This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                  • DIP 1 2017
                                                                                                                                                                                                  • DIP 02 (2017)

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Mask mode radiography ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                                    h(x y) the mask is an X-ray image of a region of a patientrsquos body captured by an

                                                                                                                                                                                                    intensified TV camera (instead of traditional X-ray film) located opposite an X-ray

                                                                                                                                                                                                    source The procedure consists of injecting an X-ray contrast medium into the patientrsquos

                                                                                                                                                                                                    bloodstream taking a series of images called live images (denoted f(x y)) of the same

                                                                                                                                                                                                    anatomical region as h(x y) and subtracting the mask from the series of incoming live

                                                                                                                                                                                                    images after injection of the contrast medium

                                                                                                                                                                                                    In g(x y) we can find the differences between h and f as enhanced detail

                                                                                                                                                                                                    Images being captured at TV rates we obtain a movie showing how the contrast medium

                                                                                                                                                                                                    propagates through the various arteries in the area being observed

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    An important application of image multiplication (and division) is shading correction

                                                                                                                                                                                                    Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                                    f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                                                    When the shading function is known

                                                                                                                                                                                                    ( )( )( )

                                                                                                                                                                                                    g x yf x yh x y

                                                                                                                                                                                                    h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                                                    approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                                                    sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    (a) (b) (c)

                                                                                                                                                                                                    Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                                    operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                                    1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                                    image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                                    (a) (b) (c)

                                                                                                                                                                                                    Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                                    min( )mf f f

                                                                                                                                                                                                    0 ( 255)max( )

                                                                                                                                                                                                    ms

                                                                                                                                                                                                    m

                                                                                                                                                                                                    ff K K K

                                                                                                                                                                                                    f

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Spatial Operations

                                                                                                                                                                                                    - are performed directly on the pixels of a given image

                                                                                                                                                                                                    There are three categories of spatial operations

                                                                                                                                                                                                    single-pixel operations

                                                                                                                                                                                                    neighborhood operations

                                                                                                                                                                                                    geometric spatial transformations

                                                                                                                                                                                                    Single-pixel operations

                                                                                                                                                                                                    - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                                    where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                                    corresponding pixel in the processed image

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Neighborhood operations

                                                                                                                                                                                                    Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                                    in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                                    based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                                    rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                                    intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                                    ( )

                                                                                                                                                                                                    1( ) ( )xyr c S

                                                                                                                                                                                                    g x y f r cm n

                                                                                                                                                                                                    The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                                    used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                                    largest region of an image

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Geometric spatial transformations and image registration

                                                                                                                                                                                                    - modify the spatial relationship between pixels in an image

                                                                                                                                                                                                    - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                                    printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                                    predefined set of rules

                                                                                                                                                                                                    A geometric transformation consists of 2 basic operations

                                                                                                                                                                                                    1 a spatial transformation of coordinates

                                                                                                                                                                                                    2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                                    pixels

                                                                                                                                                                                                    The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                                    (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                                    (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                                    Affine transform

                                                                                                                                                                                                    11 1211 21 31

                                                                                                                                                                                                    21 2212 22 33

                                                                                                                                                                                                    31 32

                                                                                                                                                                                                    0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                                    1

                                                                                                                                                                                                    t tx t v t w t

                                                                                                                                                                                                    x y v w T v w t ty t v t w t

                                                                                                                                                                                                    t t

                                                                                                                                                                                                    (AT)

                                                                                                                                                                                                    This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                                    on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                                    result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                                    scaling rotation and translation matrices from Table 1

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Affine transformations

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                                    the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                                    using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                                    In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                                    forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                                    location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                                    Problems

                                                                                                                                                                                                    - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                                    the same location in the output image

                                                                                                                                                                                                    - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                                    assignment)

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                    computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                    It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                    pixel value

                                                                                                                                                                                                    Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                    numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                    In image registration we have available the input and output images but the specific

                                                                                                                                                                                                    transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                    The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                    images

                                                                                                                                                                                                    - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                    same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                    - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                    of time (satellite images)

                                                                                                                                                                                                    Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                    corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                    image

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    How to select tie points

                                                                                                                                                                                                    - interactively selecting them

                                                                                                                                                                                                    - use of algorithms that try to detect these points

                                                                                                                                                                                                    - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                    the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                    marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                    for establishing tie points

                                                                                                                                                                                                    The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                    of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                    a bilinear approximation is given by

                                                                                                                                                                                                    1 2 3 4

                                                                                                                                                                                                    5 6 7 8

                                                                                                                                                                                                    x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                    (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                    frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                    subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                    subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                    The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                    problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Probabilistic Methods

                                                                                                                                                                                                    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                    p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                    ( ) kk

                                                                                                                                                                                                    np zM N

                                                                                                                                                                                                    nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                    pixels in the image) 1

                                                                                                                                                                                                    0( ) 1

                                                                                                                                                                                                    L

                                                                                                                                                                                                    kk

                                                                                                                                                                                                    p z

                                                                                                                                                                                                    The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                    0( )

                                                                                                                                                                                                    L

                                                                                                                                                                                                    k kk

                                                                                                                                                                                                    m z p z

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    The variance of the intensities is 1

                                                                                                                                                                                                    2 2

                                                                                                                                                                                                    0( ) ( )

                                                                                                                                                                                                    L

                                                                                                                                                                                                    k kk

                                                                                                                                                                                                    z m p z

                                                                                                                                                                                                    The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                    measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                    ( ) is used

                                                                                                                                                                                                    The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                    0( ) ( ) ( )

                                                                                                                                                                                                    Ln

                                                                                                                                                                                                    n k kk

                                                                                                                                                                                                    z z m p z

                                                                                                                                                                                                    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                    3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                    ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                    mean

                                                                                                                                                                                                    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                    Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                    Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                    Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                    ( ) ( )g x y T f x y

                                                                                                                                                                                                    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                    neighborhood of (x y)

                                                                                                                                                                                                    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                    and much smaller in size than the image

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                    called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                    ( )s T r

                                                                                                                                                                                                    s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                    Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                    darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                    is called contrast stretching

                                                                                                                                                                                                    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                    thresholding function

                                                                                                                                                                                                    Some Basic Intensity Transformation Functions

                                                                                                                                                                                                    Image Negatives

                                                                                                                                                                                                    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                    - equivalent of a photographic negative

                                                                                                                                                                                                    - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                    image

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Original Negative image

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                    Some basic intensity transformation functions

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                    range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                    while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                    transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                    variations in pixel values

                                                                                                                                                                                                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Power-Law (Gamma) Transformations

                                                                                                                                                                                                    - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                    Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                    of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                    curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                    1c - identity transformation

                                                                                                                                                                                                    A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                    power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                    gamma correction

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Piecewise-Linear Transformations Functions

                                                                                                                                                                                                    Contrast stretching

                                                                                                                                                                                                    - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                    intensity range of the recording tool or display device

                                                                                                                                                                                                    a b c d Fig5

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    11

                                                                                                                                                                                                    1

                                                                                                                                                                                                    2 1 1 21 2

                                                                                                                                                                                                    2 1 2 1

                                                                                                                                                                                                    22

                                                                                                                                                                                                    2

                                                                                                                                                                                                    [0 ]

                                                                                                                                                                                                    ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                    ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                    s r r rrs r r s r rT r r r r

                                                                                                                                                                                                    r r r rs L r r r L

                                                                                                                                                                                                    L r

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                    1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                    Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                    in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                    from their original range to the full range [0 L-1]

                                                                                                                                                                                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                    2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                    The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                    image of pollen magnified approximately 700 times

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Intensity-level slicing

                                                                                                                                                                                                    - highlighting a specific range of intensities in an image

                                                                                                                                                                                                    There are two approaches for intensity-level slicing

                                                                                                                                                                                                    1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                    another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                    intensities in the image (Figure 311 (b))

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                    the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                    Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                    blockageshellip)

                                                                                                                                                                                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                    image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                    Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Bit-plane slicing

                                                                                                                                                                                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                    This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                    • DIP 1 2017
                                                                                                                                                                                                    • DIP 02 (2017)

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      a b c d Fig 5 ndash Angiography ndash subtraction example (a) ndash mask image (b) ndash live image (c) ndash difference between (a) and (b) (d) - image (c) enhanced

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      An important application of image multiplication (and division) is shading correction

                                                                                                                                                                                                      Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                                      f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                                                      When the shading function is known

                                                                                                                                                                                                      ( )( )( )

                                                                                                                                                                                                      g x yf x yh x y

                                                                                                                                                                                                      h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                                                      approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                                                      sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      (a) (b) (c)

                                                                                                                                                                                                      Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                                      operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                                      1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                                      image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                                      (a) (b) (c)

                                                                                                                                                                                                      Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                                      min( )mf f f

                                                                                                                                                                                                      0 ( 255)max( )

                                                                                                                                                                                                      ms

                                                                                                                                                                                                      m

                                                                                                                                                                                                      ff K K K

                                                                                                                                                                                                      f

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Spatial Operations

                                                                                                                                                                                                      - are performed directly on the pixels of a given image

                                                                                                                                                                                                      There are three categories of spatial operations

                                                                                                                                                                                                      single-pixel operations

                                                                                                                                                                                                      neighborhood operations

                                                                                                                                                                                                      geometric spatial transformations

                                                                                                                                                                                                      Single-pixel operations

                                                                                                                                                                                                      - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                                      where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                                      corresponding pixel in the processed image

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Neighborhood operations

                                                                                                                                                                                                      Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                                      in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                                      based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                                      rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                                      intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                                      ( )

                                                                                                                                                                                                      1( ) ( )xyr c S

                                                                                                                                                                                                      g x y f r cm n

                                                                                                                                                                                                      The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                                      used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                                      largest region of an image

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Geometric spatial transformations and image registration

                                                                                                                                                                                                      - modify the spatial relationship between pixels in an image

                                                                                                                                                                                                      - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                                      printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                                      predefined set of rules

                                                                                                                                                                                                      A geometric transformation consists of 2 basic operations

                                                                                                                                                                                                      1 a spatial transformation of coordinates

                                                                                                                                                                                                      2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                                      pixels

                                                                                                                                                                                                      The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                                      (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                                      (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                                      Affine transform

                                                                                                                                                                                                      11 1211 21 31

                                                                                                                                                                                                      21 2212 22 33

                                                                                                                                                                                                      31 32

                                                                                                                                                                                                      0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                                      1

                                                                                                                                                                                                      t tx t v t w t

                                                                                                                                                                                                      x y v w T v w t ty t v t w t

                                                                                                                                                                                                      t t

                                                                                                                                                                                                      (AT)

                                                                                                                                                                                                      This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                                      on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                                      result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                                      scaling rotation and translation matrices from Table 1

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Affine transformations

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                                      the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                                      using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                                      In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                                      forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                                      location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                                      Problems

                                                                                                                                                                                                      - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                                      the same location in the output image

                                                                                                                                                                                                      - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                                      assignment)

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                      computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                      It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                      pixel value

                                                                                                                                                                                                      Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                      numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                      In image registration we have available the input and output images but the specific

                                                                                                                                                                                                      transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                      The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                      images

                                                                                                                                                                                                      - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                      same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                      - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                      of time (satellite images)

                                                                                                                                                                                                      Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                      corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                      image

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      How to select tie points

                                                                                                                                                                                                      - interactively selecting them

                                                                                                                                                                                                      - use of algorithms that try to detect these points

                                                                                                                                                                                                      - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                      the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                      marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                      for establishing tie points

                                                                                                                                                                                                      The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                      of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                      a bilinear approximation is given by

                                                                                                                                                                                                      1 2 3 4

                                                                                                                                                                                                      5 6 7 8

                                                                                                                                                                                                      x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                      (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                      frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                      subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                      subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                      The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                      problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Probabilistic Methods

                                                                                                                                                                                                      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                      p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                      ( ) kk

                                                                                                                                                                                                      np zM N

                                                                                                                                                                                                      nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                      pixels in the image) 1

                                                                                                                                                                                                      0( ) 1

                                                                                                                                                                                                      L

                                                                                                                                                                                                      kk

                                                                                                                                                                                                      p z

                                                                                                                                                                                                      The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                      0( )

                                                                                                                                                                                                      L

                                                                                                                                                                                                      k kk

                                                                                                                                                                                                      m z p z

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      The variance of the intensities is 1

                                                                                                                                                                                                      2 2

                                                                                                                                                                                                      0( ) ( )

                                                                                                                                                                                                      L

                                                                                                                                                                                                      k kk

                                                                                                                                                                                                      z m p z

                                                                                                                                                                                                      The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                      measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                      ( ) is used

                                                                                                                                                                                                      The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                      0( ) ( ) ( )

                                                                                                                                                                                                      Ln

                                                                                                                                                                                                      n k kk

                                                                                                                                                                                                      z z m p z

                                                                                                                                                                                                      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                      3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                      ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                      mean

                                                                                                                                                                                                      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                      Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                      Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                      Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                      ( ) ( )g x y T f x y

                                                                                                                                                                                                      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                      neighborhood of (x y)

                                                                                                                                                                                                      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                      and much smaller in size than the image

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                      called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                      ( )s T r

                                                                                                                                                                                                      s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                      Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                      darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                      is called contrast stretching

                                                                                                                                                                                                      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                      thresholding function

                                                                                                                                                                                                      Some Basic Intensity Transformation Functions

                                                                                                                                                                                                      Image Negatives

                                                                                                                                                                                                      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                      - equivalent of a photographic negative

                                                                                                                                                                                                      - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                      image

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Original Negative image

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                      Some basic intensity transformation functions

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                      range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                      while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                      transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                      variations in pixel values

                                                                                                                                                                                                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Power-Law (Gamma) Transformations

                                                                                                                                                                                                      - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                      Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                      of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                      curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                      1c - identity transformation

                                                                                                                                                                                                      A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                      power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                      gamma correction

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Piecewise-Linear Transformations Functions

                                                                                                                                                                                                      Contrast stretching

                                                                                                                                                                                                      - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                      intensity range of the recording tool or display device

                                                                                                                                                                                                      a b c d Fig5

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      11

                                                                                                                                                                                                      1

                                                                                                                                                                                                      2 1 1 21 2

                                                                                                                                                                                                      2 1 2 1

                                                                                                                                                                                                      22

                                                                                                                                                                                                      2

                                                                                                                                                                                                      [0 ]

                                                                                                                                                                                                      ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                      ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                      s r r rrs r r s r rT r r r r

                                                                                                                                                                                                      r r r rs L r r r L

                                                                                                                                                                                                      L r

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                      1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                      Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                      in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                      from their original range to the full range [0 L-1]

                                                                                                                                                                                                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                      2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                      The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                      image of pollen magnified approximately 700 times

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Intensity-level slicing

                                                                                                                                                                                                      - highlighting a specific range of intensities in an image

                                                                                                                                                                                                      There are two approaches for intensity-level slicing

                                                                                                                                                                                                      1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                      another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                      intensities in the image (Figure 311 (b))

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                      the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                      Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                      blockageshellip)

                                                                                                                                                                                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                      image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                      Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Bit-plane slicing

                                                                                                                                                                                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                      This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                      • DIP 1 2017
                                                                                                                                                                                                      • DIP 02 (2017)

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        An important application of image multiplication (and division) is shading correction

                                                                                                                                                                                                        Suppose that an imaging sensor produces images in the form ( ) ( ) ( )g x y f x y h x y

                                                                                                                                                                                                        f(x y) ndash the ldquoperfect imagerdquo h(x y) ndash the shading function

                                                                                                                                                                                                        When the shading function is known

                                                                                                                                                                                                        ( )( )( )

                                                                                                                                                                                                        g x yf x yh x y

                                                                                                                                                                                                        h(x y) is unknown but we have access to the imaging system we can obtain an

                                                                                                                                                                                                        approximation to the shading function by imaging a target of constant intensity When the

                                                                                                                                                                                                        sensor is not available often the shading pattern can be estimated from the image

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        (a) (b) (c)

                                                                                                                                                                                                        Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                                        operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                                        1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                                        image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                                        (a) (b) (c)

                                                                                                                                                                                                        Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                                        min( )mf f f

                                                                                                                                                                                                        0 ( 255)max( )

                                                                                                                                                                                                        ms

                                                                                                                                                                                                        m

                                                                                                                                                                                                        ff K K K

                                                                                                                                                                                                        f

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Spatial Operations

                                                                                                                                                                                                        - are performed directly on the pixels of a given image

                                                                                                                                                                                                        There are three categories of spatial operations

                                                                                                                                                                                                        single-pixel operations

                                                                                                                                                                                                        neighborhood operations

                                                                                                                                                                                                        geometric spatial transformations

                                                                                                                                                                                                        Single-pixel operations

                                                                                                                                                                                                        - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                                        where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                                        corresponding pixel in the processed image

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Neighborhood operations

                                                                                                                                                                                                        Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                                        in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                                        based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                                        rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                                        intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                                        ( )

                                                                                                                                                                                                        1( ) ( )xyr c S

                                                                                                                                                                                                        g x y f r cm n

                                                                                                                                                                                                        The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                                        used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                                        largest region of an image

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Geometric spatial transformations and image registration

                                                                                                                                                                                                        - modify the spatial relationship between pixels in an image

                                                                                                                                                                                                        - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                                        printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                                        predefined set of rules

                                                                                                                                                                                                        A geometric transformation consists of 2 basic operations

                                                                                                                                                                                                        1 a spatial transformation of coordinates

                                                                                                                                                                                                        2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                                        pixels

                                                                                                                                                                                                        The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                                        (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                                        (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                                        Affine transform

                                                                                                                                                                                                        11 1211 21 31

                                                                                                                                                                                                        21 2212 22 33

                                                                                                                                                                                                        31 32

                                                                                                                                                                                                        0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                                        1

                                                                                                                                                                                                        t tx t v t w t

                                                                                                                                                                                                        x y v w T v w t ty t v t w t

                                                                                                                                                                                                        t t

                                                                                                                                                                                                        (AT)

                                                                                                                                                                                                        This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                                        on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                                        result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                                        scaling rotation and translation matrices from Table 1

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Affine transformations

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                                        the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                                        using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                                        In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                                        forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                                        location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                                        Problems

                                                                                                                                                                                                        - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                                        the same location in the output image

                                                                                                                                                                                                        - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                                        assignment)

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                        computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                        It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                        pixel value

                                                                                                                                                                                                        Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                        numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                        In image registration we have available the input and output images but the specific

                                                                                                                                                                                                        transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                        The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                        images

                                                                                                                                                                                                        - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                        same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                        - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                        of time (satellite images)

                                                                                                                                                                                                        Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                        corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                        image

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        How to select tie points

                                                                                                                                                                                                        - interactively selecting them

                                                                                                                                                                                                        - use of algorithms that try to detect these points

                                                                                                                                                                                                        - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                        the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                        marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                        for establishing tie points

                                                                                                                                                                                                        The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                        of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                        a bilinear approximation is given by

                                                                                                                                                                                                        1 2 3 4

                                                                                                                                                                                                        5 6 7 8

                                                                                                                                                                                                        x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                        (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                        frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                        subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                        subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                        The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                        problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Probabilistic Methods

                                                                                                                                                                                                        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                        p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                        ( ) kk

                                                                                                                                                                                                        np zM N

                                                                                                                                                                                                        nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                        pixels in the image) 1

                                                                                                                                                                                                        0( ) 1

                                                                                                                                                                                                        L

                                                                                                                                                                                                        kk

                                                                                                                                                                                                        p z

                                                                                                                                                                                                        The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                        0( )

                                                                                                                                                                                                        L

                                                                                                                                                                                                        k kk

                                                                                                                                                                                                        m z p z

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        The variance of the intensities is 1

                                                                                                                                                                                                        2 2

                                                                                                                                                                                                        0( ) ( )

                                                                                                                                                                                                        L

                                                                                                                                                                                                        k kk

                                                                                                                                                                                                        z m p z

                                                                                                                                                                                                        The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                        measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                        ( ) is used

                                                                                                                                                                                                        The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                        0( ) ( ) ( )

                                                                                                                                                                                                        Ln

                                                                                                                                                                                                        n k kk

                                                                                                                                                                                                        z z m p z

                                                                                                                                                                                                        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                        3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                        ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                        mean

                                                                                                                                                                                                        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                        Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                        Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                        Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                        ( ) ( )g x y T f x y

                                                                                                                                                                                                        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                        neighborhood of (x y)

                                                                                                                                                                                                        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                        and much smaller in size than the image

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                        called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                        ( )s T r

                                                                                                                                                                                                        s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                        Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                        darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                        is called contrast stretching

                                                                                                                                                                                                        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                        thresholding function

                                                                                                                                                                                                        Some Basic Intensity Transformation Functions

                                                                                                                                                                                                        Image Negatives

                                                                                                                                                                                                        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                        - equivalent of a photographic negative

                                                                                                                                                                                                        - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                        image

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Original Negative image

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                        Some basic intensity transformation functions

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                        range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                        while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                        transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                        variations in pixel values

                                                                                                                                                                                                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Power-Law (Gamma) Transformations

                                                                                                                                                                                                        - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                        Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                        of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                        curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                        1c - identity transformation

                                                                                                                                                                                                        A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                        power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                        gamma correction

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Piecewise-Linear Transformations Functions

                                                                                                                                                                                                        Contrast stretching

                                                                                                                                                                                                        - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                        intensity range of the recording tool or display device

                                                                                                                                                                                                        a b c d Fig5

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        11

                                                                                                                                                                                                        1

                                                                                                                                                                                                        2 1 1 21 2

                                                                                                                                                                                                        2 1 2 1

                                                                                                                                                                                                        22

                                                                                                                                                                                                        2

                                                                                                                                                                                                        [0 ]

                                                                                                                                                                                                        ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                        ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                        s r r rrs r r s r rT r r r r

                                                                                                                                                                                                        r r r rs L r r r L

                                                                                                                                                                                                        L r

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                        1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                        Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                        in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                        from their original range to the full range [0 L-1]

                                                                                                                                                                                                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                        2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                        The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                        image of pollen magnified approximately 700 times

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Intensity-level slicing

                                                                                                                                                                                                        - highlighting a specific range of intensities in an image

                                                                                                                                                                                                        There are two approaches for intensity-level slicing

                                                                                                                                                                                                        1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                        another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                        intensities in the image (Figure 311 (b))

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                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                        the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                        Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                        blockageshellip)

                                                                                                                                                                                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                        image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                        Fig 6 - Aortic angiogram and intensity sliced versions

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                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Bit-plane slicing

                                                                                                                                                                                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                        This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                        • DIP 1 2017
                                                                                                                                                                                                        • DIP 02 (2017)

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                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          (a) (b) (c)

                                                                                                                                                                                                          Fig 6 Shading correction (a) ndash Shaded image of a tungsten filament magnified 130 times (b) - shading pattern (c) corrected image

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                                          operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                                          1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                                          image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                                          (a) (b) (c)

                                                                                                                                                                                                          Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                                          min( )mf f f

                                                                                                                                                                                                          0 ( 255)max( )

                                                                                                                                                                                                          ms

                                                                                                                                                                                                          m

                                                                                                                                                                                                          ff K K K

                                                                                                                                                                                                          f

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                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Spatial Operations

                                                                                                                                                                                                          - are performed directly on the pixels of a given image

                                                                                                                                                                                                          There are three categories of spatial operations

                                                                                                                                                                                                          single-pixel operations

                                                                                                                                                                                                          neighborhood operations

                                                                                                                                                                                                          geometric spatial transformations

                                                                                                                                                                                                          Single-pixel operations

                                                                                                                                                                                                          - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                                          where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                                          corresponding pixel in the processed image

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Neighborhood operations

                                                                                                                                                                                                          Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                                          in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                                          based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                                          rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                                          intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                                          ( )

                                                                                                                                                                                                          1( ) ( )xyr c S

                                                                                                                                                                                                          g x y f r cm n

                                                                                                                                                                                                          The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                                          used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                                          largest region of an image

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Geometric spatial transformations and image registration

                                                                                                                                                                                                          - modify the spatial relationship between pixels in an image

                                                                                                                                                                                                          - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                                          printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                                          predefined set of rules

                                                                                                                                                                                                          A geometric transformation consists of 2 basic operations

                                                                                                                                                                                                          1 a spatial transformation of coordinates

                                                                                                                                                                                                          2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                                          pixels

                                                                                                                                                                                                          The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                                          (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                                          (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                                          Affine transform

                                                                                                                                                                                                          11 1211 21 31

                                                                                                                                                                                                          21 2212 22 33

                                                                                                                                                                                                          31 32

                                                                                                                                                                                                          0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                                          1

                                                                                                                                                                                                          t tx t v t w t

                                                                                                                                                                                                          x y v w T v w t ty t v t w t

                                                                                                                                                                                                          t t

                                                                                                                                                                                                          (AT)

                                                                                                                                                                                                          This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                                          on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                                          result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                                          scaling rotation and translation matrices from Table 1

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Affine transformations

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                                          the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                                          using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                                          In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                                          forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                                          location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                                          Problems

                                                                                                                                                                                                          - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                                          the same location in the output image

                                                                                                                                                                                                          - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                                          assignment)

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                          computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                          It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                          pixel value

                                                                                                                                                                                                          Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                          numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                          In image registration we have available the input and output images but the specific

                                                                                                                                                                                                          transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                          The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                          images

                                                                                                                                                                                                          - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                          same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                          - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                          of time (satellite images)

                                                                                                                                                                                                          Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                          corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                          image

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          How to select tie points

                                                                                                                                                                                                          - interactively selecting them

                                                                                                                                                                                                          - use of algorithms that try to detect these points

                                                                                                                                                                                                          - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                          the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                          marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                          for establishing tie points

                                                                                                                                                                                                          The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                          of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                          a bilinear approximation is given by

                                                                                                                                                                                                          1 2 3 4

                                                                                                                                                                                                          5 6 7 8

                                                                                                                                                                                                          x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                          (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                          frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                          subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                          subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                          The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                          problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Probabilistic Methods

                                                                                                                                                                                                          zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                          p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                          ( ) kk

                                                                                                                                                                                                          np zM N

                                                                                                                                                                                                          nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                          pixels in the image) 1

                                                                                                                                                                                                          0( ) 1

                                                                                                                                                                                                          L

                                                                                                                                                                                                          kk

                                                                                                                                                                                                          p z

                                                                                                                                                                                                          The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                          0( )

                                                                                                                                                                                                          L

                                                                                                                                                                                                          k kk

                                                                                                                                                                                                          m z p z

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          The variance of the intensities is 1

                                                                                                                                                                                                          2 2

                                                                                                                                                                                                          0( ) ( )

                                                                                                                                                                                                          L

                                                                                                                                                                                                          k kk

                                                                                                                                                                                                          z m p z

                                                                                                                                                                                                          The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                          measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                          ( ) is used

                                                                                                                                                                                                          The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                          0( ) ( ) ( )

                                                                                                                                                                                                          Ln

                                                                                                                                                                                                          n k kk

                                                                                                                                                                                                          z z m p z

                                                                                                                                                                                                          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                          3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                          ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                          mean

                                                                                                                                                                                                          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                          Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                          Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                          Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                          ( ) ( )g x y T f x y

                                                                                                                                                                                                          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                          neighborhood of (x y)

                                                                                                                                                                                                          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                          and much smaller in size than the image

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                          called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                          ( )s T r

                                                                                                                                                                                                          s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                          Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                          darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                          is called contrast stretching

                                                                                                                                                                                                          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                          thresholding function

                                                                                                                                                                                                          Some Basic Intensity Transformation Functions

                                                                                                                                                                                                          Image Negatives

                                                                                                                                                                                                          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                          - equivalent of a photographic negative

                                                                                                                                                                                                          - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                          image

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Original Negative image

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                          Some basic intensity transformation functions

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                          range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                          while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                          transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                          variations in pixel values

                                                                                                                                                                                                          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Power-Law (Gamma) Transformations

                                                                                                                                                                                                          - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                          Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                          of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                          curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                          1c - identity transformation

                                                                                                                                                                                                          A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                          power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                          gamma correction

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Piecewise-Linear Transformations Functions

                                                                                                                                                                                                          Contrast stretching

                                                                                                                                                                                                          - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                          intensity range of the recording tool or display device

                                                                                                                                                                                                          a b c d Fig5

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          11

                                                                                                                                                                                                          1

                                                                                                                                                                                                          2 1 1 21 2

                                                                                                                                                                                                          2 1 2 1

                                                                                                                                                                                                          22

                                                                                                                                                                                                          2

                                                                                                                                                                                                          [0 ]

                                                                                                                                                                                                          ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                          ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                          s r r rrs r r s r rT r r r r

                                                                                                                                                                                                          r r r rs L r r r L

                                                                                                                                                                                                          L r

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                          1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                          Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                          in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                          from their original range to the full range [0 L-1]

                                                                                                                                                                                                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                          2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                          The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                          image of pollen magnified approximately 700 times

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Intensity-level slicing

                                                                                                                                                                                                          - highlighting a specific range of intensities in an image

                                                                                                                                                                                                          There are two approaches for intensity-level slicing

                                                                                                                                                                                                          1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                          another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                          intensities in the image (Figure 311 (b))

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                          the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                          Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                          blockageshellip)

                                                                                                                                                                                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                          image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                          Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Bit-plane slicing

                                                                                                                                                                                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                          This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                          • DIP 1 2017
                                                                                                                                                                                                          • DIP 02 (2017)

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Another use of image multiplication is in masking also called region of interest (ROI)

                                                                                                                                                                                                            operations The process consists of multiplying a given image by a mask image that has

                                                                                                                                                                                                            1rsquos (white) in the ROI and 0rsquos elsewhere There can be more than one ROI in the mask

                                                                                                                                                                                                            image and the shape of the ROI can be arbitrary but usually is a rectangular shape

                                                                                                                                                                                                            (a) (b) (c)

                                                                                                                                                                                                            Fig 7 (a) ndash digital dental X-ray image (b) - ROI mask for teeth with fillings (c) product of (a) and (b)

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                                            min( )mf f f

                                                                                                                                                                                                            0 ( 255)max( )

                                                                                                                                                                                                            ms

                                                                                                                                                                                                            m

                                                                                                                                                                                                            ff K K K

                                                                                                                                                                                                            f

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Spatial Operations

                                                                                                                                                                                                            - are performed directly on the pixels of a given image

                                                                                                                                                                                                            There are three categories of spatial operations

                                                                                                                                                                                                            single-pixel operations

                                                                                                                                                                                                            neighborhood operations

                                                                                                                                                                                                            geometric spatial transformations

                                                                                                                                                                                                            Single-pixel operations

                                                                                                                                                                                                            - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                                            where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                                            corresponding pixel in the processed image

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Neighborhood operations

                                                                                                                                                                                                            Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                                            in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                                            based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                                            rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                                            intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                                            ( )

                                                                                                                                                                                                            1( ) ( )xyr c S

                                                                                                                                                                                                            g x y f r cm n

                                                                                                                                                                                                            The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                                            used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                                            largest region of an image

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Geometric spatial transformations and image registration

                                                                                                                                                                                                            - modify the spatial relationship between pixels in an image

                                                                                                                                                                                                            - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                                            printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                                            predefined set of rules

                                                                                                                                                                                                            A geometric transformation consists of 2 basic operations

                                                                                                                                                                                                            1 a spatial transformation of coordinates

                                                                                                                                                                                                            2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                                            pixels

                                                                                                                                                                                                            The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                                            (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                                            (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                                            Affine transform

                                                                                                                                                                                                            11 1211 21 31

                                                                                                                                                                                                            21 2212 22 33

                                                                                                                                                                                                            31 32

                                                                                                                                                                                                            0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                                            1

                                                                                                                                                                                                            t tx t v t w t

                                                                                                                                                                                                            x y v w T v w t ty t v t w t

                                                                                                                                                                                                            t t

                                                                                                                                                                                                            (AT)

                                                                                                                                                                                                            This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                                            on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                                            result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                                            scaling rotation and translation matrices from Table 1

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Affine transformations

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                                            the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                                            using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                                            In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                                            forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                                            location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                                            Problems

                                                                                                                                                                                                            - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                                            the same location in the output image

                                                                                                                                                                                                            - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                                            assignment)

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                            computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                            It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                            pixel value

                                                                                                                                                                                                            Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                            numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                            In image registration we have available the input and output images but the specific

                                                                                                                                                                                                            transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                            The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                            images

                                                                                                                                                                                                            - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                            same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                            - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                            of time (satellite images)

                                                                                                                                                                                                            Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                            corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                            image

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            How to select tie points

                                                                                                                                                                                                            - interactively selecting them

                                                                                                                                                                                                            - use of algorithms that try to detect these points

                                                                                                                                                                                                            - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                            the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                            marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                            for establishing tie points

                                                                                                                                                                                                            The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                            of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                            a bilinear approximation is given by

                                                                                                                                                                                                            1 2 3 4

                                                                                                                                                                                                            5 6 7 8

                                                                                                                                                                                                            x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                            (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                            frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                            subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                            subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                            The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                            problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Probabilistic Methods

                                                                                                                                                                                                            zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                            p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                            ( ) kk

                                                                                                                                                                                                            np zM N

                                                                                                                                                                                                            nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                            pixels in the image) 1

                                                                                                                                                                                                            0( ) 1

                                                                                                                                                                                                            L

                                                                                                                                                                                                            kk

                                                                                                                                                                                                            p z

                                                                                                                                                                                                            The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                            0( )

                                                                                                                                                                                                            L

                                                                                                                                                                                                            k kk

                                                                                                                                                                                                            m z p z

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            The variance of the intensities is 1

                                                                                                                                                                                                            2 2

                                                                                                                                                                                                            0( ) ( )

                                                                                                                                                                                                            L

                                                                                                                                                                                                            k kk

                                                                                                                                                                                                            z m p z

                                                                                                                                                                                                            The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                            measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                            ( ) is used

                                                                                                                                                                                                            The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                            0( ) ( ) ( )

                                                                                                                                                                                                            Ln

                                                                                                                                                                                                            n k kk

                                                                                                                                                                                                            z z m p z

                                                                                                                                                                                                            ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                            3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                            ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                            mean

                                                                                                                                                                                                            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                            Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                            Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                            Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                            ( ) ( )g x y T f x y

                                                                                                                                                                                                            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                            neighborhood of (x y)

                                                                                                                                                                                                            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                            and much smaller in size than the image

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                            called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                            ( )s T r

                                                                                                                                                                                                            s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                            Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                            darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                            is called contrast stretching

                                                                                                                                                                                                            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                            thresholding function

                                                                                                                                                                                                            Some Basic Intensity Transformation Functions

                                                                                                                                                                                                            Image Negatives

                                                                                                                                                                                                            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                            - equivalent of a photographic negative

                                                                                                                                                                                                            - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                            image

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Original Negative image

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                            Some basic intensity transformation functions

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                            range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                            while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                            transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                            variations in pixel values

                                                                                                                                                                                                            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Power-Law (Gamma) Transformations

                                                                                                                                                                                                            - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                            Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                            of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                            curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                            1c - identity transformation

                                                                                                                                                                                                            A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                            power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                            gamma correction

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Piecewise-Linear Transformations Functions

                                                                                                                                                                                                            Contrast stretching

                                                                                                                                                                                                            - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                            intensity range of the recording tool or display device

                                                                                                                                                                                                            a b c d Fig5

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            11

                                                                                                                                                                                                            1

                                                                                                                                                                                                            2 1 1 21 2

                                                                                                                                                                                                            2 1 2 1

                                                                                                                                                                                                            22

                                                                                                                                                                                                            2

                                                                                                                                                                                                            [0 ]

                                                                                                                                                                                                            ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                            ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                            s r r rrs r r s r rT r r r r

                                                                                                                                                                                                            r r r rs L r r r L

                                                                                                                                                                                                            L r

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                            1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                            Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                            in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                            from their original range to the full range [0 L-1]

                                                                                                                                                                                                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                            2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                            The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                            image of pollen magnified approximately 700 times

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Intensity-level slicing

                                                                                                                                                                                                            - highlighting a specific range of intensities in an image

                                                                                                                                                                                                            There are two approaches for intensity-level slicing

                                                                                                                                                                                                            1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                            another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                            intensities in the image (Figure 311 (b))

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                            the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                            Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                            blockageshellip)

                                                                                                                                                                                                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                            image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                            Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Bit-plane slicing

                                                                                                                                                                                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                            This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                            • DIP 1 2017
                                                                                                                                                                                                            • DIP 02 (2017)

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              In practice most images are displayed using 8 bits the image values are in the range [0255] TIFF JPEG images ndash conversion to this range is automatic The conversion depends on the system used Difference of two images can produce image with values in the range [-255255] Addition of two images ndash range [0510] Many software packages simply set the negative values to 0 and set to 255 all values greater than 255 A more appropriate procedure compute

                                                                                                                                                                                                              min( )mf f f

                                                                                                                                                                                                              0 ( 255)max( )

                                                                                                                                                                                                              ms

                                                                                                                                                                                                              m

                                                                                                                                                                                                              ff K K K

                                                                                                                                                                                                              f

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Spatial Operations

                                                                                                                                                                                                              - are performed directly on the pixels of a given image

                                                                                                                                                                                                              There are three categories of spatial operations

                                                                                                                                                                                                              single-pixel operations

                                                                                                                                                                                                              neighborhood operations

                                                                                                                                                                                                              geometric spatial transformations

                                                                                                                                                                                                              Single-pixel operations

                                                                                                                                                                                                              - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                                              where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                                              corresponding pixel in the processed image

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Neighborhood operations

                                                                                                                                                                                                              Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                                              in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                                              based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                                              rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                                              intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                                              ( )

                                                                                                                                                                                                              1( ) ( )xyr c S

                                                                                                                                                                                                              g x y f r cm n

                                                                                                                                                                                                              The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                                              used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                                              largest region of an image

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Geometric spatial transformations and image registration

                                                                                                                                                                                                              - modify the spatial relationship between pixels in an image

                                                                                                                                                                                                              - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                                              printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                                              predefined set of rules

                                                                                                                                                                                                              A geometric transformation consists of 2 basic operations

                                                                                                                                                                                                              1 a spatial transformation of coordinates

                                                                                                                                                                                                              2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                                              pixels

                                                                                                                                                                                                              The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                                              (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                                              (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                                              Affine transform

                                                                                                                                                                                                              11 1211 21 31

                                                                                                                                                                                                              21 2212 22 33

                                                                                                                                                                                                              31 32

                                                                                                                                                                                                              0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                                              1

                                                                                                                                                                                                              t tx t v t w t

                                                                                                                                                                                                              x y v w T v w t ty t v t w t

                                                                                                                                                                                                              t t

                                                                                                                                                                                                              (AT)

                                                                                                                                                                                                              This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                                              on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                                              result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                                              scaling rotation and translation matrices from Table 1

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Affine transformations

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                                              the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                                              using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                                              In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                                              forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                                              location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                                              Problems

                                                                                                                                                                                                              - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                                              the same location in the output image

                                                                                                                                                                                                              - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                                              assignment)

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                              computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                              It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                              pixel value

                                                                                                                                                                                                              Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                              numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                              In image registration we have available the input and output images but the specific

                                                                                                                                                                                                              transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                              The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                              images

                                                                                                                                                                                                              - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                              same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                              - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                              of time (satellite images)

                                                                                                                                                                                                              Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                              corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                              image

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              How to select tie points

                                                                                                                                                                                                              - interactively selecting them

                                                                                                                                                                                                              - use of algorithms that try to detect these points

                                                                                                                                                                                                              - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                              the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                              marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                              for establishing tie points

                                                                                                                                                                                                              The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                              of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                              a bilinear approximation is given by

                                                                                                                                                                                                              1 2 3 4

                                                                                                                                                                                                              5 6 7 8

                                                                                                                                                                                                              x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                              (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                              frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                              subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                              subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                              The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                              problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Probabilistic Methods

                                                                                                                                                                                                              zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                              p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                              ( ) kk

                                                                                                                                                                                                              np zM N

                                                                                                                                                                                                              nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                              pixels in the image) 1

                                                                                                                                                                                                              0( ) 1

                                                                                                                                                                                                              L

                                                                                                                                                                                                              kk

                                                                                                                                                                                                              p z

                                                                                                                                                                                                              The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                              0( )

                                                                                                                                                                                                              L

                                                                                                                                                                                                              k kk

                                                                                                                                                                                                              m z p z

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              The variance of the intensities is 1

                                                                                                                                                                                                              2 2

                                                                                                                                                                                                              0( ) ( )

                                                                                                                                                                                                              L

                                                                                                                                                                                                              k kk

                                                                                                                                                                                                              z m p z

                                                                                                                                                                                                              The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                              measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                              ( ) is used

                                                                                                                                                                                                              The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                              0( ) ( ) ( )

                                                                                                                                                                                                              Ln

                                                                                                                                                                                                              n k kk

                                                                                                                                                                                                              z z m p z

                                                                                                                                                                                                              ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                              3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                              ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                              mean

                                                                                                                                                                                                              Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                              Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                              Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                              Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                              ( ) ( )g x y T f x y

                                                                                                                                                                                                              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                              neighborhood of (x y)

                                                                                                                                                                                                              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                              and much smaller in size than the image

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                              called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                              ( )s T r

                                                                                                                                                                                                              s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                              Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                              darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                              is called contrast stretching

                                                                                                                                                                                                              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                              thresholding function

                                                                                                                                                                                                              Some Basic Intensity Transformation Functions

                                                                                                                                                                                                              Image Negatives

                                                                                                                                                                                                              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                              - equivalent of a photographic negative

                                                                                                                                                                                                              - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                              image

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Original Negative image

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                              Some basic intensity transformation functions

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                              range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                              while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                              transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                              variations in pixel values

                                                                                                                                                                                                              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Power-Law (Gamma) Transformations

                                                                                                                                                                                                              - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                              Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                              of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                              curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                              1c - identity transformation

                                                                                                                                                                                                              A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                              power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                              gamma correction

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Piecewise-Linear Transformations Functions

                                                                                                                                                                                                              Contrast stretching

                                                                                                                                                                                                              - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                              intensity range of the recording tool or display device

                                                                                                                                                                                                              a b c d Fig5

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              11

                                                                                                                                                                                                              1

                                                                                                                                                                                                              2 1 1 21 2

                                                                                                                                                                                                              2 1 2 1

                                                                                                                                                                                                              22

                                                                                                                                                                                                              2

                                                                                                                                                                                                              [0 ]

                                                                                                                                                                                                              ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                              ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                              s r r rrs r r s r rT r r r r

                                                                                                                                                                                                              r r r rs L r r r L

                                                                                                                                                                                                              L r

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                              1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                              Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                              in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                              from their original range to the full range [0 L-1]

                                                                                                                                                                                                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                              2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                              The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                              image of pollen magnified approximately 700 times

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Intensity-level slicing

                                                                                                                                                                                                              - highlighting a specific range of intensities in an image

                                                                                                                                                                                                              There are two approaches for intensity-level slicing

                                                                                                                                                                                                              1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                              another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                              intensities in the image (Figure 311 (b))

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                              the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                              Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                              blockageshellip)

                                                                                                                                                                                                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                              image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                              Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Bit-plane slicing

                                                                                                                                                                                                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                              This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                              • DIP 1 2017
                                                                                                                                                                                                              • DIP 02 (2017)

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Spatial Operations

                                                                                                                                                                                                                - are performed directly on the pixels of a given image

                                                                                                                                                                                                                There are three categories of spatial operations

                                                                                                                                                                                                                single-pixel operations

                                                                                                                                                                                                                neighborhood operations

                                                                                                                                                                                                                geometric spatial transformations

                                                                                                                                                                                                                Single-pixel operations

                                                                                                                                                                                                                - change the values of intensity for the individual pixels ( )s T z

                                                                                                                                                                                                                where z is the intensity of a pixel in the original image and s is the intensity of the

                                                                                                                                                                                                                corresponding pixel in the processed image

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Neighborhood operations

                                                                                                                                                                                                                Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                                                in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                                                based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                                                rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                                                intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                                                ( )

                                                                                                                                                                                                                1( ) ( )xyr c S

                                                                                                                                                                                                                g x y f r cm n

                                                                                                                                                                                                                The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                                                used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                                                largest region of an image

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Geometric spatial transformations and image registration

                                                                                                                                                                                                                - modify the spatial relationship between pixels in an image

                                                                                                                                                                                                                - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                                                printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                                                predefined set of rules

                                                                                                                                                                                                                A geometric transformation consists of 2 basic operations

                                                                                                                                                                                                                1 a spatial transformation of coordinates

                                                                                                                                                                                                                2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                                                pixels

                                                                                                                                                                                                                The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                                                (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                                                (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                                                Affine transform

                                                                                                                                                                                                                11 1211 21 31

                                                                                                                                                                                                                21 2212 22 33

                                                                                                                                                                                                                31 32

                                                                                                                                                                                                                0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                                                1

                                                                                                                                                                                                                t tx t v t w t

                                                                                                                                                                                                                x y v w T v w t ty t v t w t

                                                                                                                                                                                                                t t

                                                                                                                                                                                                                (AT)

                                                                                                                                                                                                                This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                                                on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                                                result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                                                scaling rotation and translation matrices from Table 1

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Affine transformations

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                                                the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                                                using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                                                In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                                                forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                                                location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                                                Problems

                                                                                                                                                                                                                - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                                                the same location in the output image

                                                                                                                                                                                                                - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                                                assignment)

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                                computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                                It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                                pixel value

                                                                                                                                                                                                                Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                                numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                                In image registration we have available the input and output images but the specific

                                                                                                                                                                                                                transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                                The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                                images

                                                                                                                                                                                                                - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                                same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                                - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                                of time (satellite images)

                                                                                                                                                                                                                Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                                corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                                image

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                How to select tie points

                                                                                                                                                                                                                - interactively selecting them

                                                                                                                                                                                                                - use of algorithms that try to detect these points

                                                                                                                                                                                                                - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                                the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                                marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                                for establishing tie points

                                                                                                                                                                                                                The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                                of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                                a bilinear approximation is given by

                                                                                                                                                                                                                1 2 3 4

                                                                                                                                                                                                                5 6 7 8

                                                                                                                                                                                                                x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                                (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                                frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                                subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                                subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                                The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                                problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Probabilistic Methods

                                                                                                                                                                                                                zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                                p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                                ( ) kk

                                                                                                                                                                                                                np zM N

                                                                                                                                                                                                                nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                                pixels in the image) 1

                                                                                                                                                                                                                0( ) 1

                                                                                                                                                                                                                L

                                                                                                                                                                                                                kk

                                                                                                                                                                                                                p z

                                                                                                                                                                                                                The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                                0( )

                                                                                                                                                                                                                L

                                                                                                                                                                                                                k kk

                                                                                                                                                                                                                m z p z

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                The variance of the intensities is 1

                                                                                                                                                                                                                2 2

                                                                                                                                                                                                                0( ) ( )

                                                                                                                                                                                                                L

                                                                                                                                                                                                                k kk

                                                                                                                                                                                                                z m p z

                                                                                                                                                                                                                The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                                measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                                ( ) is used

                                                                                                                                                                                                                The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                                0( ) ( ) ( )

                                                                                                                                                                                                                Ln

                                                                                                                                                                                                                n k kk

                                                                                                                                                                                                                z z m p z

                                                                                                                                                                                                                ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                                3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                                ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                mean

                                                                                                                                                                                                                Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                ( ) ( )g x y T f x y

                                                                                                                                                                                                                f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                neighborhood of (x y)

                                                                                                                                                                                                                - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                and much smaller in size than the image

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                ( )s T r

                                                                                                                                                                                                                s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                is called contrast stretching

                                                                                                                                                                                                                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                thresholding function

                                                                                                                                                                                                                Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                Image Negatives

                                                                                                                                                                                                                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                - equivalent of a photographic negative

                                                                                                                                                                                                                - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                image

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Original Negative image

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                Some basic intensity transformation functions

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                variations in pixel values

                                                                                                                                                                                                                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Power-Law (Gamma) Transformations

                                                                                                                                                                                                                - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                1c - identity transformation

                                                                                                                                                                                                                A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                gamma correction

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                Contrast stretching

                                                                                                                                                                                                                - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                intensity range of the recording tool or display device

                                                                                                                                                                                                                a b c d Fig5

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                11

                                                                                                                                                                                                                1

                                                                                                                                                                                                                2 1 1 21 2

                                                                                                                                                                                                                2 1 2 1

                                                                                                                                                                                                                22

                                                                                                                                                                                                                2

                                                                                                                                                                                                                [0 ]

                                                                                                                                                                                                                ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                r r r rs L r r r L

                                                                                                                                                                                                                L r

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                from their original range to the full range [0 L-1]

                                                                                                                                                                                                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                image of pollen magnified approximately 700 times

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Intensity-level slicing

                                                                                                                                                                                                                - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                There are two approaches for intensity-level slicing

                                                                                                                                                                                                                1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                blockageshellip)

                                                                                                                                                                                                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Bit-plane slicing

                                                                                                                                                                                                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                • DIP 1 2017
                                                                                                                                                                                                                • DIP 02 (2017)

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Neighborhood operations

                                                                                                                                                                                                                  Let Sxy denote a set of coordinates of a neighborhood centered on an arbitrary point (x y)

                                                                                                                                                                                                                  in an image f Neighborhood processing generates new intensity level at point (x y)

                                                                                                                                                                                                                  based on the values of the intensities of the points in Sxy For example if Sxy is a

                                                                                                                                                                                                                  rectangular neighborhood of size m x n centered in (x y) we can assign the new value of

                                                                                                                                                                                                                  intensity by computing the average value of the pixels in Sxy

                                                                                                                                                                                                                  ( )

                                                                                                                                                                                                                  1( ) ( )xyr c S

                                                                                                                                                                                                                  g x y f r cm n

                                                                                                                                                                                                                  The net effect is to perform local blurring in the original image This type of process is

                                                                                                                                                                                                                  used for example to eliminate small details and thus render ldquoblobsrdquo corresponding to the

                                                                                                                                                                                                                  largest region of an image

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Geometric spatial transformations and image registration

                                                                                                                                                                                                                  - modify the spatial relationship between pixels in an image

                                                                                                                                                                                                                  - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                                                  printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                                                  predefined set of rules

                                                                                                                                                                                                                  A geometric transformation consists of 2 basic operations

                                                                                                                                                                                                                  1 a spatial transformation of coordinates

                                                                                                                                                                                                                  2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                                                  pixels

                                                                                                                                                                                                                  The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                                                  (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                                                  (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                                                  Affine transform

                                                                                                                                                                                                                  11 1211 21 31

                                                                                                                                                                                                                  21 2212 22 33

                                                                                                                                                                                                                  31 32

                                                                                                                                                                                                                  0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                                                  1

                                                                                                                                                                                                                  t tx t v t w t

                                                                                                                                                                                                                  x y v w T v w t ty t v t w t

                                                                                                                                                                                                                  t t

                                                                                                                                                                                                                  (AT)

                                                                                                                                                                                                                  This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                                                  on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                                                  result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                                                  scaling rotation and translation matrices from Table 1

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Affine transformations

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                                                  the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                                                  using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                                                  In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                                                  forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                                                  location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                                                  Problems

                                                                                                                                                                                                                  - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                                                  the same location in the output image

                                                                                                                                                                                                                  - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                                                  assignment)

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                                  computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                                  It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                                  pixel value

                                                                                                                                                                                                                  Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                                  numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                                  In image registration we have available the input and output images but the specific

                                                                                                                                                                                                                  transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                                  The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                                  images

                                                                                                                                                                                                                  - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                                  same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                                  - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                                  of time (satellite images)

                                                                                                                                                                                                                  Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                                  corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                                  image

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  How to select tie points

                                                                                                                                                                                                                  - interactively selecting them

                                                                                                                                                                                                                  - use of algorithms that try to detect these points

                                                                                                                                                                                                                  - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                                  the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                                  marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                                  for establishing tie points

                                                                                                                                                                                                                  The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                                  of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                                  a bilinear approximation is given by

                                                                                                                                                                                                                  1 2 3 4

                                                                                                                                                                                                                  5 6 7 8

                                                                                                                                                                                                                  x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                                  (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                                  frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                                  subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                                  subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                                  The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                                  problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Probabilistic Methods

                                                                                                                                                                                                                  zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                                  p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                                  ( ) kk

                                                                                                                                                                                                                  np zM N

                                                                                                                                                                                                                  nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                                  pixels in the image) 1

                                                                                                                                                                                                                  0( ) 1

                                                                                                                                                                                                                  L

                                                                                                                                                                                                                  kk

                                                                                                                                                                                                                  p z

                                                                                                                                                                                                                  The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                                  0( )

                                                                                                                                                                                                                  L

                                                                                                                                                                                                                  k kk

                                                                                                                                                                                                                  m z p z

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  The variance of the intensities is 1

                                                                                                                                                                                                                  2 2

                                                                                                                                                                                                                  0( ) ( )

                                                                                                                                                                                                                  L

                                                                                                                                                                                                                  k kk

                                                                                                                                                                                                                  z m p z

                                                                                                                                                                                                                  The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                                  measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                                  ( ) is used

                                                                                                                                                                                                                  The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                                  0( ) ( ) ( )

                                                                                                                                                                                                                  Ln

                                                                                                                                                                                                                  n k kk

                                                                                                                                                                                                                  z z m p z

                                                                                                                                                                                                                  ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                                  3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                                  ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                  mean

                                                                                                                                                                                                                  Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                  Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                  Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                  Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                  ( ) ( )g x y T f x y

                                                                                                                                                                                                                  f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                  neighborhood of (x y)

                                                                                                                                                                                                                  - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                  and much smaller in size than the image

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                  called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                  ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                  ( )s T r

                                                                                                                                                                                                                  s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                  Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                  darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                  is called contrast stretching

                                                                                                                                                                                                                  Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                  thresholding function

                                                                                                                                                                                                                  Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                  Image Negatives

                                                                                                                                                                                                                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                  - equivalent of a photographic negative

                                                                                                                                                                                                                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                  image

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Original Negative image

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                  Some basic intensity transformation functions

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                  range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                  while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                  transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                  variations in pixel values

                                                                                                                                                                                                                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Power-Law (Gamma) Transformations

                                                                                                                                                                                                                  - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                  Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                  of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                  curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                  1c - identity transformation

                                                                                                                                                                                                                  A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                  power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                  gamma correction

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                  Contrast stretching

                                                                                                                                                                                                                  - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                  intensity range of the recording tool or display device

                                                                                                                                                                                                                  a b c d Fig5

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  11

                                                                                                                                                                                                                  1

                                                                                                                                                                                                                  2 1 1 21 2

                                                                                                                                                                                                                  2 1 2 1

                                                                                                                                                                                                                  22

                                                                                                                                                                                                                  2

                                                                                                                                                                                                                  [0 ]

                                                                                                                                                                                                                  ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                  ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                  s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                  r r r rs L r r r L

                                                                                                                                                                                                                  L r

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                  1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                  Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                  in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                  from their original range to the full range [0 L-1]

                                                                                                                                                                                                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                  2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                  The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                  image of pollen magnified approximately 700 times

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Intensity-level slicing

                                                                                                                                                                                                                  - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                  There are two approaches for intensity-level slicing

                                                                                                                                                                                                                  1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                  another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                  intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                  the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                  Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                  blockageshellip)

                                                                                                                                                                                                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                  image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                  Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Bit-plane slicing

                                                                                                                                                                                                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                  This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                  • DIP 1 2017
                                                                                                                                                                                                                  • DIP 02 (2017)

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Geometric spatial transformations and image registration

                                                                                                                                                                                                                    - modify the spatial relationship between pixels in an image

                                                                                                                                                                                                                    - these transformations are often called rubber-sheet transformations (analogous to

                                                                                                                                                                                                                    printing an image on a sheet of rubber and then stretching the sheet according to a

                                                                                                                                                                                                                    predefined set of rules

                                                                                                                                                                                                                    A geometric transformation consists of 2 basic operations

                                                                                                                                                                                                                    1 a spatial transformation of coordinates

                                                                                                                                                                                                                    2 intensity interpolation that assign intensity values to the spatial transformed

                                                                                                                                                                                                                    pixels

                                                                                                                                                                                                                    The coordinate system transformation ( ) [( )]x y T v w

                                                                                                                                                                                                                    (v w) ndash pixel coordinates in the original image

                                                                                                                                                                                                                    (x y) ndash pixel coordinates in the transformed image

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                                                    Affine transform

                                                                                                                                                                                                                    11 1211 21 31

                                                                                                                                                                                                                    21 2212 22 33

                                                                                                                                                                                                                    31 32

                                                                                                                                                                                                                    0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                                                    1

                                                                                                                                                                                                                    t tx t v t w t

                                                                                                                                                                                                                    x y v w T v w t ty t v t w t

                                                                                                                                                                                                                    t t

                                                                                                                                                                                                                    (AT)

                                                                                                                                                                                                                    This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                                                    on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                                                    result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                                                    scaling rotation and translation matrices from Table 1

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Affine transformations

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                                                    the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                                                    using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                                                    In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                                                    forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                                                    location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                                                    Problems

                                                                                                                                                                                                                    - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                                                    the same location in the output image

                                                                                                                                                                                                                    - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                                                    assignment)

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                                    computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                                    It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                                    pixel value

                                                                                                                                                                                                                    Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                                    numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                                    In image registration we have available the input and output images but the specific

                                                                                                                                                                                                                    transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                                    The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                                    images

                                                                                                                                                                                                                    - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                                    same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                                    - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                                    of time (satellite images)

                                                                                                                                                                                                                    Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                                    corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                                    image

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    How to select tie points

                                                                                                                                                                                                                    - interactively selecting them

                                                                                                                                                                                                                    - use of algorithms that try to detect these points

                                                                                                                                                                                                                    - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                                    the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                                    marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                                    for establishing tie points

                                                                                                                                                                                                                    The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                                    of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                                    a bilinear approximation is given by

                                                                                                                                                                                                                    1 2 3 4

                                                                                                                                                                                                                    5 6 7 8

                                                                                                                                                                                                                    x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                                    (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                                    frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                                    subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                                    subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                                    The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                                    problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Probabilistic Methods

                                                                                                                                                                                                                    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                                    p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                                    ( ) kk

                                                                                                                                                                                                                    np zM N

                                                                                                                                                                                                                    nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                                    pixels in the image) 1

                                                                                                                                                                                                                    0( ) 1

                                                                                                                                                                                                                    L

                                                                                                                                                                                                                    kk

                                                                                                                                                                                                                    p z

                                                                                                                                                                                                                    The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                                    0( )

                                                                                                                                                                                                                    L

                                                                                                                                                                                                                    k kk

                                                                                                                                                                                                                    m z p z

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    The variance of the intensities is 1

                                                                                                                                                                                                                    2 2

                                                                                                                                                                                                                    0( ) ( )

                                                                                                                                                                                                                    L

                                                                                                                                                                                                                    k kk

                                                                                                                                                                                                                    z m p z

                                                                                                                                                                                                                    The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                                    measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                                    ( ) is used

                                                                                                                                                                                                                    The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                                    0( ) ( ) ( )

                                                                                                                                                                                                                    Ln

                                                                                                                                                                                                                    n k kk

                                                                                                                                                                                                                    z z m p z

                                                                                                                                                                                                                    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                                    3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                                    ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                    mean

                                                                                                                                                                                                                    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                    Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                    Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                    Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                    ( ) ( )g x y T f x y

                                                                                                                                                                                                                    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                    neighborhood of (x y)

                                                                                                                                                                                                                    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                    and much smaller in size than the image

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                    called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                    ( )s T r

                                                                                                                                                                                                                    s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                    Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                    darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                    is called contrast stretching

                                                                                                                                                                                                                    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                    thresholding function

                                                                                                                                                                                                                    Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                    Image Negatives

                                                                                                                                                                                                                    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                    - equivalent of a photographic negative

                                                                                                                                                                                                                    - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                    image

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Original Negative image

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                    Some basic intensity transformation functions

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                    range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                    while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                    transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                    variations in pixel values

                                                                                                                                                                                                                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Power-Law (Gamma) Transformations

                                                                                                                                                                                                                    - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                    Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                    of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                    curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                    1c - identity transformation

                                                                                                                                                                                                                    A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                    power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                    gamma correction

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                    Contrast stretching

                                                                                                                                                                                                                    - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                    intensity range of the recording tool or display device

                                                                                                                                                                                                                    a b c d Fig5

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    11

                                                                                                                                                                                                                    1

                                                                                                                                                                                                                    2 1 1 21 2

                                                                                                                                                                                                                    2 1 2 1

                                                                                                                                                                                                                    22

                                                                                                                                                                                                                    2

                                                                                                                                                                                                                    [0 ]

                                                                                                                                                                                                                    ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                    ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                    s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                    r r r rs L r r r L

                                                                                                                                                                                                                    L r

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                    1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                    Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                    in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                    from their original range to the full range [0 L-1]

                                                                                                                                                                                                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                    2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                    The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                    image of pollen magnified approximately 700 times

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Intensity-level slicing

                                                                                                                                                                                                                    - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                    There are two approaches for intensity-level slicing

                                                                                                                                                                                                                    1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                    another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                    intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                    the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                    Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                    blockageshellip)

                                                                                                                                                                                                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                    image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                    Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Bit-plane slicing

                                                                                                                                                                                                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                    This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                    • DIP 1 2017
                                                                                                                                                                                                                    • DIP 02 (2017)

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      [( )] ( )2 2v wT v w shrinks the original image half its size in both spatial directions

                                                                                                                                                                                                                      Affine transform

                                                                                                                                                                                                                      11 1211 21 31

                                                                                                                                                                                                                      21 2212 22 33

                                                                                                                                                                                                                      31 32

                                                                                                                                                                                                                      0[ 1] [ 1] [ 1] 0

                                                                                                                                                                                                                      1

                                                                                                                                                                                                                      t tx t v t w t

                                                                                                                                                                                                                      x y v w T v w t ty t v t w t

                                                                                                                                                                                                                      t t

                                                                                                                                                                                                                      (AT)

                                                                                                                                                                                                                      This transform can scale rotate translate or shear a set of coordinate points depending

                                                                                                                                                                                                                      on the elements of the matrix T If we want to resize an image rotate it and move the

                                                                                                                                                                                                                      result to some location we simply form a 3x3 matrix equal to the matrix product of the

                                                                                                                                                                                                                      scaling rotation and translation matrices from Table 1

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Affine transformations

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                                                      the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                                                      using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                                                      In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                                                      forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                                                      location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                                                      Problems

                                                                                                                                                                                                                      - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                                                      the same location in the output image

                                                                                                                                                                                                                      - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                                                      assignment)

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                                      computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                                      It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                                      pixel value

                                                                                                                                                                                                                      Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                                      numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                                      In image registration we have available the input and output images but the specific

                                                                                                                                                                                                                      transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                                      The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                                      images

                                                                                                                                                                                                                      - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                                      same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                                      - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                                      of time (satellite images)

                                                                                                                                                                                                                      Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                                      corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                                      image

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      How to select tie points

                                                                                                                                                                                                                      - interactively selecting them

                                                                                                                                                                                                                      - use of algorithms that try to detect these points

                                                                                                                                                                                                                      - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                                      the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                                      marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                                      for establishing tie points

                                                                                                                                                                                                                      The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                                      of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                                      a bilinear approximation is given by

                                                                                                                                                                                                                      1 2 3 4

                                                                                                                                                                                                                      5 6 7 8

                                                                                                                                                                                                                      x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                                      (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                                      frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                                      subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                                      subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                                      The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                                      problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Probabilistic Methods

                                                                                                                                                                                                                      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                                      p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                                      ( ) kk

                                                                                                                                                                                                                      np zM N

                                                                                                                                                                                                                      nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                                      pixels in the image) 1

                                                                                                                                                                                                                      0( ) 1

                                                                                                                                                                                                                      L

                                                                                                                                                                                                                      kk

                                                                                                                                                                                                                      p z

                                                                                                                                                                                                                      The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                                      0( )

                                                                                                                                                                                                                      L

                                                                                                                                                                                                                      k kk

                                                                                                                                                                                                                      m z p z

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      The variance of the intensities is 1

                                                                                                                                                                                                                      2 2

                                                                                                                                                                                                                      0( ) ( )

                                                                                                                                                                                                                      L

                                                                                                                                                                                                                      k kk

                                                                                                                                                                                                                      z m p z

                                                                                                                                                                                                                      The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                                      measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                                      ( ) is used

                                                                                                                                                                                                                      The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                                      0( ) ( ) ( )

                                                                                                                                                                                                                      Ln

                                                                                                                                                                                                                      n k kk

                                                                                                                                                                                                                      z z m p z

                                                                                                                                                                                                                      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                                      3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                                      ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                      mean

                                                                                                                                                                                                                      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                      Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                      Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                      Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                      ( ) ( )g x y T f x y

                                                                                                                                                                                                                      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                      neighborhood of (x y)

                                                                                                                                                                                                                      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                      and much smaller in size than the image

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                      called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                      ( )s T r

                                                                                                                                                                                                                      s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                      Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                      darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                      is called contrast stretching

                                                                                                                                                                                                                      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                      thresholding function

                                                                                                                                                                                                                      Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                      Image Negatives

                                                                                                                                                                                                                      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                      - equivalent of a photographic negative

                                                                                                                                                                                                                      - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                      image

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Original Negative image

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                      Some basic intensity transformation functions

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                      range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                      while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                      transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                      variations in pixel values

                                                                                                                                                                                                                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Power-Law (Gamma) Transformations

                                                                                                                                                                                                                      - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                      Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                      of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                      curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                      1c - identity transformation

                                                                                                                                                                                                                      A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                      power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                      gamma correction

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                      Contrast stretching

                                                                                                                                                                                                                      - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                      intensity range of the recording tool or display device

                                                                                                                                                                                                                      a b c d Fig5

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      11

                                                                                                                                                                                                                      1

                                                                                                                                                                                                                      2 1 1 21 2

                                                                                                                                                                                                                      2 1 2 1

                                                                                                                                                                                                                      22

                                                                                                                                                                                                                      2

                                                                                                                                                                                                                      [0 ]

                                                                                                                                                                                                                      ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                      ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                      s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                      r r r rs L r r r L

                                                                                                                                                                                                                      L r

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                      1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                      Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                      in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                      from their original range to the full range [0 L-1]

                                                                                                                                                                                                                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                      2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                      The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                      image of pollen magnified approximately 700 times

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Intensity-level slicing

                                                                                                                                                                                                                      - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                      There are two approaches for intensity-level slicing

                                                                                                                                                                                                                      1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                      another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                      intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                      the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                      Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                      blockageshellip)

                                                                                                                                                                                                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                      image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                      Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Bit-plane slicing

                                                                                                                                                                                                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                      This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                      • DIP 1 2017
                                                                                                                                                                                                                      • DIP 02 (2017)

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Affine transformations

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                                                        the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                                                        using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                                                        In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                                                        forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                                                        location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                                                        Problems

                                                                                                                                                                                                                        - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                                                        the same location in the output image

                                                                                                                                                                                                                        - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                                                        assignment)

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                                        computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                                        It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                                        pixel value

                                                                                                                                                                                                                        Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                                        numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                                        In image registration we have available the input and output images but the specific

                                                                                                                                                                                                                        transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                                        The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                                        images

                                                                                                                                                                                                                        - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                                        same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                                        - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                                        of time (satellite images)

                                                                                                                                                                                                                        Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                                        corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                                        image

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        How to select tie points

                                                                                                                                                                                                                        - interactively selecting them

                                                                                                                                                                                                                        - use of algorithms that try to detect these points

                                                                                                                                                                                                                        - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                                        the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                                        marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                                        for establishing tie points

                                                                                                                                                                                                                        The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                                        of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                                        a bilinear approximation is given by

                                                                                                                                                                                                                        1 2 3 4

                                                                                                                                                                                                                        5 6 7 8

                                                                                                                                                                                                                        x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                                        (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                                        frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                                        subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                                        subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                                        The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                                        problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Probabilistic Methods

                                                                                                                                                                                                                        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                                        p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                                        ( ) kk

                                                                                                                                                                                                                        np zM N

                                                                                                                                                                                                                        nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                                        pixels in the image) 1

                                                                                                                                                                                                                        0( ) 1

                                                                                                                                                                                                                        L

                                                                                                                                                                                                                        kk

                                                                                                                                                                                                                        p z

                                                                                                                                                                                                                        The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                                        0( )

                                                                                                                                                                                                                        L

                                                                                                                                                                                                                        k kk

                                                                                                                                                                                                                        m z p z

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        The variance of the intensities is 1

                                                                                                                                                                                                                        2 2

                                                                                                                                                                                                                        0( ) ( )

                                                                                                                                                                                                                        L

                                                                                                                                                                                                                        k kk

                                                                                                                                                                                                                        z m p z

                                                                                                                                                                                                                        The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                                        measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                                        ( ) is used

                                                                                                                                                                                                                        The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                                        0( ) ( ) ( )

                                                                                                                                                                                                                        Ln

                                                                                                                                                                                                                        n k kk

                                                                                                                                                                                                                        z z m p z

                                                                                                                                                                                                                        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                                        3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                                        ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                        mean

                                                                                                                                                                                                                        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                        Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                        Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                        Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                        ( ) ( )g x y T f x y

                                                                                                                                                                                                                        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                        neighborhood of (x y)

                                                                                                                                                                                                                        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                        and much smaller in size than the image

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                        called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                        ( )s T r

                                                                                                                                                                                                                        s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                        Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                        darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                        is called contrast stretching

                                                                                                                                                                                                                        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                        thresholding function

                                                                                                                                                                                                                        Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                        Image Negatives

                                                                                                                                                                                                                        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                        - equivalent of a photographic negative

                                                                                                                                                                                                                        - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                        image

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Original Negative image

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                        Some basic intensity transformation functions

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                        range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                        while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                        transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                        variations in pixel values

                                                                                                                                                                                                                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Power-Law (Gamma) Transformations

                                                                                                                                                                                                                        - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                        Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                        of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                        curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                        1c - identity transformation

                                                                                                                                                                                                                        A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                        power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                        gamma correction

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                        Contrast stretching

                                                                                                                                                                                                                        - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                        intensity range of the recording tool or display device

                                                                                                                                                                                                                        a b c d Fig5

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        11

                                                                                                                                                                                                                        1

                                                                                                                                                                                                                        2 1 1 21 2

                                                                                                                                                                                                                        2 1 2 1

                                                                                                                                                                                                                        22

                                                                                                                                                                                                                        2

                                                                                                                                                                                                                        [0 ]

                                                                                                                                                                                                                        ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                        ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                        s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                        r r r rs L r r r L

                                                                                                                                                                                                                        L r

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                        1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                        Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                        in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                        from their original range to the full range [0 L-1]

                                                                                                                                                                                                                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                        2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                        The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                        image of pollen magnified approximately 700 times

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Intensity-level slicing

                                                                                                                                                                                                                        - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                        There are two approaches for intensity-level slicing

                                                                                                                                                                                                                        1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                        another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                        intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                        the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                        Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                        blockageshellip)

                                                                                                                                                                                                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                        image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                        Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Bit-plane slicing

                                                                                                                                                                                                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                        This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                        • DIP 1 2017
                                                                                                                                                                                                                        • DIP 02 (2017)

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          The preceding transformations relocate pixels on an image to new locations To complete

                                                                                                                                                                                                                          the process we have to assign intensity values to those locations This task is done by

                                                                                                                                                                                                                          using intensity interpolation (like nearest neighbor bilinear bi-cubic interpolation)

                                                                                                                                                                                                                          In practice we can use equation (AT) in two basic ways

                                                                                                                                                                                                                          forward mapping scan the pixels of the input image (v w) compute the new spatial

                                                                                                                                                                                                                          location (x y) of the corresponding pixel in the new image using (AT) directly

                                                                                                                                                                                                                          Problems

                                                                                                                                                                                                                          - intensity assignment when 2 or more pixels in the original image are transformed to

                                                                                                                                                                                                                          the same location in the output image

                                                                                                                                                                                                                          - some output locations have no correspondent in the original image (no intensity

                                                                                                                                                                                                                          assignment)

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                                          computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                                          It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                                          pixel value

                                                                                                                                                                                                                          Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                                          numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                                          In image registration we have available the input and output images but the specific

                                                                                                                                                                                                                          transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                                          The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                                          images

                                                                                                                                                                                                                          - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                                          same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                                          - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                                          of time (satellite images)

                                                                                                                                                                                                                          Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                                          corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                                          image

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          How to select tie points

                                                                                                                                                                                                                          - interactively selecting them

                                                                                                                                                                                                                          - use of algorithms that try to detect these points

                                                                                                                                                                                                                          - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                                          the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                                          marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                                          for establishing tie points

                                                                                                                                                                                                                          The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                                          of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                                          a bilinear approximation is given by

                                                                                                                                                                                                                          1 2 3 4

                                                                                                                                                                                                                          5 6 7 8

                                                                                                                                                                                                                          x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                                          (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                                          frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                                          subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                                          subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                                          The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                                          problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          Probabilistic Methods

                                                                                                                                                                                                                          zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                                          p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                                          ( ) kk

                                                                                                                                                                                                                          np zM N

                                                                                                                                                                                                                          nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                                          pixels in the image) 1

                                                                                                                                                                                                                          0( ) 1

                                                                                                                                                                                                                          L

                                                                                                                                                                                                                          kk

                                                                                                                                                                                                                          p z

                                                                                                                                                                                                                          The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                                          0( )

                                                                                                                                                                                                                          L

                                                                                                                                                                                                                          k kk

                                                                                                                                                                                                                          m z p z

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          The variance of the intensities is 1

                                                                                                                                                                                                                          2 2

                                                                                                                                                                                                                          0( ) ( )

                                                                                                                                                                                                                          L

                                                                                                                                                                                                                          k kk

                                                                                                                                                                                                                          z m p z

                                                                                                                                                                                                                          The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                                          measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                                          ( ) is used

                                                                                                                                                                                                                          The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                                          0( ) ( ) ( )

                                                                                                                                                                                                                          Ln

                                                                                                                                                                                                                          n k kk

                                                                                                                                                                                                                          z z m p z

                                                                                                                                                                                                                          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                                          3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                                          ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                          mean

                                                                                                                                                                                                                          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                          Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                          Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                          Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                          ( ) ( )g x y T f x y

                                                                                                                                                                                                                          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                          neighborhood of (x y)

                                                                                                                                                                                                                          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                          and much smaller in size than the image

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                          called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                          ( )s T r

                                                                                                                                                                                                                          s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                          Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                          darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                          is called contrast stretching

                                                                                                                                                                                                                          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                          thresholding function

                                                                                                                                                                                                                          Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                          Image Negatives

                                                                                                                                                                                                                          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                          - equivalent of a photographic negative

                                                                                                                                                                                                                          - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                          image

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          Original Negative image

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                          Some basic intensity transformation functions

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                          range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                          while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                          transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                          variations in pixel values

                                                                                                                                                                                                                          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          Power-Law (Gamma) Transformations

                                                                                                                                                                                                                          - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                          Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                          of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                          curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                          1c - identity transformation

                                                                                                                                                                                                                          A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                          power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                          gamma correction

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                          Contrast stretching

                                                                                                                                                                                                                          - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                          intensity range of the recording tool or display device

                                                                                                                                                                                                                          a b c d Fig5

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          11

                                                                                                                                                                                                                          1

                                                                                                                                                                                                                          2 1 1 21 2

                                                                                                                                                                                                                          2 1 2 1

                                                                                                                                                                                                                          22

                                                                                                                                                                                                                          2

                                                                                                                                                                                                                          [0 ]

                                                                                                                                                                                                                          ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                          ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                          s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                          r r r rs L r r r L

                                                                                                                                                                                                                          L r

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                          1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                          Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                          in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                          from their original range to the full range [0 L-1]

                                                                                                                                                                                                                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                          2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                          The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                          image of pollen magnified approximately 700 times

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          Intensity-level slicing

                                                                                                                                                                                                                          - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                          There are two approaches for intensity-level slicing

                                                                                                                                                                                                                          1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                          another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                          intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                          the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                          Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                          blockageshellip)

                                                                                                                                                                                                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                          image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                          Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          Bit-plane slicing

                                                                                                                                                                                                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                          This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                          • DIP 1 2017
                                                                                                                                                                                                                          • DIP 02 (2017)

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            inverse mapping scans the output pixel locations and at each location (x y)

                                                                                                                                                                                                                            computes the corresponding location in the input image (v w) 1( ) ( )v w T x y

                                                                                                                                                                                                                            It then interpolates among the nearest input pixels to determine the intensity of the output

                                                                                                                                                                                                                            pixel value

                                                                                                                                                                                                                            Inverse mappings are more efficient to implement than forward mappings and are used in

                                                                                                                                                                                                                            numerous commercial implementations of spatial transformations (MATLAB for ex)

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                                            In image registration we have available the input and output images but the specific

                                                                                                                                                                                                                            transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                                            The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                                            images

                                                                                                                                                                                                                            - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                                            same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                                            - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                                            of time (satellite images)

                                                                                                                                                                                                                            Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                                            corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                                            image

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            How to select tie points

                                                                                                                                                                                                                            - interactively selecting them

                                                                                                                                                                                                                            - use of algorithms that try to detect these points

                                                                                                                                                                                                                            - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                                            the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                                            marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                                            for establishing tie points

                                                                                                                                                                                                                            The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                                            of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                                            a bilinear approximation is given by

                                                                                                                                                                                                                            1 2 3 4

                                                                                                                                                                                                                            5 6 7 8

                                                                                                                                                                                                                            x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                                            (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                                            frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                                            subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                                            subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                                            The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                                            problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            Probabilistic Methods

                                                                                                                                                                                                                            zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                                            p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                                            ( ) kk

                                                                                                                                                                                                                            np zM N

                                                                                                                                                                                                                            nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                                            pixels in the image) 1

                                                                                                                                                                                                                            0( ) 1

                                                                                                                                                                                                                            L

                                                                                                                                                                                                                            kk

                                                                                                                                                                                                                            p z

                                                                                                                                                                                                                            The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                                            0( )

                                                                                                                                                                                                                            L

                                                                                                                                                                                                                            k kk

                                                                                                                                                                                                                            m z p z

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            The variance of the intensities is 1

                                                                                                                                                                                                                            2 2

                                                                                                                                                                                                                            0( ) ( )

                                                                                                                                                                                                                            L

                                                                                                                                                                                                                            k kk

                                                                                                                                                                                                                            z m p z

                                                                                                                                                                                                                            The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                                            measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                                            ( ) is used

                                                                                                                                                                                                                            The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                                            0( ) ( ) ( )

                                                                                                                                                                                                                            Ln

                                                                                                                                                                                                                            n k kk

                                                                                                                                                                                                                            z z m p z

                                                                                                                                                                                                                            ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                                            3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                                            ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                            mean

                                                                                                                                                                                                                            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                            Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                            Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                            Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                            ( ) ( )g x y T f x y

                                                                                                                                                                                                                            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                            neighborhood of (x y)

                                                                                                                                                                                                                            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                            and much smaller in size than the image

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                            called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                            ( )s T r

                                                                                                                                                                                                                            s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                            Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                            darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                            is called contrast stretching

                                                                                                                                                                                                                            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                            thresholding function

                                                                                                                                                                                                                            Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                            Image Negatives

                                                                                                                                                                                                                            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                            - equivalent of a photographic negative

                                                                                                                                                                                                                            - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                            image

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            Original Negative image

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                            Some basic intensity transformation functions

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                            range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                            while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                            transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                            variations in pixel values

                                                                                                                                                                                                                            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            Power-Law (Gamma) Transformations

                                                                                                                                                                                                                            - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                            Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                            of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                            curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                            1c - identity transformation

                                                                                                                                                                                                                            A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                            power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                            gamma correction

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                            Contrast stretching

                                                                                                                                                                                                                            - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                            intensity range of the recording tool or display device

                                                                                                                                                                                                                            a b c d Fig5

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            11

                                                                                                                                                                                                                            1

                                                                                                                                                                                                                            2 1 1 21 2

                                                                                                                                                                                                                            2 1 2 1

                                                                                                                                                                                                                            22

                                                                                                                                                                                                                            2

                                                                                                                                                                                                                            [0 ]

                                                                                                                                                                                                                            ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                            ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                            s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                            r r r rs L r r r L

                                                                                                                                                                                                                            L r

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                            1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                            Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                            in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                            from their original range to the full range [0 L-1]

                                                                                                                                                                                                                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                            2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                            The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                            image of pollen magnified approximately 700 times

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            Intensity-level slicing

                                                                                                                                                                                                                            - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                            There are two approaches for intensity-level slicing

                                                                                                                                                                                                                            1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                            another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                            intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                            the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                            Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                            blockageshellip)

                                                                                                                                                                                                                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                            image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                            Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            Bit-plane slicing

                                                                                                                                                                                                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                            This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                            • DIP 1 2017
                                                                                                                                                                                                                            • DIP 02 (2017)

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                                              In image registration we have available the input and output images but the specific

                                                                                                                                                                                                                              transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                                              The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                                              images

                                                                                                                                                                                                                              - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                                              same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                                              - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                                              of time (satellite images)

                                                                                                                                                                                                                              Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                                              corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                                              image

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              How to select tie points

                                                                                                                                                                                                                              - interactively selecting them

                                                                                                                                                                                                                              - use of algorithms that try to detect these points

                                                                                                                                                                                                                              - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                                              the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                                              marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                                              for establishing tie points

                                                                                                                                                                                                                              The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                                              of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                                              a bilinear approximation is given by

                                                                                                                                                                                                                              1 2 3 4

                                                                                                                                                                                                                              5 6 7 8

                                                                                                                                                                                                                              x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                                              (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                                              frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                                              subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                                              subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                                              The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                                              problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              Probabilistic Methods

                                                                                                                                                                                                                              zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                                              p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                                              ( ) kk

                                                                                                                                                                                                                              np zM N

                                                                                                                                                                                                                              nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                                              pixels in the image) 1

                                                                                                                                                                                                                              0( ) 1

                                                                                                                                                                                                                              L

                                                                                                                                                                                                                              kk

                                                                                                                                                                                                                              p z

                                                                                                                                                                                                                              The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                                              0( )

                                                                                                                                                                                                                              L

                                                                                                                                                                                                                              k kk

                                                                                                                                                                                                                              m z p z

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              The variance of the intensities is 1

                                                                                                                                                                                                                              2 2

                                                                                                                                                                                                                              0( ) ( )

                                                                                                                                                                                                                              L

                                                                                                                                                                                                                              k kk

                                                                                                                                                                                                                              z m p z

                                                                                                                                                                                                                              The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                                              measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                                              ( ) is used

                                                                                                                                                                                                                              The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                                              0( ) ( ) ( )

                                                                                                                                                                                                                              Ln

                                                                                                                                                                                                                              n k kk

                                                                                                                                                                                                                              z z m p z

                                                                                                                                                                                                                              ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                                              3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                                              ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                              mean

                                                                                                                                                                                                                              Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                              Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                              Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                              Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                              ( ) ( )g x y T f x y

                                                                                                                                                                                                                              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                              neighborhood of (x y)

                                                                                                                                                                                                                              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                              and much smaller in size than the image

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                              called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                              ( )s T r

                                                                                                                                                                                                                              s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                              Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                              darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                              is called contrast stretching

                                                                                                                                                                                                                              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                              thresholding function

                                                                                                                                                                                                                              Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                              Image Negatives

                                                                                                                                                                                                                              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                              - equivalent of a photographic negative

                                                                                                                                                                                                                              - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                              image

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              Original Negative image

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                              Some basic intensity transformation functions

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                              range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                              while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                              transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                              variations in pixel values

                                                                                                                                                                                                                              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              Power-Law (Gamma) Transformations

                                                                                                                                                                                                                              - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                              Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                              of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                              curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                              1c - identity transformation

                                                                                                                                                                                                                              A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                              power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                              gamma correction

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                              Contrast stretching

                                                                                                                                                                                                                              - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                              intensity range of the recording tool or display device

                                                                                                                                                                                                                              a b c d Fig5

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              11

                                                                                                                                                                                                                              1

                                                                                                                                                                                                                              2 1 1 21 2

                                                                                                                                                                                                                              2 1 2 1

                                                                                                                                                                                                                              22

                                                                                                                                                                                                                              2

                                                                                                                                                                                                                              [0 ]

                                                                                                                                                                                                                              ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                              ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                              s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                              r r r rs L r r r L

                                                                                                                                                                                                                              L r

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                              1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                              Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                              in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                              from their original range to the full range [0 L-1]

                                                                                                                                                                                                                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                              2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                              The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                              image of pollen magnified approximately 700 times

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              Intensity-level slicing

                                                                                                                                                                                                                              - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                              There are two approaches for intensity-level slicing

                                                                                                                                                                                                                              1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                              another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                              intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                              the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                              Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                              blockageshellip)

                                                                                                                                                                                                                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                              image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                              Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              Bit-plane slicing

                                                                                                                                                                                                                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                              This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                              • DIP 1 2017
                                                                                                                                                                                                                              • DIP 02 (2017)

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                Image registration ndash align two or more images of the same scene

                                                                                                                                                                                                                                In image registration we have available the input and output images but the specific

                                                                                                                                                                                                                                transformation that produced the output image from the input is generally unknown

                                                                                                                                                                                                                                The problem is to estimate the transformation function and then use it to register the two

                                                                                                                                                                                                                                images

                                                                                                                                                                                                                                - it may be of interest to align (register) two or more image taken at approximately the

                                                                                                                                                                                                                                same time but using different imaging systems (MRI scanner and a PET scanner)

                                                                                                                                                                                                                                - align images of a given location taken by the same instrument at different moments

                                                                                                                                                                                                                                of time (satellite images)

                                                                                                                                                                                                                                Solving the problem using tie points (also called control points) which are

                                                                                                                                                                                                                                corresponding points whose locations are known precisely in the input and reference

                                                                                                                                                                                                                                image

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                How to select tie points

                                                                                                                                                                                                                                - interactively selecting them

                                                                                                                                                                                                                                - use of algorithms that try to detect these points

                                                                                                                                                                                                                                - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                                                the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                                                marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                                                for establishing tie points

                                                                                                                                                                                                                                The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                                                of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                                                a bilinear approximation is given by

                                                                                                                                                                                                                                1 2 3 4

                                                                                                                                                                                                                                5 6 7 8

                                                                                                                                                                                                                                x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                                                (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                                                frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                                                subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                                                subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                                                The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                                                problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                Probabilistic Methods

                                                                                                                                                                                                                                zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                                                p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                                                ( ) kk

                                                                                                                                                                                                                                np zM N

                                                                                                                                                                                                                                nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                                                pixels in the image) 1

                                                                                                                                                                                                                                0( ) 1

                                                                                                                                                                                                                                L

                                                                                                                                                                                                                                kk

                                                                                                                                                                                                                                p z

                                                                                                                                                                                                                                The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                                                0( )

                                                                                                                                                                                                                                L

                                                                                                                                                                                                                                k kk

                                                                                                                                                                                                                                m z p z

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                The variance of the intensities is 1

                                                                                                                                                                                                                                2 2

                                                                                                                                                                                                                                0( ) ( )

                                                                                                                                                                                                                                L

                                                                                                                                                                                                                                k kk

                                                                                                                                                                                                                                z m p z

                                                                                                                                                                                                                                The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                                                measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                                                ( ) is used

                                                                                                                                                                                                                                The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                                                0( ) ( ) ( )

                                                                                                                                                                                                                                Ln

                                                                                                                                                                                                                                n k kk

                                                                                                                                                                                                                                z z m p z

                                                                                                                                                                                                                                ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                                                3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                                                ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                                mean

                                                                                                                                                                                                                                Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                                Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                                Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                                Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                                ( ) ( )g x y T f x y

                                                                                                                                                                                                                                f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                                neighborhood of (x y)

                                                                                                                                                                                                                                - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                                and much smaller in size than the image

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                                called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                                ( )s T r

                                                                                                                                                                                                                                s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                                Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                                darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                                is called contrast stretching

                                                                                                                                                                                                                                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                                thresholding function

                                                                                                                                                                                                                                Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                                Image Negatives

                                                                                                                                                                                                                                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                                - equivalent of a photographic negative

                                                                                                                                                                                                                                - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                                image

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                Original Negative image

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                                Some basic intensity transformation functions

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                                range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                                while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                                transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                                variations in pixel values

                                                                                                                                                                                                                                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                Power-Law (Gamma) Transformations

                                                                                                                                                                                                                                - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                                Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                1c - identity transformation

                                                                                                                                                                                                                                A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                gamma correction

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                Contrast stretching

                                                                                                                                                                                                                                - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                intensity range of the recording tool or display device

                                                                                                                                                                                                                                a b c d Fig5

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                11

                                                                                                                                                                                                                                1

                                                                                                                                                                                                                                2 1 1 21 2

                                                                                                                                                                                                                                2 1 2 1

                                                                                                                                                                                                                                22

                                                                                                                                                                                                                                2

                                                                                                                                                                                                                                [0 ]

                                                                                                                                                                                                                                ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                r r r rs L r r r L

                                                                                                                                                                                                                                L r

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                Intensity-level slicing

                                                                                                                                                                                                                                - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                blockageshellip)

                                                                                                                                                                                                                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                Bit-plane slicing

                                                                                                                                                                                                                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                • DIP 1 2017
                                                                                                                                                                                                                                • DIP 02 (2017)

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  How to select tie points

                                                                                                                                                                                                                                  - interactively selecting them

                                                                                                                                                                                                                                  - use of algorithms that try to detect these points

                                                                                                                                                                                                                                  - some imaging systems have physical artifacts (small metallic objects) embedded in

                                                                                                                                                                                                                                  the imaging sensors These objects produce a set of known points (called reseau

                                                                                                                                                                                                                                  marks) directly on all images captured by the system which can be used as guides

                                                                                                                                                                                                                                  for establishing tie points

                                                                                                                                                                                                                                  The problem of estimating the transformation is one of modeling Suppose we have a set

                                                                                                                                                                                                                                  of 4 tie points both on the input image and the reference image A simple model based on

                                                                                                                                                                                                                                  a bilinear approximation is given by

                                                                                                                                                                                                                                  1 2 3 4

                                                                                                                                                                                                                                  5 6 7 8

                                                                                                                                                                                                                                  x c v c w c v w cy c v c w c v w c

                                                                                                                                                                                                                                  (v w) and (x y) are the coordinates of the tie points (we get a 8x8 linear system for ci )

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                                                  frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                                                  subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                                                  subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                                                  The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                                                  problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  Probabilistic Methods

                                                                                                                                                                                                                                  zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                                                  p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                                                  ( ) kk

                                                                                                                                                                                                                                  np zM N

                                                                                                                                                                                                                                  nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                                                  pixels in the image) 1

                                                                                                                                                                                                                                  0( ) 1

                                                                                                                                                                                                                                  L

                                                                                                                                                                                                                                  kk

                                                                                                                                                                                                                                  p z

                                                                                                                                                                                                                                  The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                                                  0( )

                                                                                                                                                                                                                                  L

                                                                                                                                                                                                                                  k kk

                                                                                                                                                                                                                                  m z p z

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  The variance of the intensities is 1

                                                                                                                                                                                                                                  2 2

                                                                                                                                                                                                                                  0( ) ( )

                                                                                                                                                                                                                                  L

                                                                                                                                                                                                                                  k kk

                                                                                                                                                                                                                                  z m p z

                                                                                                                                                                                                                                  The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                                                  measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                                                  ( ) is used

                                                                                                                                                                                                                                  The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                                                  0( ) ( ) ( )

                                                                                                                                                                                                                                  Ln

                                                                                                                                                                                                                                  n k kk

                                                                                                                                                                                                                                  z z m p z

                                                                                                                                                                                                                                  ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                                                  3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                                                  ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                                  mean

                                                                                                                                                                                                                                  Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                                  Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                                  Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                                  Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                                  ( ) ( )g x y T f x y

                                                                                                                                                                                                                                  f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                                  neighborhood of (x y)

                                                                                                                                                                                                                                  - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                                  and much smaller in size than the image

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                                  called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                                  ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                                  ( )s T r

                                                                                                                                                                                                                                  s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                                  Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                                  darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                                  is called contrast stretching

                                                                                                                                                                                                                                  Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                                  thresholding function

                                                                                                                                                                                                                                  Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                                  Image Negatives

                                                                                                                                                                                                                                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                                  - equivalent of a photographic negative

                                                                                                                                                                                                                                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                                  image

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  Original Negative image

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                                  Some basic intensity transformation functions

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                                  range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                                  while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                                  transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                                  variations in pixel values

                                                                                                                                                                                                                                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  Power-Law (Gamma) Transformations

                                                                                                                                                                                                                                  - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                                  Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                  of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                  curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                  1c - identity transformation

                                                                                                                                                                                                                                  A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                  power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                  gamma correction

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                  Contrast stretching

                                                                                                                                                                                                                                  - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                  intensity range of the recording tool or display device

                                                                                                                                                                                                                                  a b c d Fig5

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  11

                                                                                                                                                                                                                                  1

                                                                                                                                                                                                                                  2 1 1 21 2

                                                                                                                                                                                                                                  2 1 2 1

                                                                                                                                                                                                                                  22

                                                                                                                                                                                                                                  2

                                                                                                                                                                                                                                  [0 ]

                                                                                                                                                                                                                                  ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                  ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                  s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                  r r r rs L r r r L

                                                                                                                                                                                                                                  L r

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                  1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                  Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                  in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                  from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                  2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                  The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                  image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  Intensity-level slicing

                                                                                                                                                                                                                                  - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                  There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                  1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                  another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                  intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                  the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                  Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                  blockageshellip)

                                                                                                                                                                                                                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                  image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                  Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  Bit-plane slicing

                                                                                                                                                                                                                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                  This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                  • DIP 1 2017
                                                                                                                                                                                                                                  • DIP 02 (2017)

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    When 4 tie points are insufficient to obtain satisfactory registration an approach used

                                                                                                                                                                                                                                    frequently is to select a larger number of tie points and using this new set of tie points

                                                                                                                                                                                                                                    subdivide the image in rectangular regions marked by groups of 4 tie points On the

                                                                                                                                                                                                                                    subregions marked by 4 tie points we applied the transformation model described above

                                                                                                                                                                                                                                    The number of tie points and the sophistication of the model required to solve the register

                                                                                                                                                                                                                                    problem depend on the severity of the geometrical distortion

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    Probabilistic Methods

                                                                                                                                                                                                                                    zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                                                    p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                                                    ( ) kk

                                                                                                                                                                                                                                    np zM N

                                                                                                                                                                                                                                    nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                                                    pixels in the image) 1

                                                                                                                                                                                                                                    0( ) 1

                                                                                                                                                                                                                                    L

                                                                                                                                                                                                                                    kk

                                                                                                                                                                                                                                    p z

                                                                                                                                                                                                                                    The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                                                    0( )

                                                                                                                                                                                                                                    L

                                                                                                                                                                                                                                    k kk

                                                                                                                                                                                                                                    m z p z

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    The variance of the intensities is 1

                                                                                                                                                                                                                                    2 2

                                                                                                                                                                                                                                    0( ) ( )

                                                                                                                                                                                                                                    L

                                                                                                                                                                                                                                    k kk

                                                                                                                                                                                                                                    z m p z

                                                                                                                                                                                                                                    The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                                                    measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                                                    ( ) is used

                                                                                                                                                                                                                                    The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                                                    0( ) ( ) ( )

                                                                                                                                                                                                                                    Ln

                                                                                                                                                                                                                                    n k kk

                                                                                                                                                                                                                                    z z m p z

                                                                                                                                                                                                                                    ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                                                    3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                                                    ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                                    mean

                                                                                                                                                                                                                                    Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                                    Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                                    Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                                    Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                                    ( ) ( )g x y T f x y

                                                                                                                                                                                                                                    f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                                    neighborhood of (x y)

                                                                                                                                                                                                                                    - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                                    and much smaller in size than the image

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                                    called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                                    ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                                    ( )s T r

                                                                                                                                                                                                                                    s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                                    Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                                    darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                                    is called contrast stretching

                                                                                                                                                                                                                                    Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                                    thresholding function

                                                                                                                                                                                                                                    Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                                    Image Negatives

                                                                                                                                                                                                                                    The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                                    - equivalent of a photographic negative

                                                                                                                                                                                                                                    - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                                    image

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    Original Negative image

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                                    Some basic intensity transformation functions

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                                    range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                                    while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                                    transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                                    variations in pixel values

                                                                                                                                                                                                                                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    Power-Law (Gamma) Transformations

                                                                                                                                                                                                                                    - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                                    Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                    of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                    curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                    1c - identity transformation

                                                                                                                                                                                                                                    A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                    power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                    gamma correction

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                    Contrast stretching

                                                                                                                                                                                                                                    - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                    intensity range of the recording tool or display device

                                                                                                                                                                                                                                    a b c d Fig5

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    11

                                                                                                                                                                                                                                    1

                                                                                                                                                                                                                                    2 1 1 21 2

                                                                                                                                                                                                                                    2 1 2 1

                                                                                                                                                                                                                                    22

                                                                                                                                                                                                                                    2

                                                                                                                                                                                                                                    [0 ]

                                                                                                                                                                                                                                    ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                    ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                    s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                    r r r rs L r r r L

                                                                                                                                                                                                                                    L r

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                    1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                    Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                    in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                    from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                    2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                    The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                    image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    Intensity-level slicing

                                                                                                                                                                                                                                    - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                    There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                    1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                    another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                    intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                    the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                    Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                    blockageshellip)

                                                                                                                                                                                                                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                    image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                    Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    Bit-plane slicing

                                                                                                                                                                                                                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                    This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                    • DIP 1 2017
                                                                                                                                                                                                                                    • DIP 02 (2017)

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      a b c d (a) ndash reference image (b) ndash geometrically distorted image (c) - registered image (d) ndash difference between (a) and (c)

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      Probabilistic Methods

                                                                                                                                                                                                                                      zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                                                      p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                                                      ( ) kk

                                                                                                                                                                                                                                      np zM N

                                                                                                                                                                                                                                      nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                                                      pixels in the image) 1

                                                                                                                                                                                                                                      0( ) 1

                                                                                                                                                                                                                                      L

                                                                                                                                                                                                                                      kk

                                                                                                                                                                                                                                      p z

                                                                                                                                                                                                                                      The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                                                      0( )

                                                                                                                                                                                                                                      L

                                                                                                                                                                                                                                      k kk

                                                                                                                                                                                                                                      m z p z

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      The variance of the intensities is 1

                                                                                                                                                                                                                                      2 2

                                                                                                                                                                                                                                      0( ) ( )

                                                                                                                                                                                                                                      L

                                                                                                                                                                                                                                      k kk

                                                                                                                                                                                                                                      z m p z

                                                                                                                                                                                                                                      The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                                                      measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                                                      ( ) is used

                                                                                                                                                                                                                                      The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                                                      0( ) ( ) ( )

                                                                                                                                                                                                                                      Ln

                                                                                                                                                                                                                                      n k kk

                                                                                                                                                                                                                                      z z m p z

                                                                                                                                                                                                                                      ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                                                      3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                                                      ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                                      mean

                                                                                                                                                                                                                                      Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                                      Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                                      Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                                      Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                                      ( ) ( )g x y T f x y

                                                                                                                                                                                                                                      f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                                      neighborhood of (x y)

                                                                                                                                                                                                                                      - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                                      and much smaller in size than the image

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                                      called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                                      ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                                      ( )s T r

                                                                                                                                                                                                                                      s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                                      Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                                      darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                                      is called contrast stretching

                                                                                                                                                                                                                                      Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                                      thresholding function

                                                                                                                                                                                                                                      Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                                      Image Negatives

                                                                                                                                                                                                                                      The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                                      - equivalent of a photographic negative

                                                                                                                                                                                                                                      - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                                      image

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      Original Negative image

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                                      Some basic intensity transformation functions

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                                      range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                                      while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                                      transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                                      variations in pixel values

                                                                                                                                                                                                                                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      Power-Law (Gamma) Transformations

                                                                                                                                                                                                                                      - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                                      Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                      of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                      curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                      1c - identity transformation

                                                                                                                                                                                                                                      A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                      power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                      gamma correction

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                      Contrast stretching

                                                                                                                                                                                                                                      - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                      intensity range of the recording tool or display device

                                                                                                                                                                                                                                      a b c d Fig5

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      11

                                                                                                                                                                                                                                      1

                                                                                                                                                                                                                                      2 1 1 21 2

                                                                                                                                                                                                                                      2 1 2 1

                                                                                                                                                                                                                                      22

                                                                                                                                                                                                                                      2

                                                                                                                                                                                                                                      [0 ]

                                                                                                                                                                                                                                      ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                      ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                      s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                      r r r rs L r r r L

                                                                                                                                                                                                                                      L r

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                      1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                      Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                      in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                      from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                      2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                      The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                      image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      Intensity-level slicing

                                                                                                                                                                                                                                      - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                      There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                      1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                      another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                      intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                      the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                      Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                      blockageshellip)

                                                                                                                                                                                                                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                      image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                      Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      Bit-plane slicing

                                                                                                                                                                                                                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                      This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                      • DIP 1 2017
                                                                                                                                                                                                                                      • DIP 02 (2017)

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        Probabilistic Methods

                                                                                                                                                                                                                                        zi = the values of all possible intensities in an MtimesN digital image i=01hellipL-1

                                                                                                                                                                                                                                        p(zk) = the probability that the intensity level zk occurs in the given image

                                                                                                                                                                                                                                        ( ) kk

                                                                                                                                                                                                                                        np zM N

                                                                                                                                                                                                                                        nk = the number of times that intensity zk occurs in the image (MN is the total number of

                                                                                                                                                                                                                                        pixels in the image) 1

                                                                                                                                                                                                                                        0( ) 1

                                                                                                                                                                                                                                        L

                                                                                                                                                                                                                                        kk

                                                                                                                                                                                                                                        p z

                                                                                                                                                                                                                                        The mean (average) intensity of an image is given by 1

                                                                                                                                                                                                                                        0( )

                                                                                                                                                                                                                                        L

                                                                                                                                                                                                                                        k kk

                                                                                                                                                                                                                                        m z p z

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        The variance of the intensities is 1

                                                                                                                                                                                                                                        2 2

                                                                                                                                                                                                                                        0( ) ( )

                                                                                                                                                                                                                                        L

                                                                                                                                                                                                                                        k kk

                                                                                                                                                                                                                                        z m p z

                                                                                                                                                                                                                                        The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                                                        measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                                                        ( ) is used

                                                                                                                                                                                                                                        The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                                                        0( ) ( ) ( )

                                                                                                                                                                                                                                        Ln

                                                                                                                                                                                                                                        n k kk

                                                                                                                                                                                                                                        z z m p z

                                                                                                                                                                                                                                        ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                                                        3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                                                        ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                                        mean

                                                                                                                                                                                                                                        Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                                        Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                                        Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                                        Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                                        ( ) ( )g x y T f x y

                                                                                                                                                                                                                                        f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                                        neighborhood of (x y)

                                                                                                                                                                                                                                        - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                                        and much smaller in size than the image

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                                        called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                                        ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                                        ( )s T r

                                                                                                                                                                                                                                        s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                                        Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                                        darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                                        is called contrast stretching

                                                                                                                                                                                                                                        Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                                        thresholding function

                                                                                                                                                                                                                                        Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                                        Image Negatives

                                                                                                                                                                                                                                        The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                                        - equivalent of a photographic negative

                                                                                                                                                                                                                                        - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                                        image

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        Original Negative image

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                                        Some basic intensity transformation functions

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                                        range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                                        while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                                        transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                                        variations in pixel values

                                                                                                                                                                                                                                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        Power-Law (Gamma) Transformations

                                                                                                                                                                                                                                        - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                                        Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                        of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                        curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                        1c - identity transformation

                                                                                                                                                                                                                                        A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                        power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                        gamma correction

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                        Contrast stretching

                                                                                                                                                                                                                                        - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                        intensity range of the recording tool or display device

                                                                                                                                                                                                                                        a b c d Fig5

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        11

                                                                                                                                                                                                                                        1

                                                                                                                                                                                                                                        2 1 1 21 2

                                                                                                                                                                                                                                        2 1 2 1

                                                                                                                                                                                                                                        22

                                                                                                                                                                                                                                        2

                                                                                                                                                                                                                                        [0 ]

                                                                                                                                                                                                                                        ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                        ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                        s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                        r r r rs L r r r L

                                                                                                                                                                                                                                        L r

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                        1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                        Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                        in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                        from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                        2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                        The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                        image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        Intensity-level slicing

                                                                                                                                                                                                                                        - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                        There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                        1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                        another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                        intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                        the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                        Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                        blockageshellip)

                                                                                                                                                                                                                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                        image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                        Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        Bit-plane slicing

                                                                                                                                                                                                                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                        This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                        • DIP 1 2017
                                                                                                                                                                                                                                        • DIP 02 (2017)

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          The variance of the intensities is 1

                                                                                                                                                                                                                                          2 2

                                                                                                                                                                                                                                          0( ) ( )

                                                                                                                                                                                                                                          L

                                                                                                                                                                                                                                          k kk

                                                                                                                                                                                                                                          z m p z

                                                                                                                                                                                                                                          The variance is a measure of the spread of the values of z about the mean so it is a

                                                                                                                                                                                                                                          measure of image contrast Usually for measuring image contrast the standard deviation

                                                                                                                                                                                                                                          ( ) is used

                                                                                                                                                                                                                                          The n-th moment of a random variable z about the mean is defined as 1

                                                                                                                                                                                                                                          0( ) ( ) ( )

                                                                                                                                                                                                                                          Ln

                                                                                                                                                                                                                                          n k kk

                                                                                                                                                                                                                                          z z m p z

                                                                                                                                                                                                                                          ( 20 1 2( ) 1 ( ) 0 ( )z z z )

                                                                                                                                                                                                                                          3( ) 0z the intensities are biased to values higher than the mean

                                                                                                                                                                                                                                          ( 3( ) 0z the intensities are biased to values lower than the mean

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                                          mean

                                                                                                                                                                                                                                          Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                                          Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                                          Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                                          Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                                          ( ) ( )g x y T f x y

                                                                                                                                                                                                                                          f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                                          neighborhood of (x y)

                                                                                                                                                                                                                                          - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                                          and much smaller in size than the image

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                                          called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                                          ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                                          ( )s T r

                                                                                                                                                                                                                                          s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                                          Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                                          darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                                          is called contrast stretching

                                                                                                                                                                                                                                          Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                                          thresholding function

                                                                                                                                                                                                                                          Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                                          Image Negatives

                                                                                                                                                                                                                                          The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                                          - equivalent of a photographic negative

                                                                                                                                                                                                                                          - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                                          image

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          Original Negative image

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                                          Some basic intensity transformation functions

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                                          range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                                          while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                                          transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                                          variations in pixel values

                                                                                                                                                                                                                                          Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                                          Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                                          a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          Power-Law (Gamma) Transformations

                                                                                                                                                                                                                                          - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                                          Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                          of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                          curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                          1c - identity transformation

                                                                                                                                                                                                                                          A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                          power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                          gamma correction

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                          Contrast stretching

                                                                                                                                                                                                                                          - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                          intensity range of the recording tool or display device

                                                                                                                                                                                                                                          a b c d Fig5

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          11

                                                                                                                                                                                                                                          1

                                                                                                                                                                                                                                          2 1 1 21 2

                                                                                                                                                                                                                                          2 1 2 1

                                                                                                                                                                                                                                          22

                                                                                                                                                                                                                                          2

                                                                                                                                                                                                                                          [0 ]

                                                                                                                                                                                                                                          ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                          ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                          s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                          r r r rs L r r r L

                                                                                                                                                                                                                                          L r

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                          1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                          Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                          in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                          from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                          2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                          The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                          image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          Intensity-level slicing

                                                                                                                                                                                                                                          - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                          There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                          1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                          another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                          intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                          the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                          Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                          blockageshellip)

                                                                                                                                                                                                                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                          image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                          Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          Bit-plane slicing

                                                                                                                                                                                                                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                          This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                          • DIP 1 2017
                                                                                                                                                                                                                                          • DIP 02 (2017)

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            3( ) 0z the intensities are distributed approximately equally on both side of the

                                                                                                                                                                                                                                            mean

                                                                                                                                                                                                                                            Fig1 (a) Low contrast (b) medium contrast (c) high contrast

                                                                                                                                                                                                                                            Figure 1(a) ndash standard deviation 143 (variance = 2045)

                                                                                                                                                                                                                                            Figure 1(b) ndash standard deviation 316 (variance = 9986)

                                                                                                                                                                                                                                            Figure 1(c) ndash standard deviation 492 (variance = 24206)

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                                            ( ) ( )g x y T f x y

                                                                                                                                                                                                                                            f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                                            neighborhood of (x y)

                                                                                                                                                                                                                                            - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                                            and much smaller in size than the image

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                                            called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                                            ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                                            ( )s T r

                                                                                                                                                                                                                                            s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                                            Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                                            darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                                            is called contrast stretching

                                                                                                                                                                                                                                            Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                                            thresholding function

                                                                                                                                                                                                                                            Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                                            Image Negatives

                                                                                                                                                                                                                                            The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                                            - equivalent of a photographic negative

                                                                                                                                                                                                                                            - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                                            image

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            Original Negative image

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                                            Some basic intensity transformation functions

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                                            range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                                            while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                                            transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                                            variations in pixel values

                                                                                                                                                                                                                                            Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                                            Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                                            a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            Power-Law (Gamma) Transformations

                                                                                                                                                                                                                                            - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                                            Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                            of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                            curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                            1c - identity transformation

                                                                                                                                                                                                                                            A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                            power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                            gamma correction

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                            Contrast stretching

                                                                                                                                                                                                                                            - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                            intensity range of the recording tool or display device

                                                                                                                                                                                                                                            a b c d Fig5

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            11

                                                                                                                                                                                                                                            1

                                                                                                                                                                                                                                            2 1 1 21 2

                                                                                                                                                                                                                                            2 1 2 1

                                                                                                                                                                                                                                            22

                                                                                                                                                                                                                                            2

                                                                                                                                                                                                                                            [0 ]

                                                                                                                                                                                                                                            ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                            ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                            s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                            r r r rs L r r r L

                                                                                                                                                                                                                                            L r

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                            1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                            Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                            in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                            from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                            2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                            The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                            image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            Intensity-level slicing

                                                                                                                                                                                                                                            - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                            There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                            1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                            another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                            intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                            the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                            Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                            blockageshellip)

                                                                                                                                                                                                                                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                            image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                            Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            Bit-plane slicing

                                                                                                                                                                                                                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                            This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                            • DIP 1 2017
                                                                                                                                                                                                                                            • DIP 02 (2017)

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              Intensity Transformations and Spatial Filtering

                                                                                                                                                                                                                                              ( ) ( )g x y T f x y

                                                                                                                                                                                                                                              f(x y) ndash input image g(x y) ndash output image T ndash an operator on f defined over a

                                                                                                                                                                                                                                              neighborhood of (x y)

                                                                                                                                                                                                                                              - the neighborhood of the point (x y) Sxy usually is rectangular centered on (x y)

                                                                                                                                                                                                                                              and much smaller in size than the image

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                                              called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                                              ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                                              ( )s T r

                                                                                                                                                                                                                                              s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                                              Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                                              darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                                              is called contrast stretching

                                                                                                                                                                                                                                              Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                                              thresholding function

                                                                                                                                                                                                                                              Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                                              Image Negatives

                                                                                                                                                                                                                                              The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                                              - equivalent of a photographic negative

                                                                                                                                                                                                                                              - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                                              image

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              Original Negative image

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                                              Some basic intensity transformation functions

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                                              range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                                              while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                                              transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                                              variations in pixel values

                                                                                                                                                                                                                                              Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                                              Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                                              a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              Power-Law (Gamma) Transformations

                                                                                                                                                                                                                                              - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                                              Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                              of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                              curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                              1c - identity transformation

                                                                                                                                                                                                                                              A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                              power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                              gamma correction

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                              Contrast stretching

                                                                                                                                                                                                                                              - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                              intensity range of the recording tool or display device

                                                                                                                                                                                                                                              a b c d Fig5

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              11

                                                                                                                                                                                                                                              1

                                                                                                                                                                                                                                              2 1 1 21 2

                                                                                                                                                                                                                                              2 1 2 1

                                                                                                                                                                                                                                              22

                                                                                                                                                                                                                                              2

                                                                                                                                                                                                                                              [0 ]

                                                                                                                                                                                                                                              ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                              ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                              s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                              r r r rs L r r r L

                                                                                                                                                                                                                                              L r

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                              1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                              Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                              in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                              from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                              2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                              The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                              image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              Intensity-level slicing

                                                                                                                                                                                                                                              - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                              There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                              1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                              another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                              intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                              the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                              Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                              blockageshellip)

                                                                                                                                                                                                                                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                              image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                              Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              Bit-plane slicing

                                                                                                                                                                                                                                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                              This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                              • DIP 1 2017
                                                                                                                                                                                                                                              • DIP 02 (2017)

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                - spatial filtering the operator T (the neighborhood and the operation applied on it) is

                                                                                                                                                                                                                                                called spatial filter (spatial mask kernel template or window)

                                                                                                                                                                                                                                                ( )xyS x y T becomes an intensity (gray-level or mapping) transformation function

                                                                                                                                                                                                                                                ( )s T r

                                                                                                                                                                                                                                                s and r are denoting respectively the intensity of g and f at (x y)

                                                                                                                                                                                                                                                Figure 2 left - T produces an output image of higher contrast than the original by

                                                                                                                                                                                                                                                darkening the intensity levels below k and brightening the levels above k ndash this technique

                                                                                                                                                                                                                                                is called contrast stretching

                                                                                                                                                                                                                                                Fig 2 Intensity transformation functions left - contrast stretching right - thresholding function

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                                                thresholding function

                                                                                                                                                                                                                                                Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                                                Image Negatives

                                                                                                                                                                                                                                                The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                                                - equivalent of a photographic negative

                                                                                                                                                                                                                                                - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                                                image

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                Original Negative image

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                                                Some basic intensity transformation functions

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                                                range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                                                while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                                                transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                                                variations in pixel values

                                                                                                                                                                                                                                                Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                                                Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                                                a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                Power-Law (Gamma) Transformations

                                                                                                                                                                                                                                                - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                                                Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                                of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                                curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                                1c - identity transformation

                                                                                                                                                                                                                                                A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                                power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                                gamma correction

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                                Contrast stretching

                                                                                                                                                                                                                                                - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                                intensity range of the recording tool or display device

                                                                                                                                                                                                                                                a b c d Fig5

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                11

                                                                                                                                                                                                                                                1

                                                                                                                                                                                                                                                2 1 1 21 2

                                                                                                                                                                                                                                                2 1 2 1

                                                                                                                                                                                                                                                22

                                                                                                                                                                                                                                                2

                                                                                                                                                                                                                                                [0 ]

                                                                                                                                                                                                                                                ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                                ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                                s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                                r r r rs L r r r L

                                                                                                                                                                                                                                                L r

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                                1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                                Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                                in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                                from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                                2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                                The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                                image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                Intensity-level slicing

                                                                                                                                                                                                                                                - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                                There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                                1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                                another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                                intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                                the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                                Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                                blockageshellip)

                                                                                                                                                                                                                                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                                image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                                Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                Bit-plane slicing

                                                                                                                                                                                                                                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                • DIP 1 2017
                                                                                                                                                                                                                                                • DIP 02 (2017)

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  Figure 2 right - T produces a binary output image A mapping of this form is called

                                                                                                                                                                                                                                                  thresholding function

                                                                                                                                                                                                                                                  Some Basic Intensity Transformation Functions

                                                                                                                                                                                                                                                  Image Negatives

                                                                                                                                                                                                                                                  The negative of an image with intensity levels in [0 L-1] is obtain using the function ( ) 1s T r L r

                                                                                                                                                                                                                                                  - equivalent of a photographic negative

                                                                                                                                                                                                                                                  - technique suited for enhancing white or gray detail embedded in dark regions of an

                                                                                                                                                                                                                                                  image

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  Original Negative image

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                                                  Some basic intensity transformation functions

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                                                  range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                                                  while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                                                  transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                                                  variations in pixel values

                                                                                                                                                                                                                                                  Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                                                  Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                                                  a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  Power-Law (Gamma) Transformations

                                                                                                                                                                                                                                                  - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                                                  Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                                  of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                                  curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                                  1c - identity transformation

                                                                                                                                                                                                                                                  A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                                  power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                                  gamma correction

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                                  Contrast stretching

                                                                                                                                                                                                                                                  - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                                  intensity range of the recording tool or display device

                                                                                                                                                                                                                                                  a b c d Fig5

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  11

                                                                                                                                                                                                                                                  1

                                                                                                                                                                                                                                                  2 1 1 21 2

                                                                                                                                                                                                                                                  2 1 2 1

                                                                                                                                                                                                                                                  22

                                                                                                                                                                                                                                                  2

                                                                                                                                                                                                                                                  [0 ]

                                                                                                                                                                                                                                                  ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                                  ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                                  s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                                  r r r rs L r r r L

                                                                                                                                                                                                                                                  L r

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                                  1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                                  Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                                  in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                                  from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                                  2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                                  The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                                  image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  Intensity-level slicing

                                                                                                                                                                                                                                                  - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                                  There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                                  1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                                  another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                                  intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                                  the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                                  Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                                  blockageshellip)

                                                                                                                                                                                                                                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                                  image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                                  Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  Bit-plane slicing

                                                                                                                                                                                                                                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                  This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                  • DIP 1 2017
                                                                                                                                                                                                                                                  • DIP 02 (2017)

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    Original Negative image

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                                                    Some basic intensity transformation functions

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                                                    range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                                                    while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                                                    transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                                                    variations in pixel values

                                                                                                                                                                                                                                                    Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                                                    Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                                                    a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    Power-Law (Gamma) Transformations

                                                                                                                                                                                                                                                    - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                                                    Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                                    of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                                    curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                                    1c - identity transformation

                                                                                                                                                                                                                                                    A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                                    power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                                    gamma correction

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                                    Contrast stretching

                                                                                                                                                                                                                                                    - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                                    intensity range of the recording tool or display device

                                                                                                                                                                                                                                                    a b c d Fig5

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    11

                                                                                                                                                                                                                                                    1

                                                                                                                                                                                                                                                    2 1 1 21 2

                                                                                                                                                                                                                                                    2 1 2 1

                                                                                                                                                                                                                                                    22

                                                                                                                                                                                                                                                    2

                                                                                                                                                                                                                                                    [0 ]

                                                                                                                                                                                                                                                    ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                                    ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                                    s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                                    r r r rs L r r r L

                                                                                                                                                                                                                                                    L r

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                                    1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                                    Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                                    in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                                    from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                                    2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                                    The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                                    image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    Intensity-level slicing

                                                                                                                                                                                                                                                    - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                                    There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                                    1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                                    another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                                    intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                                    the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                                    Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                                    blockageshellip)

                                                                                                                                                                                                                                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                                    image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                                    Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    Bit-plane slicing

                                                                                                                                                                                                                                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                    This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                    • DIP 1 2017
                                                                                                                                                                                                                                                    • DIP 02 (2017)

                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                      Log Transformations - constant ( ) log(1 ) 0s T r c r c r

                                                                                                                                                                                                                                                      Some basic intensity transformation functions

                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                      This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                                                      range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                                                      while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                                                      transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                                                      variations in pixel values

                                                                                                                                                                                                                                                      Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                                                      Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                                                      a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                      Power-Law (Gamma) Transformations

                                                                                                                                                                                                                                                      - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                                                      Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                      Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                                      of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                                      curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                                      1c - identity transformation

                                                                                                                                                                                                                                                      A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                                      power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                                      gamma correction

                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                      a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                      Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                                      Contrast stretching

                                                                                                                                                                                                                                                      - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                                      intensity range of the recording tool or display device

                                                                                                                                                                                                                                                      a b c d Fig5

                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                      11

                                                                                                                                                                                                                                                      1

                                                                                                                                                                                                                                                      2 1 1 21 2

                                                                                                                                                                                                                                                      2 1 2 1

                                                                                                                                                                                                                                                      22

                                                                                                                                                                                                                                                      2

                                                                                                                                                                                                                                                      [0 ]

                                                                                                                                                                                                                                                      ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                                      ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                                      s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                                      r r r rs L r r r L

                                                                                                                                                                                                                                                      L r

                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                      1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                                      1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                                      Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                                      Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                                      2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                                      in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                                      from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                                      Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                                      2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                                      The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                                      image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                      Intensity-level slicing

                                                                                                                                                                                                                                                      - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                                      There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                                      1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                                      another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                                      intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                                      the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                                      Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                                      blockageshellip)

                                                                                                                                                                                                                                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                                      image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                                      Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                      Bit-plane slicing

                                                                                                                                                                                                                                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                      This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                      • DIP 1 2017
                                                                                                                                                                                                                                                      • DIP 02 (2017)

                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                        This transformation maps a narrow range of low intensity values in the input into a wider

                                                                                                                                                                                                                                                        range An operator of this type is used to expand the values of dark pixels in an image

                                                                                                                                                                                                                                                        while compressing the higher-level values The opposite is true for the inverse log

                                                                                                                                                                                                                                                        transformation The log functions compress the dynamic range of images with large

                                                                                                                                                                                                                                                        variations in pixel values

                                                                                                                                                                                                                                                        Figure 4(a) ndash intensity values in the range 0 to 15 x 106

                                                                                                                                                                                                                                                        Figure 4(b) = log transformation of Figure 4(a) with c=1 ndash range 0 to 62

                                                                                                                                                                                                                                                        a b (a) ndash Fourier spectrum (b) ndash log transformation applied to (a) c=1 Fig 4

                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                        Power-Law (Gamma) Transformations

                                                                                                                                                                                                                                                        - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                                                        Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                        Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                                        of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                                        curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                                        1c - identity transformation

                                                                                                                                                                                                                                                        A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                                        power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                                        gamma correction

                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                        a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                        Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                                        Contrast stretching

                                                                                                                                                                                                                                                        - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                                        intensity range of the recording tool or display device

                                                                                                                                                                                                                                                        a b c d Fig5

                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                        11

                                                                                                                                                                                                                                                        1

                                                                                                                                                                                                                                                        2 1 1 21 2

                                                                                                                                                                                                                                                        2 1 2 1

                                                                                                                                                                                                                                                        22

                                                                                                                                                                                                                                                        2

                                                                                                                                                                                                                                                        [0 ]

                                                                                                                                                                                                                                                        ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                                        ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                                        s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                                        r r r rs L r r r L

                                                                                                                                                                                                                                                        L r

                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                        1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                                        1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                                        Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                                        Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                                        2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                                        in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                                        from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                                        Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                                        2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                                        The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                                        image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                        Intensity-level slicing

                                                                                                                                                                                                                                                        - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                                        There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                                        1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                                        another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                                        2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                                        intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                                        the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                                        Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                                        blockageshellip)

                                                                                                                                                                                                                                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                                        image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                                        Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                        Bit-plane slicing

                                                                                                                                                                                                                                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                        This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                        • DIP 1 2017
                                                                                                                                                                                                                                                        • DIP 02 (2017)

                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                          Power-Law (Gamma) Transformations

                                                                                                                                                                                                                                                          - positive constants( ) ( ( ) )s T r c r c s c r

                                                                                                                                                                                                                                                          Plots of gamma transformation for different values of γ (c=1)

                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                          Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                                          of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                                          curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                                          1c - identity transformation

                                                                                                                                                                                                                                                          A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                                          power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                                          gamma correction

                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                          a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                          Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                                          Contrast stretching

                                                                                                                                                                                                                                                          - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                                          intensity range of the recording tool or display device

                                                                                                                                                                                                                                                          a b c d Fig5

                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                          11

                                                                                                                                                                                                                                                          1

                                                                                                                                                                                                                                                          2 1 1 21 2

                                                                                                                                                                                                                                                          2 1 2 1

                                                                                                                                                                                                                                                          22

                                                                                                                                                                                                                                                          2

                                                                                                                                                                                                                                                          [0 ]

                                                                                                                                                                                                                                                          ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                                          ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                                          s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                                          r r r rs L r r r L

                                                                                                                                                                                                                                                          L r

                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                          1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                                          1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                                          Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                                          Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                                          2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                                          in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                                          from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                                          Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                                          2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                                          The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                                          image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                          Intensity-level slicing

                                                                                                                                                                                                                                                          - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                                          There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                                          1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                                          another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                                          2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                                          intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                          Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                                          highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                                          medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                                          the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                                          Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                                          Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                                          blockageshellip)

                                                                                                                                                                                                                                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                                          image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                                          Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                          Bit-plane slicing

                                                                                                                                                                                                                                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                          This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                          • DIP 1 2017
                                                                                                                                                                                                                                                          • DIP 02 (2017)

                                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                                            Power-law curves with 1 map a narrow range of dark input values into a wider range

                                                                                                                                                                                                                                                            of output values with the opposite being true for higher values of input values The

                                                                                                                                                                                                                                                            curves with 1 have the opposite effect of those generated with values of 1

                                                                                                                                                                                                                                                            1c - identity transformation

                                                                                                                                                                                                                                                            A variety of devices used for image capture printing and display respond according to a

                                                                                                                                                                                                                                                            power law The process used to correct these power-law response phenomena is called

                                                                                                                                                                                                                                                            gamma correction

                                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                                            a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                                            Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                                            Contrast stretching

                                                                                                                                                                                                                                                            - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                                            intensity range of the recording tool or display device

                                                                                                                                                                                                                                                            a b c d Fig5

                                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                                            11

                                                                                                                                                                                                                                                            1

                                                                                                                                                                                                                                                            2 1 1 21 2

                                                                                                                                                                                                                                                            2 1 2 1

                                                                                                                                                                                                                                                            22

                                                                                                                                                                                                                                                            2

                                                                                                                                                                                                                                                            [0 ]

                                                                                                                                                                                                                                                            ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                                            ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                                            s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                                            r r r rs L r r r L

                                                                                                                                                                                                                                                            L r

                                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                                            1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                                            1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                                            Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                                            Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                                            2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                                            in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                                            from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                                            Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                                            2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                                            The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                                            image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                                            Intensity-level slicing

                                                                                                                                                                                                                                                            - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                                            There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                                            1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                                            another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                                            2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                                            intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                                            Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                                            highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                                            medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                                            the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                                            Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                                            Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                                            which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                                            blockageshellip)

                                                                                                                                                                                                                                                            In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                                            image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                                            Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                                            Bit-plane slicing

                                                                                                                                                                                                                                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                            This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                            • DIP 1 2017
                                                                                                                                                                                                                                                            • DIP 02 (2017)

                                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                                              a b c d (a) ndash aerial image (b) ndash (d) ndash results of applying gamma transformation with c=1 and γ=30 40 and 50 respectively

                                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                                              Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                                              Contrast stretching

                                                                                                                                                                                                                                                              - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                                              intensity range of the recording tool or display device

                                                                                                                                                                                                                                                              a b c d Fig5

                                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                                              11

                                                                                                                                                                                                                                                              1

                                                                                                                                                                                                                                                              2 1 1 21 2

                                                                                                                                                                                                                                                              2 1 2 1

                                                                                                                                                                                                                                                              22

                                                                                                                                                                                                                                                              2

                                                                                                                                                                                                                                                              [0 ]

                                                                                                                                                                                                                                                              ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                                              ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                                              s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                                              r r r rs L r r r L

                                                                                                                                                                                                                                                              L r

                                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                                              1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                                              1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                                              Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                                              Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                                              2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                                              in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                                              from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                                              Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                                              2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                                              The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                                              image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                                              Intensity-level slicing

                                                                                                                                                                                                                                                              - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                                              There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                                              1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                                              another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                                              2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                                              intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                                              Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                                              highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                                              medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                                              the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                                              Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                                              Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                                              which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                                              blockageshellip)

                                                                                                                                                                                                                                                              In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                                              image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                                              Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                                              Bit-plane slicing

                                                                                                                                                                                                                                                              For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                              This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                              of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                              (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                              • DIP 1 2017
                                                                                                                                                                                                                                                              • DIP 02 (2017)

                                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                                Piecewise-Linear Transformations Functions

                                                                                                                                                                                                                                                                Contrast stretching

                                                                                                                                                                                                                                                                - a process that expands the range of intensity levels in an image so it spans the full

                                                                                                                                                                                                                                                                intensity range of the recording tool or display device

                                                                                                                                                                                                                                                                a b c d Fig5

                                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                                11

                                                                                                                                                                                                                                                                1

                                                                                                                                                                                                                                                                2 1 1 21 2

                                                                                                                                                                                                                                                                2 1 2 1

                                                                                                                                                                                                                                                                22

                                                                                                                                                                                                                                                                2

                                                                                                                                                                                                                                                                [0 ]

                                                                                                                                                                                                                                                                ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                                                ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                                                s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                                                r r r rs L r r r L

                                                                                                                                                                                                                                                                L r

                                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                                1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                                                1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                                                Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                                                Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                                                2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                                                in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                                                from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                                                Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                                                2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                                                The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                                                image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                                Intensity-level slicing

                                                                                                                                                                                                                                                                - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                                                There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                                                1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                                                another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                                                2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                                                intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                                Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                                                highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                                                medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                                                the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                                                Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                                                Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                                which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                                                blockageshellip)

                                                                                                                                                                                                                                                                In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                                                image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                                                Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                                Bit-plane slicing

                                                                                                                                                                                                                                                                For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                                This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                                of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                                (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                                • DIP 1 2017
                                                                                                                                                                                                                                                                • DIP 02 (2017)

                                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                                  11

                                                                                                                                                                                                                                                                  1

                                                                                                                                                                                                                                                                  2 1 1 21 2

                                                                                                                                                                                                                                                                  2 1 2 1

                                                                                                                                                                                                                                                                  22

                                                                                                                                                                                                                                                                  2

                                                                                                                                                                                                                                                                  [0 ]

                                                                                                                                                                                                                                                                  ( ) ( )( ) [ ]( ) ( )

                                                                                                                                                                                                                                                                  ( 1 ) [ 1]( 1 )

                                                                                                                                                                                                                                                                  s r r rrs r r s r rT r r r r

                                                                                                                                                                                                                                                                  r r r rs L r r r L

                                                                                                                                                                                                                                                                  L r

                                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                                  1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                                                  1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                                                  Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                                                  Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                                                  2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                                                  in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                                                  from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                                                  Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                                                  2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                                                  The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                                                  image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                                  Intensity-level slicing

                                                                                                                                                                                                                                                                  - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                                                  There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                                                  1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                                                  another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                                                  2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                                                  intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                                  Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                                                  highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                                                  medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                                                  the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                                                  Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                                                  Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                                  which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                                                  blockageshellip)

                                                                                                                                                                                                                                                                  In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                                                  image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                                                  Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                                  Bit-plane slicing

                                                                                                                                                                                                                                                                  For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                                  This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                                  of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                                  (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                                  Digital Image Processing

                                                                                                                                                                                                                                                                  Week 1

                                                                                                                                                                                                                                                                  The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                                  • DIP 1 2017
                                                                                                                                                                                                                                                                  • DIP 02 (2017)

                                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                                    1 1 2 2r s r s identity transformation (no change)

                                                                                                                                                                                                                                                                    1 2 1 2 0 1r r s s L thresholding function

                                                                                                                                                                                                                                                                    Figure 5(b) shows an 8-bit image with low contrast

                                                                                                                                                                                                                                                                    Figure 5(c) - contrast stretching obtained by setting the parameters 1 1 min 0r s r

                                                                                                                                                                                                                                                                    2 2 max 1r s r L where rmin and rmax denote the minimum and maximum gray levels

                                                                                                                                                                                                                                                                    in the image respectively Thus the transformation function stretched the levels linearly

                                                                                                                                                                                                                                                                    from their original range to the full range [0 L-1]

                                                                                                                                                                                                                                                                    Figure 5(d) - the thresholding function was used with 1 1 0r s m

                                                                                                                                                                                                                                                                    2 2 1r s m L where m is the mean gray level in the image

                                                                                                                                                                                                                                                                    The original image on which these results are based is a scanning electron microscope

                                                                                                                                                                                                                                                                    image of pollen magnified approximately 700 times

                                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                                    Intensity-level slicing

                                                                                                                                                                                                                                                                    - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                                                    There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                                                    1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                                                    another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                                                    2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                                                    intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                                    Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                                                    highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                                                    medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                                                    the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                                                    Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                                                    Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                                    which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                                                    blockageshellip)

                                                                                                                                                                                                                                                                    In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                                                    image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                                                    Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                                    Bit-plane slicing

                                                                                                                                                                                                                                                                    For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                                    This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                                    of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                                    (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                                    Digital Image Processing

                                                                                                                                                                                                                                                                    Week 1

                                                                                                                                                                                                                                                                    The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                                    • DIP 1 2017
                                                                                                                                                                                                                                                                    • DIP 02 (2017)

                                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                                      Intensity-level slicing

                                                                                                                                                                                                                                                                      - highlighting a specific range of intensities in an image

                                                                                                                                                                                                                                                                      There are two approaches for intensity-level slicing

                                                                                                                                                                                                                                                                      1 display in one value (white for example) all the values in the range of interest and in

                                                                                                                                                                                                                                                                      another (say black) all other intensities (Figure 311 (a))

                                                                                                                                                                                                                                                                      2 brighten (or darken) the desired range of intensities but leaves unchanged all other

                                                                                                                                                                                                                                                                      intensities in the image (Figure 311 (b))

                                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                                      Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                                                      highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                                                      medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                                                      the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                                                      Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                                                      Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                                      which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                                                      blockageshellip)

                                                                                                                                                                                                                                                                      In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                                                      image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                                                      Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                                      Bit-plane slicing

                                                                                                                                                                                                                                                                      For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                                      This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                                      of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                                      (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                                      Digital Image Processing

                                                                                                                                                                                                                                                                      Week 1

                                                                                                                                                                                                                                                                      The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                                      • DIP 1 2017
                                                                                                                                                                                                                                                                      • DIP 02 (2017)

                                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                                        Figure 6 (left) ndash aortic angiogram near the kidney The purpose of intensity slicing is to

                                                                                                                                                                                                                                                                        highlight the major blood vessels that appear brighter as a result of injecting a contrast

                                                                                                                                                                                                                                                                        medium Figure 6(middle) shows the result of applying the first technique for a band near

                                                                                                                                                                                                                                                                        the top of the scale of intensities This type of enhancement produces a binary image

                                                                                                                                                                                                                                                                        Highlights intensity range [A B] and reduces all other intensities to a lower level

                                                                                                                                                                                                                                                                        Highlights range [A B] and preserves all other intensities

                                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                                        which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                                                        blockageshellip)

                                                                                                                                                                                                                                                                        In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                                                        image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                                                        Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                                        Bit-plane slicing

                                                                                                                                                                                                                                                                        For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                                        This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                                        of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                                        (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                                        Digital Image Processing

                                                                                                                                                                                                                                                                        Week 1

                                                                                                                                                                                                                                                                        The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                                        • DIP 1 2017
                                                                                                                                                                                                                                                                        • DIP 02 (2017)

                                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                                          which is useful for studying the shape of the flow of the contrast substance (to detect

                                                                                                                                                                                                                                                                          blockageshellip)

                                                                                                                                                                                                                                                                          In Figure 312(right) the second technique was used a band of intensities in the mid-gray

                                                                                                                                                                                                                                                                          image around the mean intensity was set to black the other intensities remain unchanged

                                                                                                                                                                                                                                                                          Fig 6 - Aortic angiogram and intensity sliced versions

                                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                                          Bit-plane slicing

                                                                                                                                                                                                                                                                          For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                                          This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                                          of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                                          (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                                          Digital Image Processing

                                                                                                                                                                                                                                                                          Week 1

                                                                                                                                                                                                                                                                          The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                                          • DIP 1 2017
                                                                                                                                                                                                                                                                          • DIP 02 (2017)

                                                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                                                            Bit-plane slicing

                                                                                                                                                                                                                                                                            For a 8-bit image f(x y) is a number in [0255] with 8-bit representation in base 2

                                                                                                                                                                                                                                                                            This technique highlights the contribution made to the whole image appearances by each

                                                                                                                                                                                                                                                                            of the bits An 8-bit image may be considered as being composed of eight 1-bit planes

                                                                                                                                                                                                                                                                            (plane 1 ndash the lowest order bit plane 8 ndash the highest order bit)

                                                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                                                            Digital Image Processing

                                                                                                                                                                                                                                                                            Week 1

                                                                                                                                                                                                                                                                            The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                                            • DIP 1 2017
                                                                                                                                                                                                                                                                            • DIP 02 (2017)

                                                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                                                              Digital Image Processing

                                                                                                                                                                                                                                                                              Week 1

                                                                                                                                                                                                                                                                              The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                                              • DIP 1 2017
                                                                                                                                                                                                                                                                              • DIP 02 (2017)

                                                                                                                                                                                                                                                                                Digital Image Processing

                                                                                                                                                                                                                                                                                Week 1

                                                                                                                                                                                                                                                                                The binary image for the 8-th bit plane of an 8-bit image can be obtained by processing the input image with a threshold intensity transformation function that maps all the intensities between 0 and 127 to 0 and maps all levels between 128 and 255 to 1 The bit-slicing technique is useful for analyzing the relative importance of each bit in the image ndash helps in determining the proper number of bits to use when quantizing the image The technique is also useful for image compression

                                                                                                                                                                                                                                                                                • DIP 1 2017
                                                                                                                                                                                                                                                                                • DIP 02 (2017)

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